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
A stone filtered from your urine or surgically removed from your urinary tract by a doctor
Test Preparation Needed?
The Test Sample
What is being tested?
Kidney stone analysis uses one or more test methods to examine and determine the composition of a stone from the urinary tract. This is done in order to help identify the cause of the stone and, where possible, to prevent the formation of more stones.
The urinary tract consists of two kidneys, two ureters, the bladder, and the urethra. The kidneys filter wastes 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, urinary tract stones or calculi can form in the kidneys and in other parts of the urinary tract. Kidney stones can 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 a 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.
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
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., about half a million people go to the emergency room each year with kidney stones and about three million visit their doctor.
How is the sample collected for testing?
The doctor 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 doctor 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, the doctor may perform a surgical procedure to remove it and then send the stone for analysis.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
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