Accuracy of short-duration creatinine clearance determinations in predicting 24-hour creatinine clearance in critically ill and injured patients.

BACKGROUND: We hypothesized that measured 2-hour (CrCl2), 6-hour (CrCl6), and 16-hour (CrCl16) urine creatinine clearance accurately reflect measured (CrCl24meas) and calculated 24-hour CrCl (CrCl24calc) in critical illness.
METHODS: Urine was collected in consecutive specimens from 7 am to 9 am (CrCl2), 9 am to 3 pm (CrCl6), and 3 pm to 7 am (CrCl16) at surgical intensive care unit admission and weekly thereafter. CrCl2 and CrCl6 were added to obtain CrCl8, which was then added to CrCl16 to obtain CrCl24meas. CrCl24calc was estimated using the Cockcroft-Gault equation.
RESULTS: One hundred patients (45 with trauma) had 131 sets of CrCl2, CrCl6, and CrCl16. Trauma patients were younger; had a lower mean body surface area; and had higher CrCl2, CrCl6, and CrCl16 (all p < 0.0001). Correlation percentages (r2) comparing CrCl2, CrCl6, CrCl8, CrCl16, and CrCl24calc with CrCl24meas in trauma patients were 0.597, 0.760, 0.815, 0.958, and 0.670, respectively. In nontrauma patients, r2 values were 0.516, 0.693, 0.807, 0.946, and 0.649, respectively.
CONCLUSION: CrCl2, CrCl6, and CrCl24calc are unreliable for clinical decision making. A minimum collection period of at least 8 hours is recommended for determination of urine creatinine clearance.