To evaluate the composition of a kidney stone in order to determine the cause of its formation and to guide treatment
Kidney Stone Testing
When a stone has passed through or has been removed from your urinary tract
A stone or stones filtered from your urine or surgically removed from your urinary tract by a healthcare practitioner
Kidney stones are small, hard masses that form within the kidneys. Kidney stone testing uses one or more test methods to examine and determine the composition of the stone. This is done in order to help identify the cause of the stone and, where possible, to prevent the formation of more stones.
The 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 the 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 but also 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 will be required. 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 pulverize the stone in place using targeted shock waves. This process is called extracorporeal shock wave lithotripsy. 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 is no longer soluble and precipitates to form crystals. It can also occur when a person chronically fails to take in adequate amounts of fluids, resulting in highly concentrated urine. 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 what excessive amounts of chemicals are present. 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, around 11% of men and 6% of women in the U.S. will have kidney stones at least once in their life. In the U.S., more than half a million 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 healthcare practitioner or laboratory typically provides you with a clean container and a straining device that has a fine mesh. You should filter all of your urine through the fine mesh. This is necessary because there is no way to determine exactly when a stone will pass from the urinary tract. You then examine 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 you are in a hospital, then medical personnel will filter the urine. With a kidney stone that is too large to pass, a healthcare practitioner may perform a surgical procedure to remove it.
Is any test preparation needed to ensure the quality of the sample?
No personal test preparation is needed. Collection filter must be available.
How is the testing 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.
When is it ordered?
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.
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, 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.
Is there anything else I should know?
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
In addition to stone testing, what other lab tests might be done?
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
Do I really have to filter all of my urine for this test?
Can I just test my blood or urine instead of the stone?
How bad is the pain with kidney stones?
If I have had a kidney stone, should I avoid calcium?
You should work with your healthcare provider to determine the best treatment for you. If you had a stone with calcium in it, your healthcare practitioner may have you decrease your intake a modest amount, but the clinical picture is often less black and white than taking more or less. Some people absorb extra calcium from their food, but some excess calcium in the urine is related to an excess of sodium, so people who have had calcium-containing stones are often told to decrease salt intake rather than reducing calcium intake. Taking large doses of supplemental calcium may play a role in kidney stone formation. Sometimes drastic reductions in calcium can worsen stone formation as well as affect bone health.
How does a healthcare practitioner find a stone in my urinary tract?
Who performs a kidney stone analysis?
What can I do to prevent a kidney stone and prevent recurrence?
What are some of the specific methods used to analyze kidney stones?
Common methods of kidney stone analysis include infrared spectroscopy and X-ray diffractometry. You can read more about these and other methods at the web site for the Louis C. Herring and Company Kidney Stone Analysis Laboratory.