Kidney stones (also called renal calculi) form in the kidneys from substances that separate out of the urine. Stones may range in size from a grain of sand to about 1 in. (2.54 cm) across. Sometimes a stone will leave the kidney and move down an ureter into the bladder. From the bladder, the stone passes through the urethra and out of the body in urine. Passing a kidney stone through an ureter or the urethra may be painless or may cause severe pain.

Chemical analysis of a kidney stone can provide important clues about why it formed. Analysis may also suggest treatment and provide information that may prevent stones from forming in the future.

• Most kidney stones (up to 80%) are composed of calcium compounds, usually calcium oxalate, but they may also be made of calcium phosphate. These stones are typically black, gray, or white. A common cause of calcium stones is not drinking enough water on a regular basis. The tendency to get calcium stones can run in families.
• Between 5% and 20% of kidney stones are made of magnesium ammonium phosphate. These may form during a urinary tract infection caused by a certain type of bacteria. Doctors call these struvite stones, but they are also known as infection or staghorn stones, because they are often shaped like deer antlers. Their shape can make them difficult to remove. However, treating the infection and removing the stones often prevents them from forming again.
• About 5% to 10% of kidney stones are made of uric acid. These white or orange stones form in urine that is very acidic. Uric acid stones can occur in people who have gout or some types of cancer. They are difficult to see on an X-ray.
• Fewer than 2% of kidney stones are composed of a chemical called cystine. Cystine stones are greenish yellow and often flecked with shiny crystals; on an X-ray they look like sculpted wax or soap.

Kidney stones occur in 5% to 20% of Americans and are three times more common in men than in women. Kidney stones are also more common in whites than in blacks.

Kidney stone analysis is done on a kidney stone that has been passed in the urine or removed from the urinary tract during surgery.

Kidney Stone Analysis
Results

Kidney stones in the urine may mean

Several factors increase a person's risk for developing kidney stones. Many of these risk factors are related to diet.

• A common cause of kidney stones in some people is not drinking enough water on a regular basis.
• Kidney stones may also result when a person's diet is high in:
• Calcium (which may come from eating or using excessive amounts of dairy products, calcium supplements, or antacids).
• Vitamin C or vitamin D.
• Protein.
• Oxalate-rich foods.
• Salt (such as eating excessive amounts of chips, pickles, or lunch meats).
• Certain medications, such as acetazolamide (Diamox) or indinavir (Crixivan), may cause kidney stones as a side effect.

Certain risk factors that cannot be controlled increase a person's risk of developing kidney stones. These include:

• Having had a kidney stone before.
• Being a male between the ages of 20 and 50.
• Having a family history of kidney stones.
• Having certain diseases or conditions, such as inflammatory bowel disease.
• Prolonged bed rest.
• Having bladder problems caused by injury to the spinal nerves affecting the lower part of the body.
• Having a history of frequent urinary tract infections.