Evaluation of cystatin C utilization and its discordance with creatinine in a large academic medical center

Cystatin C (CysC) is a kidney biomarker that is used in the assessment of renal function and in the estimation of glomerular filtration rate (eGFR). Professional practice guidelines have recommended the expanded use of CysC as an alternative or adjunctive marker to creatinine. Post-implementation CysC utilization within our health system was evaluated over a three-year period. CysC and creatinine were measured immunoturbidimetrically and enzymatically, respectively; eGFR was calculated using recommended CKD-EPI equations. Clinical orders for CysC were extracted from the electronic health record system over a three-year period; data were stratified by setting and medical subspecialties. Paired CysC and creatinine results, and associated eGFRs (within 24 hours), were analyzed over a three-month period. Chart reviews were conducted for patients with normal creatinine and elevated CysC concentrations to evaluate potential clinical indications and patient outcomes. Between August 2021 and July 2024, 15,428 CysC tests were resulted for 9,002 unique patients. Over a three-month period, the majority of testing (76 %) was performed in patients with kidney disease. In a pairwise analysis, 47 % of patients exhibited discordant eGFR_Cr and eGFR_Cys, primarily due to elevated CysC and lower creatinine concentrations; this was mitigated using the combined eGFR_Cr-Cys equation. Patients with normal creatinine and eGFR_Cr but elevated CysC (>2.06 mg/L) were associated with higher short-term mortality risk. These findings demonstrate sustained utilization of CysC within our health system, with ordering practices aligning with current clinical guidelines. The predominance of nephrology-related indications among discordant cases highlights the role of CysC in refining renal risk stratification.