Estimation of creatinine clearance in morbidly obese patients

Purpose. Estimates of creatinine clearance (CL) based on equations and various body-size descriptors were compared with 24-hour measured CLcr values in morbidly obese patients. Methods. Patients age 18-75 years with a body mass index (BMI) of >= 40 kg/m(2) with stable serum creatinine values were enrolled. Covariates known to contribute to alteration in CLcr were used to exclude patients. Twenty-four-hour urine collection was performed to measure CLcr Bioelectric impedance analysis was used to estimate fat-free weight (FFW). Glomerular filtration rate was estimated using the four-variable Modification of Diet in Renal Disease (MDRD4) equation. CLcr was estimated using the Cockcroft-Gault and Salazar-Corcoran methods using total body weight (TBW). Body-size descriptors, such as ideal body weight (IBW), adjusted body weight (ABW), and lean body weight (LBW), and FFW were substituted in the Cockcroft-Gault equation to generate additional estimates of Cl-cr. Results. Fifty-four patients (mean +/- S.D. age, 48.4 +/- 12.9 years; TBW, 142.3 +/- 41.7 kg; BMI, 50.5 +/- 12.6 kg/m(2)) completed the study. All three equations were biased in their estimation of CLcr. Use of MDRD4 and IBW in the Cockcroft-Gault equation underestimated CLcr, while the Salazar-Corcoran equation and use of TBW or ABW in the Cockcroft-Gault equation overestimated this value. Substitution of fat-free weight or LBW in the Cockcroft-Gault equation provided unbiased estimates of CLcr. Conclusion. An LBW estimate, based on TBW and BMI, incorporated into the Cockcroft-Gault equation provided an unbiased, relatively precise, accurate, and clinically practical estimate of 24-hour measured CLcr in morbidly obese patients.