Effect of Cystatin C on Vancomycin Clearance Estimation in Critically Ill Children Using a Population Pharmacokinetic Modeling Approach.

Background: Vancomycin is eliminated by glomerular filtration, but current approaches to estimate kidney function in children are unreliable. The authors sought to compare the suitability of cystatin C (CysC)-based glomerular filtration rate equations with the most commonly used creatinine-based equation, bedside Schwartz, to estimate vancomycin clearance (CL). Methods: This prospective observational study enrolled critically ill patients (2-18 years) receiving intravenous vancomycin at the Children's Hospital of Philadelphia during December 2015-November 2017. Vancomycin levels were collected during clinical care and at 3 times during a single dosing interval. Plasma CysC was measured within 24 hours before intravenous vancomycin (baseline) initiation or immediately after enrollment and along with the third pharmacokinetic sample. Nonlinear mixed effects modeling was performed using NONMEM software. Covariate selection was used to test model fit with inclusion of the estimated glomerular filtration rate (eGFR) on CL using bedside Schwartz versus various published CysC-based equations. Results: In total, 83 vancomycin levels were obtained from 20 children. The median age was 12.7 years; 6 patients were women. A 1-compartment model best described the data; CL was allometrically scaled to 0.75. During covariate selection, inclusion of the eGFR calculated using a CysC-based equation significantly improved model fit [reduction in objective function value (OFV) range: 217.191 to 218.704] than bedside Schwartz (Delta OFV 212.820). Including the full age spectrum equation, an eGFR equation based on both creatinine and CysC, led to the largest OFV reduction (222.913); female sex was also a significant covariate of CL in the model. Final model pharmacokinetic indices were CL = 0.29 L/h/kg(0.75) and volume of distribution = 0.48 L/kg. Conclusions: CysC-based equations help better estimate vancomycin CL than bedside Schwartz in critically ill children.