Revisiting the Sarcopenic Index in Older Adults with Reduced Kidney Function: Association with EWGSOP2-Defined Probable Sarcopenia

Background: Sarcopenia is highly prevalent in older adults and in individuals with impaired kidney function, where it is associated with adverse clinical outcomes. A creatinine–cystatin C–based sarcopenic index has been proposed as a surrogate marker of muscle status; however, its association with sarcopenia as defined by the EWGSOP2 framework, particularly in the context of renal dysfunction, remains uncertain. Methods: Older adults were classified according to EWGSOP2 criteria into probable, confirmed, and severe sarcopenia. Associations between the sarcopenic index and sarcopenia phenotypes were examined using group comparisons and multivariable logistic regression analyses in the overall cohort and in a subgroup of participants with an estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m2. Results: The sarcopenic index was not independently associated with probable, confirmed, or severe sarcopenia. In contrast, age emerged as the strongest independent correlate of probable sarcopenia (OR 1.12; 95% CI 1.05–1.19, p = 0.001), while body mass index was independently associated with confirmed sarcopenia (OR 0.91; 95% CI 0.86–0.96, p < 0.001). Similar patterns were observed in participants with reduced kidney function. Conclusions: Within the present analytical framework, the sarcopenic index did not show a meaningful association with EWGSOP2-defined probable sarcopenia, the most uniformly assessable EWGSOP2 stage, in older adults, including those with reduced kidney function. Exploratory analyses of more advanced sarcopenia stages did not reveal additional associative information. These findings should be interpreted within a descriptive and associative framework rather than a formal assessment of diagnostic or clinical decision-making performance.