The validity of two-hour creatinine clearance studies in critically ill patients.

To determine the accuracy of 2-h creatinine clearance studies in critically ill surgical patients, 12 consecutive 2-h specimens of urine were collected on 30 patients in a surgical ICU. The 2-, 4-, 6-, 8-, 12-, and 24-h creatinine clearances that were calculated averaged 66 +/- 58 (SD), 59 +/- 45, 62 +/- 49, 63 +/- 48, 66 +/- 50, and 59 +/- 38 ml/min. The correlation coefficient between the 2- and 24-h creatinine clearance was 0.85 (p less than 0.001). The main factor causing differences between the 2- and 24-h creatinine clearance values was variability in urine output. When the average hourly urine output was essentially the same for the 2- and 24-h urine specimens, the correlation coefficient was 0.94. A relatively easy 2-h urine collection can provide essentially the same indication of creatinine clearance as a 24-h sample, particularly if urine output remains constant.