Creatinine clearance

In general, creatinine clearance is the removal of creatinine from the body. In renal physiology, creatinine clearance (C_Cr) is the volume of blood plasma that is cleared of creatinine per unit time. Clinically, creatinine clearance is a useful measure for estimating the glomerular filtration rate (GFR) of the kidneys.

Use in estimating GFR

Glomerular filtration rate (GFR) can be calculated by measuring any chemical that has a steady level in the blood, and is freely filtered but neither reabsorbed nor secreted by the kidneys.

Creatinine is used because it fulfills these requirements (though not perfectly) and it is produced naturally by the body (creatinine is a metabolite of creatine, which is found in muscle). It is actively secreted by the kidneys such that creatinine clearance overestimates actual GFR by 10-20%. This margin of error is acceptable considering the ease with which creatinine clearance is measured. Other more precise GFR measurements involve constant infusions of inulin or another compound, to maintain a steady state in the blood. Since creatininine is already at a steady-state concentration in the blood, measuring creatinine clearance is much less cumbersome.

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).

It is commonly believed to be the amount of liquid filtered out of the blood that gets processed by the kidneys. Physiologically, these quantities (volumetric blood flow and mass removal) are only related loosely. Clearance is a ratio of the mass generation and concentration at a steady state.

Calculation

Creatinine clearance (C_Cr) can be calculated if values for creatinine's urine concentration (U_Cr), urine flow rate (V), and creatinine's plasma concentration (P_Cr) are known. Since the product of urine concentration and urine flow rate yields creatine's excretion rate, creatinine clearance is also said to be its excretion rate (U_Cr×V) divided by its plasma concentration. This is commonly represented mathematically as

C_{Cr} = \frac { U_{Cr} \times V }{ P_{Cr} }

For example, a normal individual with a plasma creatinine concentration of 1 mg/dL, urine creatinine concentration of 60 mg/dL, and urine flow rate of 0.5 dL/h, would have a creatinine clearance given by

C_{Cr} = \frac { 60 \ \mbox{mg/dL} \times 0.5 \ \mbox{dL/h} }{ 1 \ \mbox{mg/dL} }

which yields 30 dL/h. Thus, 30 dL of blood are cleared of creatinine per hour.

Cockcroft-Gault approximation

More often, the creatinine clearance is estimated using the Cockcroft-Gault formula:

$\mbox{Creatinine clearance} = \frac { \mbox{(140 - Age)} \times \mbox{mass in kilograms}} {\mbox{72} \times \mbox{plasma creatinine}}$

The number produced by the above formula should be multiplied by .85 if female. Plasma creatinine is expected to be in mg/dL. In some countries mmol/L is used instead of mg/dL. This change of units results in the following equation:

$\mbox{Creatinine clearance} = \frac { \mbox{(140 - Age)} \times \mbox{mass in kilograms}} {\mbox{815} \times \mbox{plasma creatinine}}$

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Use in estimating GFR

Calculation

Cockcroft-Gault approximation

See also