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The Cockroft and Gault formula for estimating creatinine clearance (CCr) as a proxy for glomerular filtration rate (GFR) has been in use for clinical and research purposes since its derivation in 1976.1 Recently it has been largely superceded by the eGFR, based on the MDRD formula [[Image omitted. See PDF], (omitting factors for sex and race)] but remains a valuable bedside tool for estimating the need to adjust the doses of drugs that are cleared by the kidney in patients with renal dysfunction.2
In this paper I review the derivation of the Cockroft and Gault formula from a historical perspective and comment on its use at the bedside.
Methods
This work is based on an analysis of the paper by Donald W Cockroft and M Henry Gault in Nephron (1976)1 and some extrapolations therefrom. The formula as published was
[Image omitted. See PDF]
(Equation 1). An alternative formula without weight as a dependent variable was derived from the paper after calculating total 24 hr creatinine excretion, following the method described by Cockroft and Gault.
Derivation of the formula:
Cockroft and Gault studied 534 consecutive patients in whom creatinine clearance was measured on 2 or more occasions using serum and 24-hour urine creatinine concentrations. Ninety six percent of the patients were male. Patients (n = 29) were excluded if not in steady state (blood creatinine values differed by > 20%). A sub-group ("Group II") used for the derivation of the formula (n = 236) was formed from "Group I" by further excluding patients whose 24-hr urine creatinine values differed by more than 20% (n = 173) or was < 10 mg/Kg (n = 31) or where the records were inadequate (n = 65). Group II was augmented by re-inclusion of 23 patients who satisfied the second criteria but whose 24-hr urine volume was > 500 ml (final n = 249).
The steps in deriving the formula were:
Step 1 . The relationship between creatinine excretion expressed as mg/kg/24h (as the dependent variable) (CrUV24/kg) was plotted against age (independent variable) after aggregating data into age bands of 10 years (data given as table II in their paper).
Step 2 . The equation for the curve was obtained by linear regression:
[Image omitted. See PDF] (Equation 2)
Step...