Standardization of p-creatinine assays and use of lean body mass allow improved prediction of calculated glomerular filtration rate in adults: A new equation

Objective. To evaluate the Cockcroft-Gault (CG) equation, using various body weight expressions, and the Sawyer equation in predicting glomerular filtration rate (GFR) using an enzymatic and zero-calibrated Jaffe plasma-creatinine assay, and to derive a new robust equation in adults. Material and methods. The CG weight measures included total, ideal and adjusted body weight (ABW; lowest of total and ideal) and two lean body mass (LBM) expressions, while the Sawyer equation is based primarily on LBM. Iohexol clearance was used to measure GFR. One derivation set (n=436; enzymatic assay) was used to evaluate and bias-adjust existing equations when indicated, and to derive a new equation based on plasma-creatinine, age, gender and the body weight measure yielding the best adjusted R². All equations were then validated in a separate set (n=414; Jaffe assay). Results. The existing equations all performed similarly in both sets. Prediction errors of equations based on LBM showed no correlation with BMI. The CG_ABW and Sawyer equations performed best. The new equation with LBM yielded the highest adjusted R². In the combined set (n=850), its accuracy (86 %/98 % of estimates within 30 %/50 % of measured GFR) was significantly better than for the CG_ABW (79 %/95 %) and Sawyer equations (79 %/93 %) (p<0.001) for each 30 mL/min GFR subgroup within ±30 % and ±50 %, except within ±30 % > 120 mL/min. Prediction error did not correlate with BMI, age or gender. Conclusion. A new creatinine-based equation derived in a mainly Caucasian patient sample is a better predictor of GFR than CG-type equations irrespective of the body weight measure used or, if bias-adjusted, when using zero-calibrated creatinine assays.