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•	Highlights
•	Introduction
•	Causes
•	Risk Factors
•	Symptoms
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•	Treatment
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•	Prevention
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•	Kidney stones

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Kidney stones

Highlights

Diabetes and Kidney Stones

• New research has linked diabetes to the risk of kidney stones. Scientists reporting in the Journal of the American Society of Nephrology found that people with type 2 diabetes have highly acidic urine that can lead to kidney stones, particularly uric acid stones.
• Shock-wave treatment (lithotripsy) for kidney stones may increase one's risk of diabetes later in life. A 2006 study published in the journal Urology found that 17% of patients who received shock-wave lithotripsy eventually developed diabetes. The risk of diabetes was proportionate to the number and intensity of shocks.

High Blood Pressure and Kidney Stones

The same study that linked an increased diabetes risk to shock-wave lithotripsy found that the treatment also increased one's risk of high blood pressure. In the study, the patients who received shock-wave lithotripsy were 47% more likely to develop high blood pressure than those who had their stones treated without lithotripsy.

Tips for Kidney Stone Prevention

• If you have had more than one stone or have a family history of stones, you are likely to develop more stones.
• Drink plenty of water each day. Water washes away the substances that can form stones. There are many controversies surrounding diet and kidney stones, but all experts agree that fluids are key to prevention. Drinking 8 to 10 glasses of water a day is the best way to keep new stones from forming.
• Limit coffee, tea, and sodas with caffeine. Caffeine may cause dehydration.
• Avoid or limit red meat if you are at risk for uric acid stones. Red meat breaks down to form uric acid.
• If you are at risk for calcium oxalate stones, your health care provider may tell you to cut back on oxalate-rich foods such as beets, chocolate, spinach, strawberries, coffee, cola, and tea.

Introduction

Kidney stones are hard, solid rocks that form in the urinary tract. They are one of the most painful ailments. In many cases, the stones are very small and can pass out of the body without any problems. But if a stone (even a small one) becomes lodged and blocks the flow of urine, excruciating pain may result, and prompt medical intervention may be needed.

The process of urination begins in the kidneys. The kidneys process fluids and dissolve waste matter to produce urine. The two kidneys are located deep behind the abdomen below the ribs and toward the middle of the back.

• Each kidney contains over a million nephrons. These are the tiny filtration units of the kidney.
• Each nephron is composed of a tiny group of blood vessels (a glomerulus) enclosed in a funnel-like structure called a Bowman's capsule.
• Each glomerulus filters waste products, water, and salts out of the blood fluid (plasma) that has entered the kidney.
• About 1% of the blood fluid is converted into urine. The rest is returned into circulation to prevent dehydration. Urine's is primarily made of acids, urea, and creatine (nitrogen compounds).
• Urine passes from the Bowman's capsule into tiny tubules that lead to large collecting tubes in the center of the kidney. As the urine passes through this network it becomes increasingly concentrated.
• Urine then flows from the kidney through thin tubes called ureters into the bladder.
• The bladder's elastic walls expand to store the incoming urine until it is eliminated from the body via a tube called the urethra.

The kidneys are responsible for removing wastes from the body, regulating electrolyte balance and blood pressure, and stimulating red blood cell production.

Click the icon to see an image of the urinary tract.

Types of Kidney Stones

Occasionally, various salts build up on the inner surfaces of the kidney and form crystals. Eventually these crystals become large enough to form stones in the kidney, a condition called nephrolithiasis. Kidney stones (renal calculi) may also form in the ureter or the bladder. The salts that form these stones are made up of combinations of minerals and other chemicals, some of which are derived from a person's diet.

Click the icon to see an image of the kidney stones.

Calcium Stones. About 70 - 80% of all kidney stones are made of calcium, usually combined with oxalate, or oxalic acid. Oxalate is found in a number of common vegetables, fruits, and grains. About 6% of calcium stones are composed of calcium phosphate (called brushite).

Uric Acid Stones. About 7% of stones are made up of uric acid, which is formed from a breakdown in purine, a nitrogen compound found in protein. Uric acid is produced in the liver and enters the bloodstream, where most passes into the kidneys and is eliminated in urine. Often, uric acid stones occur with calcium stones.

Struvite Stones. Struvite stones are made of magnesium ammonium phosphate. They are almost always associated with certain urinary tract infections. Worldwide, they make up 30% of all kidney stones. In the United States, however, less than 15% of all stones are struvite. Most struvite stones occur in women. The rate of these stones may be declining in America, perhaps because of better control of urinary tract infections.

Cystine Stones. About 2% of stones in adults and up to 8% of kidney stones in children are caused by a build-up of the amino acid cystine, a building block of protein. The tendency to form these stones is inherited. Cystine stones are marked by rapid growth and recurrence, which, if not treated promptly, can eventually lead to kidney failure.

Xanthine Stones. Others are composed of xanthine, a nitrogen compound. These stones are extremely uncommon and usually occur as a result of a rare genetic disorder.

Click the icon to see an animation about kidney stones.

Causes

The key process in the development of kidney stones is supersaturation.

• Salts, including calcium oxalate, uric acid, cystine, or xanthine, are carried in urine.
• These salts can become extremely concentrated if the volume of urine is significantly reduced or if abnormally high amounts of crystal-forming salts are present.
• When concentration levels reach the point at which the salts no longer dissolve, the salts form crystals.

Different factors may be involved in either reducing urine volume or increasing the levels of the salts.

Deficiencies in Protective Factors. Normally, urine contains substances (magnesium, citrate, pyrophosphate, various proteins, enzymes) that may protect against stone formation. These substances:

• Allow salt in the urine to be at higher-than-normal concentrations without forming crystals
• Prevent crystal formation
• Coat the crystals and prevent them from sticking to the surface of kidney tubes

Deficiencies in these protective substances can cause stones.

Changes in the Acidity of the Urine. Changes in the acid balance of urine can affect stone formation.

• Uric acid and cystine stones thrive in acidic urine.
• Calcium phosphate and struvite stones thrive in alkaline.

Factors that Bind Crystals to the Kidney Tubules. Researchers are studying the cells lining the kidney tubules in order to understand how and why early crystals bind to the tubes long enough to form stones. Under investigation are elevated levels of substances that either cause crystals to stick to the tubes or deficiencies in those that prevent them from sticking.

Causes of Calcium Stones

In general, calcium stones form when there is an imbalance in the urine substances that promote and block the formation of stones. Often, the cause of calcium stones is not known. This condition is called idiopathic nephrolithiasis. Research suggests that abnormalities in metabolism (i.e., digestion and intestinal absorption of calcium or oxalate) are responsible for nearly all stones. Genetic factors may play a role in about half of these cases. A number of medical conditions and drugs can also affect digestion and intestinal absorption.

Excess Calcium in the Urine (Hypercalciuria). About 70% of calcium-containing stones are caused by hypercalciuria, a condition in which there is too much calcium in the urine. A number of conditions may produce hypercalciuria. Many are due to genetic factors, but most cases are idiopathic (due to unknown causes).

The following can lead to hypercalciuria and calcium stones.

• Too much calcium absorption in the intestines. In most of these conditions, genetic factors conspire to increase calcium absorption in the intestine. Researchers are investigating a possible defective gene that regulates calcitriol, a form of vitamin D, which, in excess levels, may increase intestinal absorption of calcium.
• Excessive chloride. Chloride has a negative charge and calcium a positive one, so they are often used by the body to balance each other. Excess chloride may lead to excess calcium. A gene known as CLCN5, which regulates chloride in the urine, is defective in many patients with calcium stones.
• Renal calcium leak. This is a condition in which the filtering processes in the kidney fail, causing an increase of calcium in the urine.
• Excessive sodium. High urinary levels of sodium result in increased levels of calcium. Defects in the kidney tubules transport system that cause imbalances in sodium and phosphate can lead to high calcium in the urine. A high salt diet can also produce this effect.

