Estimation of creatinine clearance in morbidly obese patients.

PURPOSE: Estimates of creatinine clearance (CL(cr)) based on equations and various body-size descriptors were compared with 24-hour measured CL(cr) 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 CL(cr) were used to exclude patients. Twenty-four-hour urine collection was performed to measure CL(cr). 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. CL(cr) 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 CL(cr). Use of MDRD4 and IBW in the Cockcroft-Gault equation underestimated CL(cr), 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 CL(cr).

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 CL(cr) in morbidly obese patients.