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

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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 (eGFRcysC) vs serum creatinine (eGFRcreat) 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. eGFRcreat and eGFRcysC were calculated using the CKD-EPI equations. The primary objective was to identify factors that were significantly associated with discordance between eGFRcysC vs eGFRcreat, which was expressed as the percentage difference in eGFR (eGFRpct_diff). Differences in eGFRpct_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 eGFRpct_diff.

Results: The study population included 226 patients. The mean (95% confidence interval) eGFRpct_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 eGFRcysC (R2 = 0.754) than it was with eGFRcreat (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.