Excess Oxalate in the Urine (Hyperoxaluria). Oxalate is the most common stone-forming compound. Excessive oxalate in the urine (hyperoxaluria) is responsible for about 30% of calcium stones and is a more common cause of stones than too much calcium in the urine.

Hyperoxaluria is defined as either primary or secondary.

• Primary hyperoxaluria is an inherited disorder in which excess oxalate in the urine is the main problem.
• Secondary hyperoxaluria is caused by specific conditions that result in excess urinary oxalate.

Secondary hyperoxaluria is usually caused by too much dietary oxalates (found in a number of common vegetables, fruits, and grains) or by abnormalities in the body's break down of oxalates. Such defects may be due to various factors:

• Severe vitamin B6 deficiencies (usually due to genetic disorders)
• Deficiencies in Oxalobacter formigene,an intestinal bacteria that degrades oxalate
• Short bowel syndrome, a condition that makes the intestines unable to properly absorb fat and nutrients; calcium may bind to unabsorbed fat instead of oxalates, which causes excess oxalate
• Androgens (male hormones)

Female hormones (estrogens) are linked to a lower risk. Estrogen may help prevent the formation of calcium stones by keeping urine alkaline and raising protective citrate levels.

Excessive Calcium in the Bloodstream (Hypercalcemia). Hypercalcemia generally occurs when bones break down and release too much calcium into the bloodstream. This is a process called resorption. It can occur from a number of different diseases and events:

• Hyperparathyroidism. Overactive parathyroid glands cause about 5% of calcium stones. People with this disorder have at least a 20% chance of developing kidney stones. Women are more likely to have this disorder than men.
• Immobilization. Lack of movement can lead to kidney stones.
• Renal tubular acidosis, a disorder that causes acid and alkaline imbalance. It not only increases calcium levels in the bloodstream, it also reduces citrate levels.

High Levels of Uric Acid (Hyperuricosuria). High levels of uric acid in urine are referred to as hyperuricosuria and occur in between 15 - 20% of people (mostly men) with calcium oxalate stones. (Hyperuricosuria is not related to the acidity of the urine itself.) In such cases, urate (the salt formed from uric acid) creates a crystal nidus (the nucleus of a crystal), around which calcium oxalate crystals form and grow. Such stones tend to be severe and recurrent and appear to be strongly related to a high intake of protein. (Hyperuricosuria also plays a major role in some uric acid stones.)

Low Urine Levels of Citrate (Hypocitraturia). Citrate is the main substance for removing excess calcium. It also blocks the process that turns calcium crystals into stone. Low levels of citrate in the urine ( hypocitraturia) is a significant risk factor for calcium stones. In addition, hypocitraturia also increases the risk for uric acid stones. This condition most likely contributes to about a third of all kidney stones.

Many conditions can reduce citrate levels, but often the causes of hypocitraturia-related stones are unknown. Some causes include:

• Renal tubular acidosis
• Potassium or magnesium deficiency
• Urinary tract infection
• Kidney failure
• Chronic diarrhea

Low Levels of Other Stone-Blocking Compounds. Nephrocalcin-A,uropontin, glycosaminoglycan, magnesium, and pyrophosphate in urine also prevent the formation of calcium stones. If any of these compounds are lacking, stones may develop.

Nanobacteria Infection. Nanobacteria are tiny infectious organisms that can pass from the blood into urine. They coat themselves with mineral deposits that resemble the composition of kidney stones. Cells infected with these bacteria develop mineral deposits on the inside and outside. Researchers believe that nanobacteria may form the cores of the kidney stones in many people.

Causes of Uric Acid Stones

Uric acid is produced when substances in the body called purines break down. Purines are found in human body tissue and certain foods such as dried beans, peas, and liver, and certain alcoholic drinks.

The following conditions are usually seen in patients with uric acid stones:

• Too much acid in the urine for a prolonged period (the most important cause of uric acid stones)
• Lower than normal amounts of urine produced.
• A metabolic disorder that leads to high levels of uric acid in the urine (hyperuricosuria)

Note: Hyperuricosuria can also trigger calcium stones. Therefore, a combination of calcium and uric acid stones may be present in patients with hyperuricosuria.

A number of conditions may contribute to or cause uric acid stones.

• Gout (Uric acid and other kidney stones develop in 10 -25% of patients with primary gout, a painful form of arthritis that occurs when uric acid in the blood forms crystals in one or more joints.)
• Diabetes. New research has shown that people with type 2 diabetes have highly acidic urine that can lead to kidney stones, particularly uric acid stones. The findings are published in the May 2006 Journal of the American Society of Nephrology.
• Insulin resistance
• Possibly kidney abnormalities that reduces ammonia productions, particularly in people with diabetes or insulin resistance
• Genetic factors
• Hypocitraturia, a low amount of citrate in the urine
• Diets overly rich in animal proteins (purines)
• Certain medications (chemotherapy agents, diuretics, and salicylates)
• Binge drinking
• Not eating for long periods of time (fasting)
• Lead poisoning
• Blood cancers (leukemia, multiple myeloma, and lymphomas)
• Some rare types of anemia
• Chronic diarrhea

Causes of Struvite Stones

Struvite stones are almost always caused by urinary tract infections due to bacteria that secrete certain enzymes. These enzymes raise urine concentrations of the ammonia that makes up the crystals that form struvite stones. The stone-promoting bacteria are usually Proteus, but may also include Pseudomonas, Klebsiella, Providencia, Serratia, and staphylococci. Women are twice as likely to have struvite stones as men.

Causes of Other Stones

Other stones, including cystine and xanthine stones, are usually due to genetic abnormalities.

Causes of Cystine Stones. Cystine stones develop from genetic defects that cause abnormal transport of amino acids in the kidney and gastrointestinal system leading to a build-up of cystine, one of these amino acids. Researchers have identified two genes responsible for this condition: SLC3A1 and CLC7A9.

Causes of Xanthine Stones. In some cases, xanthine stones may develop in patients being treated with allopurinol for gout.

Risk Factors

Kidney stones are one of the most common disorders of the urinary tract. They are an ancient health problem. Evidence of kidney stones has been found in an Egyptian mummy estimated to be more than 7,000 years old.

An estimated 1.3 million Americans seek medical help for kidney stones each year. At this time, studies suggest kidney stones affect over 5% of Americans and that the rate has increased since the 1970s, perhaps because of increases in animal and dietary protein intake.

Gender and Age

Men. The risk of kidney stones increases in a man's 40s and continues to rise until age 70. Caucasian men are at higher risk than other groups.

Women. The risk of kidney stones peaks in a woman's 50s. In younger women, stones are more likely to develop during the late stages of pregnancy. Pregnant women tend to have a higher calcium intake, but their kidneys do no handle the calcium as well as they did prior to pregnancy. Kidney stones are still a rare occurrence during pregnancy, however, affecting only 1 in 1,500 pregnancies.

Risk Factors in Children. Stones in the urinary tract in children are usually due to genetic factors. Most of the time, the cause is excess calcium in the urine (hypercalciuria). Deformities in the urinary tract pose a significant risk for kidney stones in children. Children with low birth weight who need to be fed intravenously are also at risk for stones.

Obesity and Weight Gain

Obesity and weight gain are both associated with an increased risk of kidney stones.

• Men who weigh more than 220 lbs are 44% more likely to develop kidney stones than men who weigh less than 150 lbs.
• Women who are obese are 90% more likely to develop kidney stones than women with a lower body mass index (BMI).

