Association between serum creatinine to cystatin C ratio and incident asthma in community-dwelling middle-aged and older adults: a prospective cohort study from CHARLS

The serum creatinine-to-cystatin C ratio (Scr/CysC) has drawn interest as a possible marker of muscle mass and metabolic health, but its relationship with new-onset asthma in older adults remains unclear. We used the China Health and Retirement Longitudinal Study (CHARLS) to examine this question and to generate hypotheses for future research. We used the 2011–2012 CHARLS baseline and follow-up waves 2–5 (2013, 2015, 2018, and 2020). Adults aged at least 45 years and free of asthma at baseline were eligible. The full eligible cohort included 7,994 participants; time-to-event analyses included 7,864 after 130 participants without a usable censoring time were excluded. Fine-Gray models with death as a competing risk served as the primary analysis, with cause-specific Cox models as supportive time-to-event analyses. Logistic and Poisson models were restricted to the complete-outcome cohort ( n = 5,443). We also performed a cause-specific Cox restricted cubic spline, subgroup analysis, eGFR adjustment, and inverse-probability-of-censoring weighted analysis. Among the 7,864 participants with a usable censoring time, mean (SD) follow-up was 8.12 (1.90) years, and 459 participants developed asthma. In Model 2, each 1-SD increase in the Scr/CysC ratio corresponded to a Fine-Gray SHR of 0.926 (95% CI 0.834–1.029; P = 0.154); cause-specific Cox estimates pointed in the same direction but were not statistically significant. In the complete-outcome cohort, the Fine-Gray SHR was 0.867 (95% CI 0.776–0.968; P = 0.011). The 4-knot cause-specific Cox restricted cubic spline showed no strong evidence of nonlinearity ( P = 0.173). We did not find a statistically significant association in the full eligible cohort. The complete-outcome sensitivity analysis showed a directionally consistent signal, but this finding should be viewed as hypothesis-generating. Confirmation will require studies with repeated Scr/CysC measurements and objectively ascertained asthma outcomes.