Evaluation of clinical factors associated with discordant estimated glomerular filtration rate values determined from creatinine vs cystatin C

Purpose: Estimated glomerular filtration rate (eGFR) is the standard for categorizing renal function. Current creatinine-based estimates have been demonstrated to have limited accuracy, with national organizations encouraging use of cystatin C to calculate eGFR. This study aimed to describe the relationship between eGFR calculated using serum cystatin C (eGFR_cysC) vs serum creatinine (eGFR_creat) using Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations. Methods: A retrospective review was performed for the period from July 2018 through June 2022 that included inpatient adults with a measured serum cystatin C level and a paired serum creatinine level obtained within 24 hours of each other. eGFR_creat and eGFR_cysC were calculated using the CKD-EPI equations. The primary objective was to identify factors that were significantly associated with discordance between eGFR_cysC vs eGFR_creat, which was expressed as the percentage difference in eGFR (eGFR_{pct_diff}). Differences in eGFR_{pct_diff} by patient subgroup were assessed using a paired t test or ANOVA as appropriate. Univariate and multivariate regression analyses were performed to further identify variables associated with eGFR_{pct_diff}. Results: The study population included 226 patients. The mean (95% confidence interval) eGFR_{pct_diff} by patient subgroup was as follows: CKD stage 4, 45.0% (25.6% to 64.4%); weight loss of greater than 10% in 1 year, –33.9% (–45.6% to –22.2%); cancer, –36.1% (–48.2% to –24.1%); and hemiplegia, –32.7% (–46.2% to –19.2%). Thirty-seven patients had at least one 24-hour urine collection for determination of creatinine clearance. Measured 24-hour creatinine clearance was better correlated with eGFR_cysC (R² = 0.754) than it was with eGFR_creat (R² = 0.557) or creatinine clearance calculated using the Cockcroft-Gault creatinine equation (R² = 0.288). Conclusion: Cautious interpretation is recommended when estimating renal function from serum creatinine alone in patients with conditions associated with loss of muscle mass, including weight loss, cancer, hemiplegia, and immobility.