To help evaluate a person's risk of recurring kidney stone formation; to help determine the underlying reason for developing a kidney stone; to determine if there is a family history of kidney stone disease; to help guide and monitor treatment
Kidney Stone Risk Panel
When you have had kidney stones and a healthcare practitioner wants to evaluate your risk of developing additional kidney stones; when you have risk factors such as a family history of kidney stones, a single functional kidney, or a transplanted kidney
A 24-hour urine collection is required for this test. Frequently, two different 24-hour urine samples are collected and tested to determine whether elevated or decreased levels of a substance eliminated in the urine are temporary or persistent. Collection containers with preservative will be provided by your healthcare practitioner or the lab performing the testing.
The kidney stone risk panel is a group of tests that measure the elevated or decreased amounts of substances in urine that are commonly associated with kidney stone formation (nephrolithiasis). In an individual who has already had kidney stones, an increased level of one of these substances, particularly calcium, can indicate both an elevated risk for developing additional stones and the likely type of stones that would form.
Kidney stone is a term for solid aggregates of minerals and salts that form in the kidneys. Typical kidney stones are composed of calcium oxalate (~75% of cases), calcium phosphate (15%), uric acid (8%), struvite (~1%), or cystine (<1%). A mixture of several types (calcium oxalate and calcium phosphate) may also occur.
Approximately 12-19% of men and 6-9% of women will develop a kidney stone within their lifetime. Fewer African-Americans (~50%) are affected than whites. The peak age in men is around 40 and in women, thirties. It is estimated that about 50% of individuals who had stone formation will have a recurring stone formation within ten years.
Kidney stone risk panels are intended to evaluate the risk of forming stones by testing for high concentrations of common stone-forming substances or low concentrations of stone-inhibiting substances. The specific tests included in a panel may vary somewhat from laboratory to laboratory but will typically include the following:
- Urine pH
- Urine calcium
- Urine oxalate (oxalic acid)
- Urine uric acid
- Urine creatinine (does not cause stones but is used to tell if all urine was collected and help identify how concentrated the urine is)
- Urine citrate (citric acid; this substance helps inhibit the formation of stones)
- Urine cystine
- Urine phosphorus
- Urine magnesium (helps inhibit stone formation)
- Urine sodium (does not directly cause stones but affects the amount of calcium in urine and thus its ability to form stones)
A high concentration of one or more of these substances in the urine can occur when a person produces and excretes an excess amount of the substance. Other contributing factors include chronically drinking an inadequate amount of fluids, becoming dehydrated to some degree, and having unusually concentrated urine with a strong odor.
For additional details, see the article on Kidney Stone Analysis.
How is it used?
A kidney stone risk panel is used to help evaluate the likelihood that an individual who has had kidney stones will develop additional ones, indicating a recurrent problem.
Testing is sometimes used to evaluate risk after an initial occurrence of a stone, especially if a person is considered to be at an elevated risk for forming more stones or for a person who is likely to develop kidney dysfunction if another kidney stone is formed.
This testing may be used to help guide and monitor the treatment of someone who has had a kidney stone.
Several other laboratory tests may also be used to help evaluate a person who has had a kidney stone, including:
- Individual 24-hour urine tests that may not be part of the kidney stone risk panel, such as cystine, sodium, potassium, chloride, or magnesium
- A urinalysis to evaluate urine constituents (substances, protein, red and white blood cells, pH, and presence of crystals or bacteria)
- Blood tests such as a blood urea nitrogen (BUN) and creatinine to evaluate kidney function, electrolytes (sodium, potassium, chloride, bicarbonate), uric acid, and calcium to evaluate health status and compare to urine results; if calcium levels are elevated, PTH and vitamin D assays may also be done.
- A urine culture If a person shows signs of a urinary tract infection
- Kidney stone analysis is usually performed to evaluate the specific substances present in the stone, if the kidney stone is able to be collected.
When is it ordered?
A kidney stone risk panel is usually performed several weeks after a kidney stone has been resolved to determine whether elevated levels of a substance are temporary or persistent.
Specifically, guidelines from the American Urological Association recommend that a person diagnosed with a first time kidney stone should have one or two 24-hour urine tests. Follow-up testing should include a single 24-hour urine test within 6 months of the start of treatment and a 24-hour urine sample tested annually or more frequently depending on whether the person has developed additional kidney stones.
Testing may be ordered after a first stone when a person is considered to be at an increased risk of stone formation or kidney dysfunction. This may include people who:
- Formed stones as a child
- Have a family history of kidney stones
- Had multiple kidney stones at the same time as their "initial" stone
- Have a single functional kidney, including those who have had a kidney transplant
Sometimes testing may be performed when a person has made changes to their lifestyle (such as recommended dietary changes or drinking more water) or medication changes in order to monitor the effectiveness of these changes.
In addition, laboratory testing may be performed when certain conditions that have been associated with kidney stone disease occur. They include obesity, hyperthyroidism, gout, renal tubular acidosis (RTA) type 1, diabetes mellitus type 2, bone disease, primary hyperparathyroidism, and malabsorptive gastrointestinal situations (bowel resection, bariatric surgery, or bowel/pancreatic disease.
