Obese patients, creatinine clearance

The decrease of renal function with age is independent of the decrease in creatinine production. Because of the difficulty of obtaining complete urine collections, creatinine clearance calculated in this way is at least as reliable as estimates based on urine collections. Fat-free mass (equation [14]) should be used for obese patients, and correction should be made for muscle wasting in severely ill patients. 

Salazar DE, Corcoran GB. Predicting creatinine clearance and renal drug clearance in obese patients from estimated fat-free body mass. Am J Med 1988 84 1053-1060. 

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