(933) Effect of Discordance Between Cystatin C and Serum Creatinine-Based GFRs on Medications in the ICU

Introduction: Serum creatinine (SCr) may not be the best marker to estimate renal function in critically ill (ICU) patients. Cystatin C (CysC) is a biomarker that may better estimate renal clearance. We evaluated the discordance between CysC and SCr-based estimated glomerular filtration rates (eGFR).

Methods: We retrospectively reviewed patients admitted to an ICU who had CysC collected from 12/2023 to 7/2024. Creatinine clearance (CrCl) was calculated using the Cockcroft-Gault Equation using ideal body weight (IBW) unless actual body weight (ABW) was less than (IBW) or adjusted body weight (AdjBW) if ABW was greater than 120% of IBW. The eGFRcys was calculated using the 2012 CKD-EPI cystatin C equation. Both CrCl and eGFRcys were capped at 120 mL/min and mL/min/1.73m2 respectively. Demographics, SCr and common renally dosed medications were recorded within 24 hours of collection of CysC. eGFRdiff was defined as eGFRcys minus CrCl. eGFRdiff was categorized into three groups: less than -15 (negative), -15 to < 15 (concordant), and >15 (positive) mL/min/1.73 m2. A Mann-Whitney U test was used to compare the calculated eGFRs.

Results: Fifty-three patients were included in the analysis, 57% were male, with a median age of 51 (IQR 37 to 62) years. Twenty-four (45%) patients had an acute kidney injury, and the median ICU length of stay prior to CysC was 6 (IQR 3 to 21) days. There was a significantly lower eGFR calculated by CysC (49.6 IQR 28.0-79.7) compared to CrCl (94.0 IQR 55.6-120.0), p< 0.05). The difference in eGFR between CysC and CrCl were negative, concordant, and positive in 68%, 30% and 2% of the cohort, respectively. Twenty-five (47%) patients had at least one medication that could have required a renal dose adjustment. Overall, 108 renally eliminated medications were identified, with 35 (32%) that would have received a different dose with CysC-based dosing, only one of which would have been a higher dose. The majority of affected medications were antimicrobials (25 of 36, 71%), with meropenem being most common (n=8).

Conclusions: In this small cohort, CrCl overestimated eGFR in comparison to eGFRcys in most patients, with many having at least one medication that would have been dosed differently. More data is needed to evaluate the accuracy of dosing medications based on CysC.