Accuracy of estimated creatinine clearance in obese patients with stable renal function in the intensive care unit

We compared agreement between creatinine clearance values in obese, critically ill patients calculated using three common empirically derived formulas and modifications thereof, with creatinine clearance obtained by conventional 24-hour urine collection. We selected the charts of 22 patients in intensive care units (86% medical, 14% surgical) according to the following criteria: actual body weight greater than 150% of ideal body weight; serum creatinine variation of less than 15% from the day of starting 24-hour urine collection to the day before or after the collection; presence of a urinary bladder catheter; no history of renal dialysis; and clinical indication for renal function assessment. Mean measured 24-hour urinary creatinine clearance for all patients was 72 +/- 64 ml/minute (range 8-248 ml/min). The method of estimating creatinine clearance that showed the least mean bias was the equation of Salazar and Corcoran using a corrected serum creatinine concentration (mean bias -2 ml/min); however, the corresponding 95% confidence intervals were wide (-133-129 ml/min). The narrowest range of 95% confidence intervals were seen with Jelliffe's equation (mean bias 25 ml/min, 95% confidence intervals -41-90 ml/min). In this sample, estimated creatinine clearances did not agree acceptably with measured values. Despite low mean bias values, none of the empirically derived equations that we studied had clinically acceptable 95% confidence intervals. We recommend using the 24-hour urine collection method when assessing creatinine clearance in obese, critically ill patients.