The Impact of Capping Creatinine Clearance on Achieving Therapeutic Vancomycin Concentrations in Neurocritically Ill Patients with Traumatic Brain Injury

Nicholas R. Nelson, Kathryn A. Morbitzer, J. Dedrick Jordan, Denise H. Rhoney

Traumatic brain injury (TBI) is associated with secondary complications, including infection, and patients with TBI often exhibit augmented renal clearance (ARC). This phenomenon has been associated with subtherapeutic levels of renally cleared drugs such as vancomycin, which is dosed based on body weight and creatinine clearance (CrCl). Many clinicians, however, cap CrCl at 120 mL/min/1.73 m2 when calculating vancomycin dosing regimens. We hypothesize that capping patient CrCl, as opposed to utilizing the non-capped CrCl, when determining vancomycin dosing schemes results in subtherapeutic serum trough concentrations in patients with TBI. This was a retrospective study of adult patients with TBI admitted between April 2014 and December 2015 who received vancomycin. Population-based pharmacokinetic (PK) parameters using non-capped calculated CrCl and capped CrCl were compared with patient-specific PK parameters based on serum trough concentrations. Thirty-two patients with TBI were included in the study. ARC was suspected in 24 (75%) patients due to a median estimated CrCl at serum trough concentration of 167.3 (127.7-197.7) mL/min. The mean dosing regimen was 17.1 (13.2-19.2) mg/kg every 8 (8-8) h. There was no difference between the median measured trough concentration and predicted value using non-capped CrCl [10.4 (7.1-15.0) vs. 11.5 (7.8-13.7) mcg/mL; p = 0.7986]. The median measured trough concentration was significantly lower than the predicted trough concentration when calculated based on capping the CrCl at 120 mL/min/1.73 m2 [16.3 (15.3-22.0) vs. 11.5 (7.8-13.7) mcg/mL; p < 0.0001]. Patients with traumatic brain injury appeared to exhibit augmented renal clearance, leading to subtherapeutic vancomycin serum trough concentrations when doses were calculated using the traditional method of capping creatinine clearance at 120 mL/min/1.73 m2. Instead, utilizing patients' non-capped creatinine clearance when determining a vancomycin dosing regimen is more accurate and provides a better estimation of vancomycin pharmacokinetics and could be applied to other renally excreted medications.