The Cockroft‐Gault Formula

To the Editor: — We read the paper by Friedman et al on the “Correlation of Estimated Renal Function Parameters versus 24-hour Creatinine Clearance in Ambulatory Elderly” with great interest.1 We previously reported “Estimating Creatinine Clearance from Serum Creatinine in Chronically Immobilized Nursing Home Residents.”2 The Cockroft-Gault equation is based on the assumption that 24-hour creatinine excretion and muscle mass can be predicted from age, weight and sex.3 Cockroft and Gault's retrospective database consisted of 249 male veterans mainly from medical wards who had relatively stable serum creatinines and were between the ages of 18 and 92. Dr Friedman studied 15 ambulatory, elderly individuals without evidence of severe or acute underlying disease who were volunteers in a study of antibiotic pharmacokinetics. The mean age of this group was 79.4 years. Friedman's ambulatory, functionally independent volunteers without evidence of severe or acute underlying disease generally had actual 24-hour creatinine clearance values higher than those calculated by the Cockroft-Gault formula. This underestimation of creatinine clearance by the Cockroft-Gault equation could have been expected since these ambulatory volunteers probably had greater muscle mass and 24-hour creatinine excretion than men of similar age seeking care at a Canadian Veterans Hospital. In our work, an overestimation of creatinine clearance was noted in two immobilized middle aged men who presumably had significantly less muscle mass than their age-matched cohort in the Cockroft and Gault sample. Cockroft and Gault noted that in patients with paraplegia, “creatinine excretion was as much as 40% below predicted … presumably due to muscle atrophy.” We would, therefore, suggest that when using the Cockroft-Gault formula that one consider how the muscle mass of a patient compares to that of a “typical” hospitalized Canadian veteran of the same age, weight and sex. Good luck.