Higher BMIs and larger waist circumferences are both risk factors for kidney stones. Researchers think that there may be a link between fat tissue, insulin resistance, and urine composition. People with larger body sizes may excrete more calcium and uric acid, which increase the risk of kidney stone formation.

Family History

A family history of kidney stones increases one's risk for the condition. Researchers are looking into markers or other factors that might predict the onset of stones in relatives, although none has yet been clearly identified. One report found that among the siblings of patients with calcium stones, sisters with higher urinary calcium levels and more acidic urine were more likely to develop stones. Brothers with high urinary calcium, low urinary potassium, and older age were more likely to have the problem. A family history of gout may also predispose a person to stones.

Ethnicity

According to a 2003 study of American ethnic groups, Caucasians have the highest incidence of kidney stones (5.9%) followed by Mexican Americans (2.6%). African Americans have the lowest risk (1.7%).

Geographic Differences

Dietary factors, minerals in local water, or both may contribute to geographic differences that have been observed in the prevalence of kidney stones. Studies have reported the highest occurrence of kidney stones in the southern region of the United States and the lowest in the west. One study suggested that the higher risk may be due to a higher rate of hypertension in the southern states and certain dietary habits, particularly lower intake of magnesium and low use of calcium supplements. Higher rates of kidney stones have been reported in areas of Australia where magnesium levels in drinking water are low. Hard water tends to have higher amounts of protective calcium and magnesium, although evidence suggests that the hardness or softness of water does not significantly affect risk.

Life Style Factors

Specific Foods. In general, certain foods increase the risk for stones only in people who have genetic or medical susceptibility. People whose diets are high in animal protein and low in fiber and fluids may be at higher risk for stones. A number of foods contain oxalic acid, but there is no proof that such foods make any major contribution to calcium oxalate stones in people without other risk factors. However, several studies have shown that increasing dietary calcium and restricting salt, animal protein, and foods rich in oxalate can help prevent calcium oxalate stones from returning.

Stress. One study reported that people who had a major, stressful life experience were more likely to develop stones than those who had not. Some experts speculate that this increased risk may be due to a hormone called vasopressin, which is released in response to stress. Vasopressin also increases the concentration of urine.

Sleep Position. Sleeping in the same position consistently may influence risk. A 2001 study reported that in people who had a history of kidney stones, recurrences tended to occur on the same side that people slept on. An earlier study suggested that people who had kidney stones were more apt to sleep on their stomachs. Movement during sleep did not appear to affect the risk.

Being Bedridden. Any medical or physical condition that keeps a person in bed or immobile increases blood levels of calcium from bone breakdown, thereby posing a risk for stone formation.

Medical Conditions

Gout. A 2002 study reported that the rate of kidney stones in patients with gout was 13%. The study strongly suggests that the two disorders may share a common action.

High Blood Pressure. Persons with high blood pressure are up to three times more likely to develop kidney stones. It is not entirely clear whether having high blood pressure increases the risk for a stone, whether stones lead to high blood pressure, or if there is an action linking both. Some experts suggest that imbalances between uric acid levels in the blood and urine and sodium excretion may put patients with high blood pressure at higher risk.

Inflammatory Bowel Disease. Crohn's disease and ulcerative colitis cause problems in intestinal absorption that significantly increase the risk for kidney stones. Men with these conditions may be at higher risk for stones than women.

Urinary Tract Infections (UTIs). Struvite stones are almost always caused by urinary tract infections.

Hyperparathyroidism. Some people with hyperparathyroidism develop kidney stones. Surgery to remove the parathyroid gland in such patients reduces the risk for stone formation, but the risk still remains high for some time after surgery.

Other Medical Conditions. Kidney disease, chronic diarrhea, certain cancers (e.g., leukemia and lymphomas), and sarcoidosis put people at higher risk for stones.

Medications

AIDS medications. Over 10% of persons with AIDS who take the medicine indinavir develop stones. The risk is even higher in patients with AIDS who also have hepatitis B, hepatitis C, or hemophilia, as well as those who are very thin or who take the antibiotic combination TMP-SMX. In one study of persons with AIDS who took a combination of indinavir, zidovudine, and lamivudine, 36% developed kidney stones.

Other Drugs. Kidney stones are a rare side effect of thyroid hormones and loop diuretics (drugs that increase urination). In fact, diuretics are also used to prevent calcium stones. Certain cancer chemotherapies can also cause kidney stones. Taking medicines for long periods that change the acidic content of urine, such as antacids, may increase susceptibility for kidney stones.

Symptoms

In many cases, kidney stones do not produce symptoms. However, if a stone becomes stuck in the ureter (the thin tube between the bladder and the kidney), symptoms can be very severe. Often, they vary depending on the stone's location and then progress.

Kidney stone attacks tend to be most common late at night or in the early morning, possibly because of minimal urine output or constriction of the ureters during the early morning hours. Kidney stone attacks are least common during the late afternoon

• Pain usually begins abruptly on one side and then usually continues as intense, constant pain. (In some cases it persists for a few minutes, resolves, and then returns after about 10 minutes.)
• The patient cannot become comfortable and usually stands, sits, paces, or reclines in a vain search for a position that will bring relief.
• If the stone is in the kidney or upper urinary tract, the pain usually starts in one flank area (to the side of the back near the waist). It typically moves to the groin as the stone passes down.
• If the stone is too large to pass easily, the pain follows the muscle contractions in the wall of the ureter as they try to squeeze the stone along into the bladder.
• Nausea and vomiting may occur.
• Blood in the urine may be present.
• As the stone passes down the ureter closer to the bladder, a person may feel the need to urinate more often or a burning sensation during urination.
• If fever and chills accompany any of these symptoms, an infection may be present.

The size of the stone does not necessarily predict the severity of the pain; a very tiny crystal with sharp edges can cause intense pain while a larger round stone may not be as distressing. Struvite stones can often occur without symptoms.

Diagnosis

The doctor will perform a physical exam. This includes pressing against abdominal areas for tender locations that might indicate the presence of the stone.

Medical History

The patient's age is a significant factor. Kidney stones that occur in children and young patients are more apt to result from inherited problems that cause cystine, xanthine, or, in some cases, calcium oxalate stones. In adult patients, calcium stones are most common.

A medical history may help predict which crystal has formed the stone. The doctor will need to know the following:

• Any previous kidney stone attacks
• Histories of cancer, sarcoidosis, or small bowel disease
• Any medications being taken, including non-prescription substances, particularly high doses of vitamins D or C and calcium-containing antacids

Ruling Out Other Disorders

Many conditions can cause symptoms similar to kidney stones. Usually the diagnosis is easily made because of the specific nature of the symptoms, but it is not always clear. Urinary tract infections can cause similar, but usually less intense, pain. In fact, infection may be present with a kidney stone. Other causes of pain that may mimic kidney stones include:

• Gallstones
• Diverticulitis
• Intestinal blockage
• Blood clots
• Irritable bowel syndrome
• Appendicitis
• Stomach ulcers
• Hiatal hernia
• Pancreatitis
• Hepatitis
• Pelvic inflammatory disease
• Inflammatory bowel disease (Crohn's and colitis)
• Heart attack

Imaging Techniques

Various imaging techniques are helpful in determining the presence of kidney stones. The best approach uses spiral (or helical) computed tomography scans. It is not always available, however, in which case ultrasound or standard x-rays are usually performed. If no stones show up but the patient has severe pain indicative of kidney stones, the next step is an intravenous pyelogram.

X-Rays. A standard x-ray of the kidneys, ureters, and bladder may be adequate as a first step for identifying many stones, since most are opaque on x-rays. Calcium stones can be identified on x-rays by their white color. Cystine crystals also can show up on x-rays.

