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Creatinine clearance is a test of kidney function. It measures the amount of fluid filtered and cleaned completely of creatinine in a minute by the kidneys.

Creatinine is made in the muscles as a breakdown product of muscle wear and tear and then filtered through the kidneys and put out into the urine.

Almost all the creatinine released from muscles makes its way into the urine as a waste product. Comparing the amount of creatinine in the blood with the amount of creatinine in the urine gives an idea of how well the kidneys are functioning.

Who is a candidate for the test?

A candidate for the test is anyone whose kidneys a healthcare professional suspects may not be working well.

How is the test performed?

This test is usually done on a urine sample that was collected over a 24- hour period. The person should follow the specific instructions from the healthcare professional on how to collect the urine sample. In general, this or a similar schedule is followed:

• Day 1: The person urinates upon arising (for example at 7 a.m.) without collecting that sample. After that, he or she collects all of his or her urine for the next 24 hours in a special container.
• Day 2: First thing the following morning (at 7 a.m.), the person urinates into the container again. Then the individual covers it and refrigerates it. The sample is brought to the doctor, who will send it to the lab for the analysis.

The person will also have a blood sample done so the blood level of creatinine can be measured and a calculation from a formula that incorporates the creatinine in the blood and urine gives the clearance value.

Until recently, all clearance was derived from 24 hour urine collections. Other tests for clearance have been developed including:

• the Cockroft-Gault equation (uses blood creatinine, weight, and age without a urine collection),
• the MDRD equation (also does not collect a urine sample and uses blood), and
• other tests that use radioactive tracers or x-ray contrast materials (used less frequently, and usually only in research studies).

What is involved in preparation for the test?

The person should request exact instructions from his or her healthcare professional. Sometimes other tests are done on the same urine sample (for protein or calcium, for instance) and there may be some dietary instructions as well.

What do the test results mean?

• males, 97 to 137 milliliters/minute (mL/min)
• females, 88 to 128 mL/min

Abnormally low glomerular filtration rates may suggest:

• acute or chronic kidney insufficiency or failure from any of multiple causes (there may also be protein and/or blood in the urine).
• heart failure
• dehydration or a fluid or blood deficit in the circulation
• shock, a life-threatening condition in which blood pressure falls too low to support life
• obstructive uropathy, a kidney disease caused by blockage of the kidney's drainage tubes
• any type of kidney disease