Creatinine Clearance
Alternate Names : GFR, Glomerular Filtration Rate
Definition
The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood, usually based on assessments of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period. Clearance is often measured as milliliters/minute (ml/min).
Because creatinine is found in stable plasma concentrations, is freely filtered and not reabsorbed, and is minimally secreted by the kidneys, creatinine clearance is used to estimate the glomerular filtration rate (GFR). The GFR in turn is the standard by which kidney function is assessed.
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Why is the Test 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 constitutes a greater percentage of the overall urine creatinine.
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