Kidney Stone Testing

A kidney stone analysis is performed to determine the chemical composition of a stone when it is filtered out of the urine or removed from the urinary tract. A laboratory will typically document the physical characteristics of a stone – its size, shape, weight, color, and texture. Often, a picture of the stone will be taken to document its size and appearance. The stone may also be fractured so that its layers can be observed. One or more tests will then be performed to determine the stone's composition.

Kidney stone analysis is ordered when you have passed a kidney stone and it has been filtered out of the urine or when a stone has been removed from your urinary tract.

Sometimes, your healthcare practitioner may suspect that you have a kidney stone and will search for a stone either in voided urine or within your body using imaging tests when you have signs and symptoms such as:

• Severe pain in the side of the back that may move to the groin (pain is the primary symptom of a kidney stone)
• Abdominal pain
• Nausea and vomiting (50% of patients will experience this)
• Bloody, cloudy, and/or foul-smelling urine
• Frequent urge to urinate
• Pain while urinating
• Fever and chills, when an infection is present
   

When you have recurrent stones, the first stone found would be tested. Subsequent stones would be analyzed as your healthcare practitioner deems necessary.

A kidney stone analysis identifies the chemical composition of the stone. Common types of kidney stones include:

• Calcium oxalate
• Calcium phosphate
• Uric acid
• Struvite (magnesium ammonium phosphate)—stones associated with a bacterial infection
   

These four types make up about 95% to 99% of kidney stones, with calcium oxalate stones being the most common.

Less common stones include:

• Cystine—stones associated with an inherited excess of cystine excretion
• Drug-related—stones that are associated with drugs such as guaifenesin, indinavir, triamterene, atazanavir, and sulfa drugs
   

However, stone analysis does not give the reason that the stone formed.

You may have an underlying disease or condition that may produce and/or release an excess of a specific chemical into the urine. Not drinking enough fluids and/or having urine with a high or low pH can contribute to your risk of forming stones. Preventing kidney stones from developing again depends upon identifying and addressing the cause of stone formation.

In general, if you have a:

• Calcium oxalate or calcium phosphate stone, then it is likely that you release excess calcium and/or oxalate (or sometimes too little citrate, which normally binds calcium but does not form stones) into your urine. Calcium phosphate stones often occur simultaneously with calcium oxalate stones. The stones may be due to:
  • An inherited tendency to absorb more than the normal amount of calcium from your diet, causing high urine calcium (hypercalcuria).
  • Dietary factors—consuming food or drink high in calcium or oxalate may result in increased amounts of these substances in the urine.
  • Factors affecting absorption of nutrients, such as inflammatory bowel disease or intestinal surgery—an imbalance in nutrient absorption may result in excess urinary calcium.
  • Too much parathyroid hormone (primary hyperparathyroidism), leading to high blood and urine calcium.
  • A variety of metabolic disorders—these may lead to some degree of metabolic acidosis, resulting in an increased amount of calcium excreted in the urine.
• Uric acid stone, then it is likely that you produce excess uric acid due to conditions such as gout or disorders of uric acid metabolism. Uric acid stones are more common in men and tend to occur in people with a diet high in animal protein.
• Struvite stone, then you likely have a specific type of bacterial infection, often in the urinary tract, that leads to the production of excess ammonia. Struvite stones are more common in women.
• Cystine stone, then you likely produce excess cystine as the result of a rare hereditary disorder (cystinuria).
• Drug-related stone, then you form stones because of a medication that you take.
   

A variety of other types of stones may be formed when an excess chemical is present and/or when the urine is very concentrated, which can occur with dehydration.

Not everyone who drinks too little liquid or who has an excess amount of a chemical in their urine will form kidney stones. Some stones will form in people for other reasons. Those who have had one kidney stone are at an increased risk for developing additional stones.

Other factors that can contribute to the formation or increased risk of kidney stones include:

• A family history of kidney stones
• Presence of a urinary tract infection
• Abnormalities in the structure of the kidneys and/or urinary tract (this may cause the flow of urine to be impeded and crystals to be deposited, leading to formation of stones)
• Kidney disorders such as polycystic kidney disease, a condition characterized by the presence of numerous cysts in the kidney

Blood, urine, and 24-hour urine tests are often ordered to determine whether you produce excess chemicals that may contribute to the formation of kidney stones and to evaluate your overall health. The test results can help distinguish between a probable stone and other conditions that may have similar symptoms but require different treatment. Testing may include:

• Blood and 24-hour urine tests for calcium, uric acid, creatinine, and sometimes oxalate, citrate, phosphate, and/or cystine
• Urinalysis - to detect red and white blood cells, crystals, signs of infection, and to measure urine pH
• Complete blood count (CBC) - to evaluate white blood cells for signs of infection