Spiral (or Helical) Computed Tomography. A computed tomography (CT) scan called a spiral or helical CT scan is currently the best method for diagnosing stones in either the kidneys or ureters. It is fast, noninvasive, and provides detailed accurate images of even very small stones. If stones are not present, it can often identify other causes of pain in the kidney area. It is superior to x-rays, ultrasound, and intravenous pyelogram--the test that was the previous standard for detecting kidney stones. Experts hope spiral CT will eventually be able to reveal the stone's composition.

Ultrasound. Ultrasound can detect translucent uric acid stones and obstruction in the urinary tract. It is not useful for finding very small stones, but some research indicates that it may be a useful first diagnostic step in the emergency room to help predict the likelihood of a stone, including suspected stones in children.

Intravenous Pyelogram. With intravenous pyelogram (IVP), the patient is injected with a dye, and x-rays are taken as the dye enters the kidneys and travels down the urinary tract. IVP is invasive but, until recently, was the most cost-effective method for detecting stones. Where it is available, spiral CT is now preferred, since it gives a faster diagnosis, is more accurate, and it is similar in cost. In any case, IVP should not be used on patients with kidney failure. There is a risk for an allergic reaction to standard dyes, although newer less allergenic ones are becoming available.

In the procedure intravenous pyelogram (IVP), the patient is injected with dye. X-rays are taken as the dye travels through the urinary tract. This procedure is done to confirm the presence of kidney stones, although some stones may be too small to see.

Magnetic Resonance Imaging. Magnetic resonance imaging (MRI) techniques are showing promise for diagnosing urinary tract obstruction but do not yet accurately reveal nonobstructive or small stones. Because no radiation is involved, however, it may prove to be a good option for pregnant women.

Urine Tests

Urine samples are required to evaluate features of the urine, including its acidity, the presence of red or white blood cells, whether infection is present, any crystals, and elevated or decreased components that inhibit or promote stone formation.

Clean-Catch Urine Sample for Culturing. Once it has been determined that a kidney stone is present, the patient is usually given a collection kit, including filters, to try to catch the stone or gravel as it passes out. A clean-catch urine sample is almost always required for culturing. To provide this, the following steps are taken:

• Patients must first wash their hands thoroughly, then wash the penis or vulva and surrounding area four times with downward strokes, using a new soapy sponge each time.
• The patient must then begin urinating into the toilet and stop after a few drops.
• The patient then positions the container to catch the middle portion of the stream. Ideally, this urine will contain only the bacteria and other evidence of the stone.
• The patient then urinates the remainder into the toilet.
• The cap must then be securely screwed on without the patient touching the inside of the rim.

Click the icon to see an image of a calcium urine test.

Twenty-Four Hour Urine Collection. A 24-hour urine collection may be needed to measure urine volume and levels of acidity, calcium, sodium, uric acid, oxalate, citrate, and creatinine.

• The patient should not change any of his or her usual eating or drinking patterns when performing this test.
• The patient discards the first urination on the day of the test.
• Afterward all urine passed over the next 24 hours is collected, including the first urination on the morning of day two.
• A second 24-hour urine collection may be needed to determine if treatment is working or if the first analysis was not conclusive and the doctor suspects a less common stone, such as a cystine or xanthine stone.

Click the icon to see an image of a uric acid urine test.

Urine tests that are used to determine the specific chemical and biologic factors causing the stone should be performed about 6 six weeks after the attack, since the attack itself may change the levels of such substances, including calcium, phosphate, and citrate. It should be noted that calcium levels in the urine may be abnormal even in many people without stones. In addition, high urinary concentrations of calcium may pose a greater or lesser risk depending on age. (In one 2001 study, middle-aged adults with high urinary calcium concentrations had a much greater risk than older adults with high levels.)

Microscopic Examination

The kidney stones obtained from the sample are examined under a microscope. The crystal formations are often specific enough so that the doctor is able to identify the substance causing the stone.

• Calcium oxalate crystals are eight-sided (octahedral), while calcium phosphate crystals tend to have irregular shapes.
• Uric acid stones are sometimes described as pear- or diamond-shaped.
• Some struvite stones have very specific shapes commonly described as "coffin lids." Struvite crystals may also occur in a formation known as a staghorn, which can be large and damaging to the kidney.

Testing the Acidity of Urine

Testing whether urine is acid or alkaline helps to identify the specific type of stone. The levels of acid or alkaline in any solution, including urine, are indicated by the pH scale:

• A pH value of 7.0 is neutral.
• A solution with a low pH (below 7.0) is acidic. (A low pH favors uric acid and cystine stones.)
• A solution with a high pH is alkaline. (A high pH favors calcium phosphate and struvite stones.)

Testing for Blood in the Urine

A dipstick for blood in the urine (called hematuria) is typically performed when patients appear in the emergency room with flank pain (the primary symptom of kidney stones). About a third of kidney stone patients, however, do not show blood in the urine, so other tests are needed.

Blood Tests

Blood Tests for Stone Factors. Blood and urine tests help determine the substance forming the crystal so that appropriate treatment and preventive measures can be taken. Blood tests may help determine levels of blood urea nitrogen, creatinine, calcium, phosphate, and uric acid for patients with known or suspected calcium oxalate stones. These tests are often scheduled about six weeks after the attack, particularly with recurrent stones, in order to measure these substances when the stone has been passed and the patient has been stabilized.

Parathyroid Tests. Tests to detect parathyroid hormone levels are given if the doctor suspects hyperparathyroidism based on other signs and symptoms.

Tests for Infection. A test result that shows a high white blood cell count might indicate infection, but such results could be misleading, since white cells could also increase in response to the extreme physical stress of a kidney stone attack.

Tests for Metabolic Problems. About half of children with stones have an identifiable metabolic disorder, which increases their risk of stone recurrence five-fold. Experts argue over whether tests for metabolic abnormalities are routinely needed once the stone composition has been determined. Studies suggest the following:

• People with recurrent calcium stones have a wide range of abnormal blood or urine chemistries indicating a variety of metabolic disorders. (For example, calcium stones in women may be due to parathyroid abnormalities.)
• Calcium phosphate stones raise the likelihood of renal tubular acidosis.
• People with non-calcium stones generally have identifiable metabolic disorders.
• Determining the stone composition may be sufficient for treatment and may help avoid unnecessary metabolic tests.

Treatment

When tests have shown there is a kidney stone, the next step is to determine treatment. The patient should be admitted to the emergency room if they have severe vomiting, fever, or symptoms of infection.

Treatment for Severe Attacks

Strong opioid painkillers such as meperidine (Demerol) are often required for a severe kidney stone attack, although doctors will usually not give such drugs until the presence of a kidney stone has been confirmed on an x-ray. In some cases, powerful nonsteroidal anti-inflammatory drugs (NSAIDs) may work just as well as opioids, and they have fewer side effects. However, they do take longer to work.

Watchful Waiting

In about 85% of patients, the kidney stones are small enough that they pass through normal urination, usually within 2 to 3 days. In some cases, a stone may take weeks to months to pass, although pain usually goes away before that.

The patient should drink plenty of water (two to three quarts a day) to help move the stone along, and take painkillers as needed. The doctor usually provides a collection kit with a filter and asks the patient to save any passed stones for testing.

If the stone has not passed in 2 to 3 days, then additional treatments are warranted. In some severe cases, hospitalization may be necessary.

General Guidelines for Surgery

Specific procedures vary depending on the size of the stone or complexity of the situation. Noninvasive procedures are proving to be very beneficial in eliminating stones, and have largely replaced invasive surgeries.

