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Kidney function tests check how well the kidneys filter and transport waste materials from the blood into the urine.

Who is a candidate for the test?

Kidney function tests are usually ordered when a healthcare provider suspects a disease that may be affecting the kidneys, when a patient has high blood pressure, or an abnormal urinalysis. These tests are also used to monitor someone who already has kidney disease.

How is the test performed?

• BUN in a blood sample
• creatinine in a blood sample
• creatinine in the urine (in a single sample or in a "24-hour urine" collection
• a "clearance" test

Creatinine and BUN are blood tests that measure metabolic breakdown products from protein in general and from breakdown protein in muscle). It is the role of the kidney to get rid of these waste products in the urine. When the kidney does not work properly, these products are not put into the urine and the levels become elevated in the blood. Urine creatinine measures the amount of creatinine that is excreted into the urine.

"Clearance" is a fairly precise way to estimate the exact amount of function a kidney has compared to normal. It requires both blood and urine measurements.

For a long time clearance was specifically creatinine clearance and a 24 urine was collected. A formula that required a calculation of blood and urine creatinine was applied.

More recently other tests for clearance have been developed including:

• the Cockcroft-Gault equation (named after the physicians who tested it)
• the MDRD (Modification of Diet in Renal Disease) equation (tested in a larger study of patients with kidney disease).

Rarely other tests including radioactive tracers or special X-ray department calculations are used. Any of these tests may be ordered to help measure kidney function. Which one is used often depends on the disease that is suspected, but many kidney specialists (nephrologists) now are using the MDRD equation.

What is involved in preparation for the test?

Specific instructions are provided by the healthcare provider.

What do the test results mean?

Please refer to the results section in each separate article for the individual tests listed above.