Creatinine/cystatin C ratio‐guided selection of renal function marker in sarcopenia

AIMS: The serum creatinine-to-cystatin C ratio (Cr/Cys-C) has recently garnered interest as a simpler surrogate marker for sarcopenia assessment. However, the reported Cr/Cys-C thresholds vary in different disease states, and Cr/Cys-C is yet to be established as a reliable diagnostic marker. This study aimed to identify a Cr/Cys-C threshold at which Cys-C-based estimated glomerular filtration rate (eGFRcys) provides more accurate renal function assessment than Cr-based eGFR (eGFRcr) for informing drug dosing strategies in sarcopenia. METHODS: This retrospective study included 172 patients who had been receiving a consistent dose of oral magnesium oxide for at least one week and had undergone same-day measurements of serum Cr, Cys-C and magnesium (Mg). Based on previously reported Cr/Cys-C thresholds (< 0.887, < 0.80 and < 0.67), patients were stratified into sarcopenia and non-sarcopenia cohorts. RESULTS: In all subjects, eGFRcr and eGFRcys each demonstrated a significant negative correlation with serum Mg level. At Cr/Cys-C thresholds of < 0.80 and < 0.67, serum Mg level showed a significant inverse correlation with eGFRcys in both sarcopenia and non-sarcopenia cohorts, but did not correlate significantly with eGFRcr in the sarcopenia cohort. ROC analysis identified an optimal Cr/Cys-C cutoff of 0.82 at which serum Mg level correlates significantly with eGFRcys but not with eGFRcr, yielding a sensitivity of 100% and specificity of 69.23%. CONCLUSION: These findings suggest that at Cr/Cys-C below 0.82, the accuracy of eGFRcr in assessing renal function may be compromised, and eGFRcys should be used in such cases.