The serum creatinine/cystatin C ratio (CCR) and sarcopenia index (SI) are emerging diagnostic markers for sarcopenia, but their values among community-dwelling older adults remain uncertain. This study evaluates the utility of SI and CCR in diagnosing sarcopenia and predicting incident frailty and disability in activities of daily living (ADL) within a substantial cohort of older Chinese adults.

We conducted a prospective cohort study using data from the from the baseline survey (2011–2012) and the third wave (2014–2015) of the China Health and Retirement Longitudinal Study (CHARLS). After applying eligibility criteria, 2,574 and 2,357 participants aged ≥60 years were included for analyses of frailty and ADL disability, respectively. Serum creatinine and cystatin C levels were measured to calculate CCR and SI. Receiver operating characteristic (ROC) curves were used to determine cutoff values and evaluate the diagnostic accuracy of these markers for sarcopenia. Multivariate logistic regression models were employed to examine the associations of SI, CCR, and sarcopenia with incident frailty and ADL disability.

Both CCR and SI exhibited significant correlations with age, muscle mass indicators, and handgrip strength. The area under the curve (AUC) for CCR was 0.61 (95% CI: 0.57-0.64) for men and 0.59 (95% CI: 0.56-0.62) for women, while for SI, it was 0.60 (95% CI: 0.56-0.64) for men and 0.63 (95% CI: 0.58-0.67) for women. The difference in AUC between CCR and SI was not statistically significant (P > 0.05). Participants in the highest quartile of SI or CCR had reduced odds of incident frailty (SI: OR = 0.24, 95% CI 0.11-0.52; CCR: OR = 0.24, 95% CI 0.11-0.51) and ADL disability (SI: OR = 0.71, 95% CI 0.54-0.94; CCR: OR = 0.69, 95% CI 0.52-0.91) compared to those in the lowest quartile. Sarcopenia defined by either CCR or SI was independently associated with increased risks of incident frailty (CCR: OR = 1.84, 95% CI: 1.20-2.83; SI: OR = 1.70, 95% CI: 1.12-2.58) and ADL disability after adjusting for confounders.

Both CCR and SI demonstrate weak diagnostic accuracy for sarcopenia, but their performance in predicting frailty and ADL disability was moderate and comparable among community-dwelling older adults. These findings support further investigation of CCR and SI as biomarkers to help clinicians identify older individuals at risk of adverse clinical outcomes.