Estimating GFR Using Creatinine, Cystatin C, or Both and the Impact on Predicted Vancomycin Half-Life

Glomerular filtration rate (GFR) is commonly estimated with equations using creatinine, cystatin C, or a combination of both. Drug dosages may differ depending on the which equation.

This study determined estimated GFRs from 4 common equations, their correlations, and the subsequent variations of predicted vancomycin half-life (t_1/2).

Retrospective cohort study of critically ill adults with GFR estimations based on Cockcroft-Gault (CrCl_CG), 2021 CKD-EPI Creatinine (eGFR_cr), 2012 CKD-EPI Cystatin C (eGFR_cys), and 2021 CKD-EPI Creatinine-Cystatin (eGFR_cr-cys). Patients were excluded if pregnant, dialysis dependent prior to admission, or sampling occurred within 24 hours of renal replacement therapy. Statistical analyses used MANOVA and Pearson correlation coefficient (r²). Bayesian modeling population estimates predicted vancomycin t_1/2.

Sixty-three patients with 104 GFR estimates were assessed. Mean age was 58.5 ± 15.2 years. Mean creatinine and cystatin C were 1.78 ± 2.1 mg/dL and 2.28 ± 1.2 mg/L, respectively. Mean estimated GFRs were 76.6 ± 59.2 mL/min, 72 ± 41.4 mL/min, 38 ± 27.5 mL/min, and 47.3 ± 32.7 mL/min for CrCl_CG, eGFR_cr, eGFR_cys, and eGFR_cr-cys, respectively (P < 0.0001). r² (all P < 0.0001) were 0.82 for CrCl_CG versus eGFR_cr, 0.54 for CrCl_CG versus eGFR_cys, 0.72 for CrCl_CG versus eGFR_cr-cys, 0.57 for eGFR_cr versus eGFR_cys, 0.77 for eGFR_cr versus eGFR_cr-cys, and 0.92 for eGFR_cys versus eGFR_cr-cys. Mean creatinine and cystatin C for vancomycin half-life estimations were 2.72 ± 3.48 mg/dL and 2.76 ± 1.1 mg/L, respectively. Mean predicted t_1/2 values from 17 patients with vancomycin monitoring were 33.3 ± 22.7 hours, 35.7 ± 27.5 hours, 43.2 ± 40.9 hours, and 38.7 ± 37.3 hours, respectively (P = 0.85).

GFR estimations varied substantially with creatinine-based equations resulting in higher values. Correlation analyses revealed moderate to strong associations between equations. Discrepancies between GFR estimations were more apparent at higher GFRs. The variable GFR estimations led to substantial differences in predicted vancomycin t_1/2. Clinicians must be aware that dosing disparities may occur based on the GFR estimation used.