Creatinine clearance

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.

Both the blood and urine will be tested in a laboratory.

How to prepare for the test:

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

How the test will feel:

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.

Why the test is performed:

The creatinine clearance is an estimate of the glomerular filtration rate, that is, the volume of filtrate made by the kidneys per minute. The urine and serum creatinine levels are measured along with the urine volume in 24 hours. The clearance rate is then calculated.

The calculation uses a correction factor for 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).

Creatinine is used for this purpose, since it is normally present in the body and very little creatinine is reabsorbed after it is filtered. The amount of filtrate made in the kidney depends on the amount of blood that passes through the glomeruli and the ability of the glomeruli to act as filters.

Because a small amount of creatinine is secreted by the kidney tubules, creatinine clearance is not exactly equivalent to the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced renal failure, where the percentage of secreted creatinine in the urine means a greater percentage of the overall urine creatinine.