Creatinine clearance is a method that estimates the glomerular filtration rate (GFR) of the kidneys. This is the amount of liquid filtered out of the blood that gets processed by the kidneys.
Creatinine clearance is the amount of creatinine in the urine, divided by the concentration in the blood, over a certain amount of time.
More often, the creatinine clearance is estimated using the Cockcroft-Gault formula:
The number produced by the above formula should be multiplied by .85 if female. Plasma creatinine is expected to be in mg/dL.
Glomerular filtration rate can be calculated by measuring any chemical that has a steady level in the blood, and is neither actively absorbed or excreted by the kidneys.
Creatinine is used because it fulfills these requirements (though not perfectly), and it is produced naturally by the body. Other methods involve constant infusions of inulin or another compound, to maintain a steady state in the blood.
The result of this test is an important gauge used in assessing excretory function of the kidneys. For example grading of chronic renal insufficiency and dosage of drugs that are primarily excreted via urine are based on GFR (creatinine clearance).
The creatinineclearance test compares the level of creatinine in urine with the creatinine level in the blood, usually based on measurements of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period.
Because creatinine is found in stable plasma concentrations, is freely filtered and not reabsorbed, and is minimally secreted by the kidneys, creatinineclearance is used to estimate the glomerular filtration rate (GFR) -- the standard by which kidney function is assessed.
The creatinineclearance is an estimate of the glomerular filtration rate, that is, the volume of filtrate made by the kidneys per minute.
Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass).
A rise in blood creatinine levels is observed only with marked damage to functioning nephrons; therefore this test is not suitable for detecting early kidney disease.
A better estimation of kidney function is given by the creatinineclearance test, which, however, is less convenient and unsuitable as a screening test because it requires the collection of urine over 24 hours.
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