• For small stones that are lodged in the lower part of the ureter, ureteroscopy or shock wave lithotripsy are the procedures of choice.
• For larger stones, ureteroscopy, percutaneous nephrolithotomy, and shock wave lithotripsy are all potentially useful, depending on location, abnormalities, and other considerations.
• In some complicated cases, standard open surgical procedures (called nephrolithotomy) may be required.

See "Other Treatments" section for more information on kidney stone surgery.

Medications

Diuretics. Diuretics are commonly used in the treatment of high blood pressure and other disorders to eliminate fluid and sodium from the body. Low doses of diuretics known as thiazides are sometimes used to reduce the amount of calcium released by the kidneys into the urine. Thiazides include:

• Hydrochlorothiazide (Esidrix, HydroDiuril)
• Chlorothiazide (Diuril)
• Trichlormethiazide (Metahydrin, Naqua)
• Chlorthalidone (Hygroton)

However, thiazides also cause potassium loss, which reduces citrate levels and can increase the risk for stones. Potassium citrate should be taken with a thiazide to prevent citrate loss. Amiloride (Midamor) is a potassium-sparing diuretic, which may be used if a thiazide does not work.

Citrates. Citrate salts are often given to people with calcium oxalate or uric acid stones:

• Potassium magnesium citrate is available over the counter. It is proving to be very beneficial in preventing kidney stones. In one study, potassium magnesium citrate reduced the risk for kidney stone recurrence by 85%.
• Potassium citrate (K-Lyte, Polycitra-K, Urocit-K) is given as the only treatment to people with normal urine calcium levels. Between 70 - 75% of patients with recurrent stones have experienced on-going remission with potassium citrate therapy. Some people cannot tolerate potassium citrate because of side effects (stomach problems).
• Magnesium citrate (Citroma, Citro-Nesia) may be useful for people who develop calcium stones from impaired intestinal absorption due to small bowel disease.

None of these products should be used by people with struvite stones, urinary tract infections, bleeding disorders, or kidney damage. Patients who take citrate supplements containing potassium should not take any other medications that either contain the mineral or prevent its loss (such as so-called potassium-sparing diuretics). People with peptic ulcers should avoid them or discuss using non-tablet forms with their doctor.

Phosphates. Phosphates help reduce the breakdown of bone that releases calcium into the bloodstream. They are also involved in reabsorption of calcium from urine by the kidney.

Phosphate compounds:

• Neutral (nonacidic) sodium or potassium phosphate (K-Phos, Neutral, Neutra-Phos) is usually taken four times a day after meals to prevent kidney stones unless otherwise directed by the doctor. Diarrhea is a possible side effect.
• Cellulose phosphate (Calcibind) is recommended only for severe hypercalciuria caused by excessive absorption of calcium from the intestines that is associated with recurrent calcium stones. However, this drug may increase oxalate levels and decrease magnesium levels, which can lead to stones. Restricting dietary oxalates, calcium, and ascorbic acid, and taking magnesium supplements may help offset these risks. Cellulose phosphate may also cause bloating.

Acidic forms of phosphate should not be used, since they increase the risks for both hypocitraturia and hypercalciuria.

Cholestyramine. Cholestyramine (Questran, Questran Light) is a drug used to reduce cholesterol levels. However, it also binds with oxalate in the intestine, so it is used to reduce elevated oxalate levels in urine (hyperoxaluria). The drug is usually taken in powder form, dissolved in water, milk, or fruit juice; it is also available as a chewable bar (Cholybar). Bloating and constipation are common side effects of this drug. The drug also interferes with other medications, including digoxin (Lanoxin) and warfarin, and may contribute to calcium loss and osteoporosis. In order to prevent such interactions, other drugs should be taken one hour before or 4 to 6 hours after taking cholestyramine. If the drug is taken for a long period of time, deficiencies of vitamins A, D, E, and K can result. Vitamin supplementation may be necessary.

Medications for Uric Acid Stones

Sodium Bicarbonate. Patients whose uric acid stones are caused by persistently acidic urine may take sodium bicarbonate to reduce acidity. Patients taking sodium bicarbonate must test their urine regularly with pH paper, which turns different colors depending on whether the urine is acidic or alkaline. Too much sodium bicarbonate can cause the urine to become overly alkaline and increase the risk for calcium phosphate stones. This treatment should not be used by patients who need to restrict sodium for other medical conditions.

Potassium Citrate. Potassium citrate, which restores citrate to the urine, is useful for patients with high levels of uric acid in the urine.

Allopurinol. Allopurinol (Lupurin, Zyloprim) is very effective in reducing high levels of uric acid and may be helpful for patients with uric acid stones. Allopurinol will not prevent calcium stones from forming. There is also a slight risk for xanthine stones with this drug. Side effects include diarrhea, headache, and fever. More severe complications include blood disorders that may produce fatigue, bleeding, or bruising. About 2% of patients experience an allergic reaction to allopurinol that causes a rash. In rare cases, the rash can become severe and widespread enough to be life threatening. Allergic individuals who had experienced only a mild rash to sodium bicarbonate may be able to build up their tolerance for the drug by undergoing a desensitization process. The drug may also increase the risk for cataracts.

Allopurinol reduces uric acid levels rapidly, so it may trigger an attack of gout in susceptible people. To prevent this, patients taking allopurinol should also take a nonsteroidal anti-inflammatory (NSAID) for 2 or 3 months. Aspirin should not be taken, since it increases uric acid levels. Patients should discuss the appropriate drug with their doctor.

Medications for Struvite Stones

Before any medical treatment is given for struvite stones, the stones must be completely removed with surgery.

Antibiotics for Eliminating Infection. Persons with struvite stones are given on-going antibiotics to keep the urine free of the bacteria that cause urinary tract infections. Careful follow-up and urine testing is extremely important. (A high pH urine indicates low acidity and an increased risk of infection.)

Acetohydroxamic Acid (AHA). Acetohydroxamic acid (AHA or Lithostat) is beneficial when used with long-term antibiotics. AHA blocks substances that are released by bacteria and has been effective in preventing stones even when bacteria are present. Side effects, however, can be severe. The drug reduces iron in the body, so anemia is a common problem. Iron supplements may be needed. Other side effects include nausea, vomiting, depression, anxiety, rash, persistent headache, and, rarely, small blood clots in the legs. Experts recommend this drug only for patients with healthy kidneys who have chronic diseases caused by these specific struvite-causing organisms. Alcohol should be avoided. Pregnant women should not take acetohydroxamic acid.

Organic Acids. Medical treatments to dissolve stones may be useful with in patients who do not respond to other medications, or in combination with surgeries. Acidic urine dissolves struvite stones, so the doctor may wash the urinary tract with a solution of organic acids (e.g., Renacidin). Candidates for irrigation must have sterile urine and healthy kidney function. In surgical patients, irrigation is performed 4 or 5 days after the operation. The urinary tract is washed with saline for 1 to 2 days and, if there are no problems, the organic acid solution is given for another 1 or 2 days until all stones are dissolved. The patient's urine should be tested on a regular basis to be sure that bacteria does not return.

Aluminum Hydroxide Gel. An aluminum hydroxide gel anti-acid may reduce phosphate levels but it carries a long-term risk of aluminum toxicity. Prolonged depletion of phosphorus can also increase the risk for calcium oxalate stones. Experts recommend limiting phosphorus through a low-protein diet.

Medications for Cystine Stones

The first-line treatment for cystine stones is increasing the alkalization of urine so the stone can dissolve. If alkalization fails, drugs such as d-penicillamine, alpha-mercaptopropionylglycine (tiopronine), or captopril may be used to lower cystine concentration. Fluid intake for cystine stones must be even more voluminous than for regular stones. The patient should uniformly drink at least four quarts of water over a 24-hour period.

