A comparison of estimates of glomerular filtration in critically ill patients with augmented renal clearance
1 Serviço de Medicina Intensiva, Hospitais da Universidade de Coimbra, EPE Praceta Prof. Mota Pinto, Av. Bissaya Barreto 3000-075 Coimbra, Portugal
2 Department of Intensive Care Medicine, Royal Brisbane and Women's Hospital, Butterfield Street, Herston 4029, Queensland, Australia
3 Burns, Trauma, and Critical Care Research Center, University of Queensland, Royal Brisbane and Women's Hospital, Butterfield Street, Herston 4029, Queensland, Australia
Critical Care 2011, 15:R139 doi:10.1186/cc10262Published: 8 June 2011
Increasingly, derived estimates of glomerular filtration, such as the modification of diet in renal disease (MDRD) equation and Cockcroft-Gault (CG) formula are being employed in the intensive care unit (ICU). To date, these estimates have not been rigorously validated in those with augmented clearances, resulting in potentially inaccurate drug prescription.
Post-hoc analysis of prospectively collected data in two tertiary level ICU's in Australia and Portugal. Patients with normal serum creatinine concentrations manifesting augmented renal clearance (ARC) (measured creatinine clearance (CLCR) > 130 ml/min/1.73 m2) were identified by chart review. Comparison between measured values and MDRD and CG estimates were then undertaken. Spearman correlation coefficients (rs) were calculated to determine goodness of fit, and precision and bias were assessed using Bland-Altman plots.
Eighty-six patients were included in analysis. The median [IQR] measured CLCR was 162 [145-190] ml/min/1.73 m2, as compared to 135 [116-171], 93 [83-110], 124[102-154], and 108 [87-135] ml/min/1.73 m2 estimated by CG, modified CG, 4-variable MDRD and 6-variable MDRD formulae. All of the equations significantly under-estimated the measured value, with CG displaying the smallest bias (39 ml/min/1.73 m2). Although a moderate correlation was noted between CLCR and CG (rs = 0.26, P = 0.017) and 4-variable MDRD (rs = 0.22, P = 0.047), neither had acceptable precision for clinical application in this setting. CG estimates had the highest sensitivity for correctly identifying patients with ARC (62%).
Derived estimates of GFR are inaccurate in the setting of ARC, and should be interpreted with caution by the physician. A measured CLCR should be performed to accurately guide drug dosing.