What does the test result mean?
The results of the kidney stone risk panel are evaluated in conjunction with other tests performed in order to help determine a person's likely risk of developing another stone.
In general, if a substance such as calcium or uric acid is present in excess in both the blood and urine, then it represents an increased risk for kidney stone formation and a condition that should be further investigated to determine the cause.
Test results cannot predict, however, who will actually develop another kidney stone or when. Some people with mild elevations, or even results within normal ranges, may form stones, while other people with significantly increased results will not. A stone may be present in a kidney for years without causing any overt signs (hematuria) or symptoms (flank pain).
In a person who has made lifestyle or medication changes, decreasing levels represent a decreased risk of stone formation.
If a person is dehydrated, then the amount of urine produced in 24-hours (urine volume) will be low and the urine will be more concentrated. This can cause an excess of a substance per volume (dissolved in less liquid) and increase the likelihood of stone formation.
Kidney stone formation may be affected by urine pH (acidic/alkaline). Uric acid and cystine crystals can only form in acidic urine, while calcium phosphate and struvite stones form in alkaline urine. Struvite stones consist of magnesium ammonium phosphate and are associated with bacterial infections.
The table below summarizes what some test results may indicate:
Test Result Indicating Increased Kidney Stone Risk Stone Formation / Comments Creatinine N/A Blood and urine creatinine levels reflect kidney function; they may be used for comparing to other substances as the level of creatinine in blood is normally stable and, in urine, it reflects how dilute or concentrated the urine is. Urine calcium High Possibility for calcium oxalate or calcium phosphate stones Urine oxalate High Possibility for calcium oxalate stones Urine uric acid High Uric acid stones may form; many who have increased uric acid also have gout Urine citric acid Low Citric acid helps inhibit stone formation.
Other less common tests are summarized below:
Test Result Indicating Increased Kidney Stone Risk Stone Formation / Comments Urine cystine High Associated with inherited condition causing excess cystine in urine; additional testing may be performed to further evaluate. Urine phosphorus High Can contribute to calcium compound stone formation Urine magnesium Low Helps inhibit stone formation Urine sodium High Sodium can cause more calcium to be excreted into the urine, increasing the risk of calcium compound stones.
How long will it take for my results?
That depends on the laboratory performing the testing. Results may be available the same day if the testing is done by a local laboratory. If the sample has to be sent to a reference laboratory for testing, results may take a few days.
How large are kidney stones?
Kidney stones may be as small as a grain of sand, as large as a golf ball, or even larger, with some filling up the entire collecting system of the kidney. These are sometimes called "staghorn" stones (calculi) because the shape of the collecting system resembles the antlers of a deer. They 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, where they can cause temporary obstruction and stretch, irritate, and/or damage the walls of the ureter. 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.
What is a struvite stone and what causes it?
Struvite (triple phosphate stones or infectious stones) is magnesium ammonium phosphate that forms stones associated with urinary tract infections. These are large stones that require surgery to remove.
How can I prevent kidney stones?
One of the easiest things to do is to make sure that you stay hydrated by consistently drinking plenty of water, approximately 8-12 glasses of water every day. If you are at an increased risk of stones, then there may be other measures recommended by your healthcare provider. Excess amounts of caffeinated, carbonated, or sweetened drinks have been associated with increased risk of stone formation. Limeade and lemonade are recommended.
If I have had a kidney stone, what foods or drinks should I avoid ?
You should work with your healthcare provider to determine the best treatment for you as it may depend on the type of stone. If you had a stone, reducing salt and overly-salted foods are recommended (a total of <2,300 mg/day). If eating oxalate-rich foods, an adequate amount of calcium should accompany the meal. Limiting the intake of foods high in purine, such as red meat, organ meat, and shellfish, and increasing fruit and vegetable consumption may minimize the risk of stone formation.
Do I really need to save all of my urine for 24 hours?
Yes, your healthcare provider needs to know how much urine you are producing over a 24-hour period (volume) and how much of each of the substances is present. Since the substances may be eliminated in the urine at varying rates, the 24-hour urine is a better representation than a single or random urine sample and is a consistent frame of reference.
Is it necessary to do both blood and urine tests?
Can stones form in other parts of the urinary tract besides the kidney?
Yes. For example, stones can form in the bladder; however, the reason for and the mechanism by which they form are different than for kidney stones. Instead of causes related to excreting excess amounts of a particular substance, bladder stones are typically caused by an inability to completely empty the bladder during urination. A bladder stone may be caused by some underlying urinary tract problem such as lack of bladder control, enlarged prostate, or urinary tract infection.
Is there anything else I should know?
Not everyone who drinks too little liquid or who has an excess amount of substances 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 stone recurrence.
Abnormalities in the structure of the kidneys and/or urinary tract may cause the flow of urine to be impeded and crystals to be deposited, leading to formation of stones.