Other Treatments

Surgery is usually needed if the stone is too large to pass on its own, if there are signs that the stone is growing, or if the stone is blocking the urine flow and causing a urinary tract infection or kidney damage.

Until recently, the procedure to remove a stone was a very painful, major surgery that required a 4- to 6-week recovery period. Today, treatments for stones are much less invasive. Major surgery is performed in less than 2% of patients.

Stone removal procedures:

• Extracorporeal shock wave lithotripsy (ESWL) is used for small stones (less than one centimeter, or slightly less than half an inch) that occur in the upper part of the ureter and do not pass on their own. One study indicated lithotripsy might even be safe and effective for patients whose stones are associated with malformed kidneys, although such patients are at higher risk for stone recurrence and should be carefully monitored.
• Percutaneous nephrolithotomy (PNL). PNL can be used for very large stones in the upper tract, when ESWL fails, for kidney transplant patients, or when there are structural abnormalities in the kidney or surrounding area. PNL is the preferred procedure for drug-resistant cystine stones, which are usually resistant to shock wave therapy.
• Ureteroscopy. For stones in the lower tract, ureteroscopy is generally the best procedure, although lithotripsy is also usually feasible and patients ordinarily prefer it.
• Standard open surgery (nephrolithotomy) may be required if any of these procedures fail or are not appropriate, or in special cases, such as when the patient is very obese.

Most procedures are more effective for calcium and uric acid stones and less effective for struvite and cystine stones, although new techniques may be improving their effects on all stones.

Extracorporeal Shock Wave Lithotripsy

Extracorporeal shock wave lithotripsy (ESWL) is a technique that uses sound waves (ultrasound) to break up simple stones in the kidney or upper urinary tract. ("Extracorporeal" means "outside the body," and "lithotripsy" means stone-breaking.) ESWL is not used for cystine stones. The procedure generally does not work for stones larger than three centimeters in diameter (which is slightly over an inch). There are several variations. The following is a typical procedure:

• Most ESWL procedures use some anesthesia, although they are often done on an outpatient basis.
• The patient is positioned in a water bath. (In some procedures the patient lies on a soft cushion.)
• The procedure uses ultrasound to generate shock waves that travel through the skin and body tissues until they hit the dense stones. (X-rays or ultrasound are used to help the surgeon pinpoint the stone during treatment.)
• The stones are crushed into tiny sand-like pieces that usually pass easily through the urinary tract.
• The shattered stone fragments may cause discomfort as they pass through the urinary tract. In such cases, the doctor may insert a small tube called a stent through the bladder into the ureter to help the fragments pass. This practice, however, has not proved to speed up passage of the stones in most cases and is not used routinely.

Extracorporeal shock wave lithotripsy (ESWL) is a procedure used to shatter simple stones in the kidney or upper urinary tract. Ultrasonic waves are passed through the body until they strike the dense stones. Pulses of sonic waves pulverize the stones, which are then more easily passed through the ureter and out of the body in the urine.

Success rates range from 50 - 90% depending on the location of the stone and the surgeon's technique and level of experience. Recovery time is short, and most people can resume normal activities in a few days.

Complications. Complications may include the following:

• The most common complication is blood in the urine, which lasts for a few days after treatment. To reduce the chances of bleeding, doctors usually tell patients to avoid taking aspirin and other NSAIDs, which can promote bleeding, for 7 to 10 days before the treatment.
• Bruising and minor discomfort due to the shock waves are common in the back or abdomen.
• Sometimes the stone is not completely fragmented with one treatment, and additional treatments then may be required. Inability to pass stone fragments may also be a particular problem in patients who have cysts or other kidney abnormalities.
• Higher risk for diabetes later. A 2006 study published in the journal Urology found that 17% of patients who received shock-wave lithotripsy developed diabetes later in life. The diabetes risk was related to the number and intensity of shocks.
• Higher risk for hypertension (high blood pressure). The same study that linked ESWL to diabetes also showed that people who received shock-wave lithotripsy treatment were 47% more likely to develop high blood pressure than those who had their stones treated without surgery.

ESWL appears to be safe for children, although a 2001 study reported temporary damage in the kidney tubules (glomeruli) after treatment. It is not known if this complication has any long-term consequences. Experts recommend using the least amount of shocks and impact possible in young people. If more than one treatment is needed, there should be a waiting period of at least 15 days.

Percutaneous Nephrolithotomy

Percutaneous nephrolithotomy may be used when ESWL is not available or effective (e.g., if the stone is very large, in an inaccessible location, or is a cystine stone). It is also preferred over ESWL for stones that have remained in the ureter for more than 4 weeks.

It is more effective that ESWL for patients with severe obesity and appears to be safe for the very elderly and the very young. Success rates have been reported to be about 98% for kidney stones and 88% for ureteral stones. They may vary according to the technique and patient group. For example, success rates are slightly lower in children, although the procedure can be done safely in young patients. Long-term effects are unknown.

A typical procedure is as follows:

• The surgeon makes a tiny incision in the back and creates a tunnel directly into the kidney.
• The surgeon then inserts an instrument called a nephroscope through the tunnel.
• The stone is located and removed. If it is large it is destroyed using ultrasound, lasers, or other devices and the fragments are then removed. (An advantage of percutaneous nephrolithotomy over ESWL is that the surgeon is able to remove the stone fragments directly instead of relying on their natural passage from the kidney.)
• Generally, patients stay in the hospital for 5 or 6 days and may need a small device called a nephrostomy tube left in the kidney during the healing process.

Devices Used to Destroy Stones. For large stones, some type of energy device may be needed to break the stone into small pieces. They are referred to as intracorporeal lithotripsy devices (meaning stone breakers within the body). The energy source may be one of the following:

• Ultrasound is employed through a rigid nephroscope and results in a stone-free rate of 94%. It is currently the preferred method.
• A more recent device uses a combination pneumatic drill and ultrasound with stone-free rates of 80 - 89%. It may be prove to be superior to ultrasound alone and to be effective against stones of all types.
• The holmium laser literally melts the stones and destroys up 100% of stones of any composition. It uses a flexible nephroscopy and has an excellent safety record. It should be used sparingly, however, and particularly cautiously with large uric acid stones until more is understood about this effect. Another device, the erbium: YAG laser, is showing promise in lithotripsy but is not currently practical.

Complications. Complication rates are about 3%, with major complications occurring in about 1% of cases. Some scarring occurs, but studies indicate that it does not impair kidney function, even if the patient requires repeat surgery. The procedure also poses a risk for blood loss during and after the procedure, which, in some cases, can be significant. Because large volumes of fluid are used during the procedure, fluid overload is a potential problem, particularly in children or patients with heart disease. In some cases infection may result. Other complications encountered are collapsed lung and injuries to areas outside the kidney but within the operative area, such as the abdomen or chest.

Ureteroscopic Stone Removal

Ureteroscopy may be used for mid- and lower ureter stones. With the advent of smaller instruments, it is also now being done successfully in children as well. The procedure involves the following:

• No incision is made in this procedure, but a general anesthetic is still required.
• The surgeon passes a small fiberoptic instrument called a ureteroscope through the urethra and bladder into the ureter.
• The surgeon then locates the stone or stones.
• Smaller ones are grasped and removed with tiny forceps. Large ones are shattered with lasers or pneumatic drill-like devices.
• A small tube, or stent, may be left in the ureter for a few days after treatment to help the lining of the ureter heal.

