Inaccuracy of estimated creatinine clearance for prediction of iothalamate glomerular filtration rate.

The clinical practice of estimating creatinine clearance (Ccr) from a patient's serum creatinine value by use of various nomograms and prediction formulas is widespread. The predictive accuracy of such Ccr estimates as substitutes for measured values of glomerular filtration rate (GFR) has not been determined. In addition, the effect of patient physical parameters on GFR prediction accuracy has not been assessed. To investigate these issues, 500 predicted Ccr values from each of four versions of the formula of Cockcroft and Gault were statistically compared with GFR values measured by sodium iothalamate clearance (Cio) in 394 human subjects representing every level of renal function. We conclude that (1) the original formula of Cockcroft and Gault is an inaccurate predictor of GFR; (2) correction of the formula for patient physical parameters does improve its accuracy for GFR prediction; (3) the best formula is not accurate enough to replace laboratory measurement of GFR; and (4) GFR prediction inaccuracy is more often associated with extremes in patient age, weight, serum creatinine, body surface area, and measured GFR but may occur for any value of each of these parameters.