Augmented Renal Clearance in the Critically Ill: How to Assess Kidney Function

BACKGROUND: Augmented renal clearance in critically ill patients can result in underdosing of life-saving drugs, potentially leading to therapeutic failure. To detect this phenomenon, correct assessment of the kidney function is essential. Currently, little is known about the validity of mathematical formulas to estimate renal function in this subset of patients. OBJECTIVE: To evaluate the validity of different methods to estimate kidney function in critically ill patients with augmented renal clearance by comparing measured renal clearance with estimated clearance using different formulas. METHODS: An observational, retrospective, single-center study was conducted in a 34-bed surgical intensive care unit (SICU) of the University Hospitals Leuven, Leuven, Belgium. Adults admitted to the SICU in 2010 with a measured creatinine clearance (CrCl) of 120 mL/min or more (based on 24-hour urinary collection) were included. The measured clearance values were compared with estimated clearance values as calculated by the Cockcroft-Gault (CrClCG) method and the re-expressed 4-variable Modification of Diet in Renal Disease estimated glomerular filtration rate (eGFR) formula. Spearman rank order correlation was performed to determine the relationship between measured and estimated clearances. Bland-Altman plots were evaluated to assess bias and limits of agreement between the 2 methods. RESULTS: Records on 1317 patients were screened. Augmented renal clearance was present in 390 patients. Spearman correlation showed fair correlation between measured and estimated clearances (r(s) = 0.343; p < 0.001 [CrClCG] and r(s) = 0.290; p < 0.001 [eGFR]). Bias was 11.2 mL/min with limits of agreement (-131.7; 109.3 mL/min [CrClCG]) and -19.9 mL/min with limits of agreement (-170.4; 130.7 mL/min [eGFR]). CONCLUSIONS: Estimated renal clearances, such as the eGFR estimated by the MDRD formula or CrCl estimated by CG, showed poor agreement with measured CrCl values in our critically ill population displaying augmented renal clearance. Clinicians should be cautious when interpreting kidney function based on estimating equations in this subset of patients. Instead, measured CrCl using urinary collection is recommended in patients suspected of displaying augmented renal clearance.