The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood. (Creatinine is a breakdown product of creatine, which is an important part of muscle.) The test helps provide information on kidney function.

See also:

• Creatinine - blood test
• Creatinine - urine test

The creatinine clearance test is used to estimate the glomerular filtration rate (GFR).

However, because a small amount of creatinine is released by the filtering tubes in the kidneys, creatinine clearance is not exactly thto the same as the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced kidney failure.

This test requires both a blood and urine sample. You will collect your urine for 24 hours, and then have blood taken.

• For information on giving a blood sample, see: venipuncture.
• For information on how to collect the urine sample, see: 24-hour urine collection.

The samples are sent to a laboratory. The laboratory specialist measures the level of creatinine in both the urine and blood samples, and looks at how much urine you collected in 24 hours.

The clearance rate is then calculated. The calculation is adjusted for your specific body size.

The creatinine clearance appears to decrease with age (each decade corresponds to a decrease of about 6.5 ml/min./1.73 m2).

If the collection is being taken from an infant, a couple of extra collection bags may be necessary.

The urine test involves only normal urination and there is no discomfort. When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.