(1447) Estimating GFR Using Cystatin C, Creatinine, or Both and Impact on Predicted Vancomycin Half-Life

Introduction: Glomerular filtration rate (GFR) is commonly estimated with equations using creatinine, cystatin C, or both. Drug dosages may differ depending on which equation is employed. This study determined estimated GFRs from four 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), 2012 CKD-EPI Cystatin C (eGFRcys), 2021 CKD-EPI Creatinine (eGFRcr), 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 ANOVA and Pearson correlation coefficient (r2). Bayesian modeling population estimates predicted vancomycin t1/2.

Results: Fifty-eight patients with 100 GFR estimates were assessed. Mean age was 58.4±15.2 years. Intensive care units were medical (n=31), cardiac (n=14), neurosurgical (n=10) and surgical / trauma (n=3). Mean creatinine and cystatin C concentrations were 1.62±1.67 mg/dL and 2.24±1.2 mg/L, respectively. Estimated GFRs were 78.8±62.1 mL/min, 39.7±28.8 mL/min, 72.5±42.3 mL/min, and 57.9±38.5 mL/min for CrClCG, eGFRcys, eGFRcr, and eGFRcr-cys, respectively (p < 0.0001). r2 (all p< 0.0001) were 0.53 for CrClCG vs eGFRcys, 0.83 for CrClCG vs. eGFRcr, 0.77 for CrClCG vs eGFRcr-cys, 0.58 for eGFRcys vs eGFRcr, 0.8 for eGFRcys vs eGFRcr-cys, and 0.89 for eGFRcr vs eGFRcr-cys. Mean predicted t1/2 values from five patients with vancomycin monitoring were 23±19.3 hours, 29.4±21.7 hours, 25.7±18.3 hours, and 40.3±26.4 hours, respectively (p=0.71).

Conclusions: GFR estimations varied based on the equation employed. Equations using only creatinine or cystatin C resulted in GFR estimations that correlated weakly. Stronger associations were demonstrated when both creatinine and cystatin C were used together. The variable GFR estimations led to substantial differences in predicted vancomycin t1/2 which could produce dosing disparities. Future studies need to assess which equation best predicts GFR and determine dosage modifications of commonly used medications.