Calculation
Creatinine clearance = (UV / P) ml per minute, where:
- concentration of creatinine in urine = U
- concentration of creatinine in plasma = P
- volume of urine collected in T minutes = V
This formula shows SI units.
- UV Amount of creatinine in 24h of urine is reported by the lab in mmol/24h; multiply by 1000 to convert to micromols
- P Serum creatinine is reported in micromols/l; multiply by 1000 to do calculation for mls
- 1440 Number of minutes in 24h CrCl(mls per min) = UVx1000x1000/(Px1440)
Alternatively, if urinary collection is difficult, the Cockcroft-Gault equation aims to predict creatinine clearance from knowledge of serum creatinine, age and weight:
- creatinine clearance = (((140 - age in years) x (wt in kg)) x 1.23) / (serum creatinine in micromol/l)
For women multiply the result of calculation by 0.85.
The second calculation is unreliable if the patient has unstable renal function, is very obese, or is oedematous.
Reference:
- Cockcroft D (1976). Nephron, 16, 31.
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