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Kidney Stone Analysis


At a Glance

Why Get Tested?

To evaluate the composition of a kidney stone, to help determine the cause of its formation and to guide treatment

When to Get Tested?

When a stone has passed through or been removed from your urinary tract

Sample Required?

A stone or stones filtered from your urine or surgically removed from your urinary tract by a health practitioner

Test Preparation Needed?

None

The Test Sample

What is being tested?

Kidney stones are small, hard masses that form within the kidneys. Kidney stone analysis uses one or more test methods to examine and determine the composition of a stone. This is done in order to help identify the cause of the stone and, where possible, to prevent the formation of more stones.

Thumbnail diagram showing kidney stones in urinary tractThumbnail diagram of normal urinary tractThe kidneys are part of the urinary tract, which also consists of two ureters, the bladder, and the urethra. The kidneys filter waste out of the blood and produce urine, which is transported from the kidneys to the bladder through tube-like ureters. Urine is eliminated from the bladder through the urethra. This is a continual process of waste filtration, urine production, and elimination.

Commonly called kidney stones, calculi can form in the kidneys and cause problems either because they grow large enough to obstruct urine flow or because they become dislodged or break off and begin to travel from a kidney through the ureter; they can cause temporary obstruction and stretch, irritate, and/or damage the walls of the ureters. This movement can cause abrupt, extremely severe pain that may be intermittent or continuous.

Many stones will eventually pass out of the body in the urine, but some are too large or have too irregular a shape for the body to expel. With very large stones, which typically cannot pass from the kidney into the ureters, and for smaller stones that get into but do not pass through the ureters, some form of treatment is needed. The stone may need to be surgically removed, often using devices that pass through the urethra and bladder to the site of the stone. With some stones, it is possible to use extracorporeal shock wave lithotripsy. This treatment pulverizes the stone in place using targeted shock waves. The smaller particles and fragments that remain can then pass through the urinary tract.

Stones can develop for several reasons, but the most common is because there is a high concentration of a particular chemical in the urine that precipitates and forms crystals. This can happen when a person produces and excretes an excess amount of the chemical. It can also occur when a person chronically takes in little liquid and has more concentrated urine because there is less water in it. Depending on how much and what type of material crystallizes and where it forms, a kidney stone may be round, sharp and pointy or irregular with branches (called a staghorn). It can range in size from a grain of sand to bigger than a golf ball. The composition of the stone depends upon the chemicals present in excess. It may be all one chemical compound or have different chemicals in different layers.

According to the National Institute of Diabetes and Digestive and Kidney Diseases, kidney stones are one of the most common urinary tract disorders. In the U.S., more than 300,000 people go to the emergency room each year with kidney stones and about a million visit their healthcare provider.

How is the sample collected for testing?

The health practitioner or laboratory typically provides a person who has kidney stone symptoms with a clean container and a straining device that has a fine mesh. The person filters all of their urine through the fine mesh. This is necessary because there is no way to determine exactly when a stone will pass out of the body. The person then examines the mesh for any particulates, keeping in mind that stones may be easily visible or as small as grains of sand. If a stone is found, it is placed into the clean container, allowed to dry, and returned to the laboratory or healthcare provider as instructed. It is important not to add anything to the stone, such as tissue or tape, as this can make testing more difficult.

If a person is in a hospital, then medical personnel will filter the urine. With a kidney stone that is too large to pass, a health practitioner may perform a surgical procedure to remove it and then send the stone for analysis.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

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.

In addition to the stone analysis, blood, urine, and 24-hour urine tests are often ordered to determine whether the individual produces excess chemicals that may contribute to the formation of kidney stones and to evaluate the person's 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

When is it ordered?

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

Signs and symptoms associated with a kidney stone may lead a health practitioner to search for a stone either in voided urine or within the body using imaging tests. A health practitioner may suspect the presence of a stone when someone has:

  • 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
  • Bloody and/or cloudy urine
  • Frequent urge to urinate
  • Fever and chills, when an infection is present

When a person has recurrent stones, the first stone found would be tested. Subsequent stones would be analyzed as the health practitioner deems necessary.

What does the test result mean?

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. About 75% of stones will contain calcium. 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.

A person with an underlying disease or condition may produce and/or excrete 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 a person's risk of forming stones. Preventing kidney stones from developing again depends upon identifying and addressing the cause of stone formation.

In general, if a person has a:

  • Calcium oxalate or calcium phosphate stone, then it is likely that the person is excreting excess calcium and/or oxalate (or sometimes too little citrate, which normally binds calcium but does not form stones) into the urine. This may be due to:
    • An inherited tendency to absorb more than the normal amount of calcium from the 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 the person is producing and excreting excess uric acid due to conditions such as gout or disorders of uric acid metabolism.
  • Struvite stone, the person likely has a specific type of bacterial infection that leads to the production of excess ammonia.
  • Cystine stone, then the person is likely producing and excreting excess cystine as the result of a rare hereditary disorder.
  • Drug-related stone, then the person is forming stones because of a medication that the person is taking.

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

Is there anything else I should know?

Not everyone who drinks too little liquid or who has an excess amount of 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.

Several factors other than an excess of certain chemicals in the urine can contribute to the formation or increased risk of kidney stones. Some of these 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


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