[Estimate of creatinine clearance with formulae: a valid option compared with conventional measurement?]

Determination of creatinine clearance is the most commonly used method to evaluate renal function, requiring collection of urine from a given period of time, usually 24 hours. Cockcroft and Gault developed a formula to estimate this parameter without urine samples, however some authors have shown the low reliability of this and similar nomograms. The present study is an attempt to evaluate the differences between the laboratory determination of this parameter and its evaluation using Cockcroft's equation. The study included 24 patients who had creatinine clearance determined through biochemical methods and with the formula proposed by Cockcroft. Mean clearance in the first instance was 81.4 mL/min and 65.5 mL/min in the second one. The estimated mean difference in creatinine clearance obtained by both methods was 16.4 mL/min. A significant difference between clearance determinations was found using the above methods and therefore it is concluded that the values obtained with Cockcroft's formula are hardly reliable since the method tends to subestimate creatinine clearance. Also, two new formulas were developed which correlate better with the results obtained in our series. To conclude, we recommend that creatinine clearance should be measured at the laboratory using serum and 24-hour creatinine determination.