Creatinine clearance
|
|
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.
<math>\mbox{GFR} = \frac { \mbox{concentration in urine} \times \mbox{volume of urine per unit time}} {\mbox{plasma concentration}}<math>
More often, the creatinine clearance is estimated using the Cockcroft-Gault formula:
<math>\mbox{Creatinine Clearance} = \frac { \mbox{(140 - Age)} \times \mbox{mass in kilograms}} {\mbox{72} \times \mbox{plasma creatinine}}<math>
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).
See also: human physiology, nephrology.