Complication rates range from 10 - 20%, with major problems occurring in between 0 - 6% of patients. In some cases, large stones are not broken up into small enough pieces that can be passed, resulting in obstruction of the urinary tract and possible kidney damage. Imaging tests such as ultrasound or spiral CT are useful within 3 months to check for residual stones, and a second procedure may be required. The risk of complications is highest when the procedure is performed by less experienced surgeons and if stones are found in the kidney. The risk for perforation of the ureter is higher the longer the operative time.

Open Surgery (Nephrolithotomy)

Open surgery involves incisions through the patient's flank and into the kidney. The kidneys are cooled down using ice. X-rays are used during the procedure to locate specific areas and the stone. The arteries in the kidney are identified and isolated away from the surgical region. The surgeon locates the collecting system and retrieves the stone. If the surgeon finds any blockage, this is corrected. The surgery is very invasive and is now restricted to the following candidates:

• Patients with very large or complex stones that cannot be removed using less invasive measures
• Very obese patients

Some centers report success with lithotripsy, however, in this patient group, so even these patients should discuss other options with their surgeon.

The procedure is not appropriate for patients with:

• Bleeding or clotting disorders
• Untreated widespread infection
• Severe and chronic kidney insufficiency (unless removing the stone will improve kidney function)

Complications

Between 70 - 90% of crystals remain tiny enough so that they can travel through the urinary tract and pass out of the body in the urine without being noticed. When they cause symptoms, however, kidney stones have been described as one of the most painful disorders to afflict humans. The pain that they cause is sometimes referred to as renal colic. ("Renal" means "kidney.")

Effects on the Urinary Tract and Kidneys

Obstruction and Infection. Although kidney stones often lead to obstruction of the urinary tract, it is usually temporary and causes no lasting damage. In some cases, however, particularly if the obstruction progresses silently, infection may occur, which can be serious and which warrants prompt attention.

Kidney Failure. It is very rare for kidney stones to cause kidney failure, although some people have risk factors that make them more susceptible to this serious complication. They include the following:

• Very frequent recurrences (such as in people with cystine stones or other inherited forms of kidney stone disorders)
• Accompanying episodes of urinary tract infections with obstruction, a particular risk with struvite stones
• A history of multiple urologic procedures for kidney stones
• Greater size of the kidney stone gravel

Long Term Outlook: Risk for Recurrence

Without preventive treatment, calcium stones recur in 10% of patients within a year of the first attack, and in half of patients within 5 to 7 years. Individual risk for recurrence, however, varies depending on the stone and the underlying condition. For example, a 15-year-old with inherited cystine stones has a very high risk for recurrence, while a middle-aged man with a first calcium oxalate stone has a good chance of never passing another.

Prevention

All individuals who have experienced kidney stones should take some specific preventive measures to prevent recurrence. The following are some general observations:

• The most important dietary recommendations for reducing the risk for calcium stones are increasing fluid intake, restricting sodium, and reducing protein intake.
• A lower risk for calcium stones is also associated with higher potassium intake.
• A high calcium diet does not appear to increase the risk for kidney stones as long as it also contains plenty of fluids and dietary potassium and phosphate. (Increasing calcium alone may pose a modest risk for stones.)
• Patients should try to correct any dietary habits that cause acidic or alkaline imbalances in the urine that promote stone formation.

Because kidney stone types may require specific dietary changes, patients should work with their doctors to develop an individualized plan. It should be stressed that nutritional considerations are very important in preventing recurrence, and patients should be vigilant in complying with the proper diet.

Fluids (Water, Juice, and Other Beverages)

Good voiding habits, particularly frequent urination, are important. Therefore, of all the preventive recommendations, drinking enough fluid is the most important guideline for people with any type of kidney stones.

• In general, patients with calcium or uric acid stones should drink at least 10 full glasses of fluid each day (at least half should be water). This includes one with each meal and drinking fluids at night, even if it means getting up from sleep. Fluid intake should produce at least two and a half quarts of urine each day.
• More water (over a gallon, or 16 8-ounce cups, every day) is needed to prevent cystine stones, and it must be drunk at regular intervals throughout the night and day.

In all cases, more fluid is needed after exertion and during times of stress. If fluid intake is sufficient, the urine should be pale and almost watery, not dark and yellow.

Water. Although water is best, it may vary depending on its source. Variations in water itself may have different impacts. One study reported that drinking hard tap water increased urinary calcium concentration by 50% compared to soft bottled water. On the other hand, mineral water containing both calcium and magnesium may reduce several risk factors for both calcium and uric acid stone formation.

Juices and Specific Effects. Other beverages have various positive or negative effects, depending on the type of stone:

• Lemon Juice. Drinking one-half cup of pure lemon juice (enough to make eight glasses of lemonade) every day raises citrate levels in the urine, which might protect against calcium stones. (While orange juice also increases citrate levels, it does not lower calcium and it raises oxalate levels. It is not recommended.)
• Cranberry and Apple Juice. Apple and cranberry juice contain oxalates, and both have been associated with a higher risk for risk for calcium oxalate stones. Cranberry juice has properties that may increase the risk for both calcium oxalate and uric acid stones. On the other hand, cranberry juice helps prevent urinary tract infections and so may be helpful for reducing the risk for struvite and brushite stones. (These stones are far less common, however.)
• Blackcurrant Juice. In one study, blackcurrant juice reduced acidity and was associated with protection against uric acid stones.
• Grapefruit Juice. A number of studies have found a risk for stones from drinking grapefruit juice. In one study, just one 8-ounce cup of grapefruit juice per day increased the risk for forming stones by 44%.

Other Beverages and Their Effects on Stone Formation.

• Soft Drinks. Cola can severely reduce citrate in the urine and should be avoided. Many soft drinks contain phosphoric acid, which increases the risk for stones. Some research shows that drinking one quart (less than three 12-ounce cans) of soda per week may increase a person's risk of developing stones by 15%.
• Alcohol. Wine may be protective against kidney stones. A study conducted in Finland, however, suggests that the risk of developing stones decreases with beer consumption. It should be noted that beer is high in oxalates. Beer and other alcoholic beverages also contain purines, which may increase the specific risk for the less common uric acid stones in susceptible people. Binge drinking, in any case, increases uric acid and the risk for stones
• Coffee and Tea. Some research as reported a lower risk for stones with tea and both regular and decaffeinated coffee.

Low-Salt and Low-Protein Diets

In a long-term 2002 study of men with calcium oxalate stones and high levels of urinary calcium, a low-sodium, low-protein diet containing normal levels of calcium dramatically reduced the recurrence of stones compared to a diet that was simply low in calcium.

Salt Restriction. Because salt intake increases the amount of calcium in urine, patients with calcium stones should restrict their sodium intake. Sodium may also elevate levels of urate, the crystalline substance that can trigger formation of recurrent calcium oxalate stones. Although the relative contribution of sodium restriction in this and other studies has not been confirmed, some researchers believe that restricting sodium along with increasing fluid intake is the most important dietary measure for preventing stones.

Protein Restriction. Protein increases uric acid, calcium, and oxalates in the urine and reduces citrate. Diets high in protein, particularly meat protein, have been consistently associated with kidney stones. (Meat protein has a higher sulfur content and generates more acid than vegetable protein.) A 2002 trial of those following a high-protein, low-carbohydrate diet, popularized in such weight-loss regimens as the Atkins diet, for example, found dramatically increased levels of urinary uric acid and calcium after just several weeks. These effects put patients at higher risk not just of kidney stones but possibly of osteoporosis as well. According to Swiss studies, about a third of people at risk for calcium stones may have a sensitivity to meat proteins that cause mild hyperoxaluria.

Whether restricting meat protein alone has any protective value without restricting sodium as well is unknown. Most studies to date have found no difference in stone development between people with low and normal meat protein diets over four years. A 2000 study reported that only dramatic reductions in meat protein had any preventive effect against stone recurrence.

