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

BACKGROUND

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.

OBJECTIVES

This study determined estimated GFRs from 4 common equations, their correlations, and the subsequent variations of predicted vancomycin half-life (t1/2).

METHODS

Retrospective cohort study of critically ill adults with GFR estimations based on Cockcroft-Gault (CrClCG), 2021 CKD-EPI Creatinine (eGFRcr), 2012 CKD-EPI Cystatin C (eGFRcys), and 2021 CKD-EPI Creatinine-Cystatin (eGFRcr-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 t1/2.

RESULTS

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 CrClCG, eGFRcr, eGFRcys, and eGFRcr-cys, respectively (P < 0.0001). r2 (all P < 0.0001) were 0.82 for CrClCG versus eGFRcr, 0.54 for CrClCG versus eGFRcys, 0.72 for CrClCG versus eGFRcr-cys, 0.57 for eGFRcr versus eGFRcys, 0.77 for eGFRcr versus eGFRcr-cys, and 0.92 for eGFRcys versus eGFRcr-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 t1/2 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).

CONCLUSIONS AND RELEVANCE: 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 t1/2. Clinicians must be aware that dosing disparities may occur based on the GFR estimation used.