Although the precise role of dietary protein in kidney stones needs further elucidation, it is reasonable for everyone to consume meat protein in moderation. People with struvite stones, who need to reduce phosphates in their diets, should also cut down on proteins.

Role of Calcium

Calcium from Foods. Dietary calcium recommendations for kidney stone prevention need to be determined on an individual basis. A doctor will suggest calcium guidelines based on a patient’s age, gender, body size, and type of stone. Most studies indicate that dietary calcium (found in milk, yogurt, and cheese) protects against many types of calcium oxalate stones. Large studies of both men and women found that those with the highest intake of calcium from foods had a much lower risk for stones than those who had little calcium in their diets. A diet containing a normal amount of calcium, but reduced amounts of animal protein and salt, may protect against stones better than a low-calcium regimen. However, calcium metabolism changes as people age. Some studies suggest that a high calcium intake protects against kidney stones in men younger than age 60, but not in older men.

Dietary calcium may actually bind the oxalate in foods, preventing it from being absorbed into the blood and excreted into the urine. In a normal healthy diet, dairy products supply almost 80% of the daily calcium requirement. For people have calcium stones associated with resorption (the breakdown of bone that releases calcium into the bloodstream), limiting calcium intake could cause further bone loss.

Calcium Supplements. Evidence on calcium supplements is mixed, although in general many studies suggest that they reduce oxalate levels and so help prevent calcium oxalate stones. One study suggested that taking 500 mg of calcium supplements a day regularly may "reprogram" the intestines to absorb less calcium and so be protective. Experts generally advise that calcium supplementation within dosage recommendations, approximately 1,200 mg per day, remains safe. In one study, however, women who took calcium supplements had a 20% higher risk for stones. Research indicates that dosages of calcium above 2,000 mg per day are clearly associated with the formation of stones. Some experts speculate that this higher risk may occur because supplements are often taken in the morning, either without food or with breakfast, which is typically low in oxalates. Taking supplements with later meals may not incur the same risk.

Calcium Restriction in Certain Cases. Some calcium stone patients may need to restrict calcium, such as those whose stones are caused by genetic defects in which the intestine over-absorbs calcium. More studies are needed to define this group precisely.

Fiber-Rich Foods and Their Compounds

Fiber may be beneficial for people with kidney stones. In addition, some fiber-rich foods may contain compounds that help protect against kidney stones. A wide variety of high-fiber plant foods contain a compound called phytate (also called inositol hexaphosphate, InsP6, or IP6), which appears to help prevent crystallization of calcium salts, both oxalate and phosphate. Phytate is found in legumes and wheat and rice bran. (Soybeans are also rich in phytate but they are also very high in oxalates, so the overall effects of soy on kidney stones are not clear.)

Purine Restriction in People at Risk for Uric Acid Stones

A high intake of purines can increase the amount of uric acid in the urine, so those at risk for uric acid stones should reduce their intake of foods that contain purines. They include beer and other alcoholic beverages, anchovies, sardines, yeast, organ meats (e.g., liver, kidneys), legumes (e.g., dried beans, peas, and soybeans), mushrooms, spinach, asparagus, cauliflower, and poultry.

Oxalate Restriction in Hyperoxaluria

Most people with calcium oxalate stones should not avoid oxalate-rich foods unless the doctor specifically recommends a restrictive diet. Oxalate binds with calcium in the intestine, which may actually reduce calcium absorption. Some studies, in fact, indicate that eating foods containing oxalates and calcium together may reduce the risk of stones. Most of the foods that contain oxalates are very important for good health. Restricting oxalates may be particularly harmful in people with bowel disorders marked by malabsorption.

• Foods high in oxalic acid include beets, soy, beet tops, black tea, chenopodium, chocolate, cocoa, dried figs, ground pepper, lamb, lime peel, nuts, parsley, poppy seeds, purslane, rhubarb, sorrel, spinach, and Swiss chard.
• Foods containing moderate amounts of oxalates include beans (green and wax), blackberries, blueberries, carrots, celery, coffee (roasted), concord grapes, currants, dandelion greens, endive, gooseberries, lemon peel, okra, green onions, oranges, green peppers, black raspberries, strawberries, and sweet potatoes.

Role of Fats

Certain fats may play a beneficial or harmful role in specific cases of kidney stones.

Restricted Fats in Patients with Stones Associated with Bowel Disease. Patients who have stones associated with short-bowel syndrome should restrict their intake of fat as well oxalates. In such cases, calcium may bind to unabsorbed fat instead of to oxalates, which increase oxalate levels.

Fish Oil. Omega-3 fatty acids, which are found in oily fish like mackerel, salmon, and albacore tuna, have many health benefits but the most current evidence suggests they do not help prevent kidney stones. A 2005 study of over 200,000 adults found that increased omega-3 fatty acid intake did not reduce kidney stone risk.

Role of Vitamins

Vitamin B6. Vitamin B6, or pyridoxine, is used to treat people with primary hyperoxaluria, a severe inherited disorder. Patients should not try to self-medicate with vitamin B6. Very high doses (500 to 2,000 mg daily over long periods) can cause nerve damage with loss of balance and numbness in the feet and hands. Food sources of vitamin B6 include meats, oily fish, poultry, whole grains, dried fortified cereals, soybeans, avocados, baked potatoes with skins, watermelon, plantains, bananas, peanuts, and brewer's yeast.

Vitamin C. Ascorbic acid (vitamin C) may convert to tiny insoluble crystals called oxalates. People with hyperoxaluria (too much oxalate in the urine) should avoid vitamin C supplements. Even for men with normal oxalate levels, higher consumption of vitamin C (more than 1000 mg a day) may increase kidney stone risk.

Role of Minerals

Magnesium and potassium may help reduce the risk for kidney stones in men.

Stress Management Techniques

Because of an association between stress and kidney stones, relaxation and stress management techniques may also be beneficial.

Preventing Recurrence

Dietary Considerations. People with kidney stones appear to be more sensitive to certain foods than people who do not form kidney stones and need to make specific changes in their diet. They should work with their doctors to develop a dietary plan that fits their individual situation. Drinking plenty of fluids is important for preventing recurrence of any kidney stone.

Indications for Drug Treatments. If dietary measures fail then drug treatments may be helpful. A number of drugs are available to prevent recurrences of calcium oxalate and other stones. Allopurinol, thiazide, potassium citrate, and potassium-magnesium citrate have all been shown to inhibit stone formation. In addition, drug treatments can sometimes also help prevent other complications related to stones, such as osteoporosis.

Correcting Underlying Conditions Known to Cause Kidney Stones. It is also important to treat and correct, if possible, any underlying disorder that may be causing stones to form. Such disorders include distal renal tubular acidosis, hyperthyroidism, sarcoidosis, and certain cancers. To prevent calcium stones that form in hyperparathyroid patients, a surgeon may remove the affected parathyroid gland (located in the neck). In most cases, only one of the glands is enlarged. Removing it ends the patient's problem with kidney stones.

Resources

• www.kidney.niddk.nih.gov -- National Kidney and Urologic Diseases Information Clearinghouse
• www.urologyhealth.org -- American Urological Association
• www.kidney.org -- National Kidney Foundation
• www.ohf.org -- Oxalosis and Hyperoxaluria Foundation

References

Curhan GC, Willett WC, Knight EL, Stampfer MJ. Dietary factors and the risk of incident kidney stones in younger women: Nurses' Health Study II. Arch Intern Med. 2004;164(8):885-891.

Straub M, Hautmann RE. Developments in stone prevention. Curr Opin Urol. 2005;15(2):119-126.

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