Development and evaluation of a cross-study comparable frailty index based on item response theory: insights from three prospective cohorts

BACKGROUND: The Frailty Index of Cumulative Deficits (FI-CD) is a key predictor of adverse health outcomes in older adults, but its utility for cross-study comparisons is limited due to assumptions of item homogeneity. METHODS: We constructed a Frailty Index based on Item Response Theory (FI-IRT) in frailty measures, including 51 health-related variables, in older adults from the China Health and Retirement Longitudinal Study (CHARLS) 2011-2020, the Health and Retirement Study (HRS) 2010-2020, and the Survey of Health, Aging, and Retirement in Europe (SHARE) 2010-2020. A two-parameter logistic (2PL) model estimated FI-IRT scores, and Cox/logistic regression assessed its associations with mortality. Several analyses tested its robustness to the variable set and population selection. RESULTS: The FI-IRT followed a slightly right-skewed, approximately normal distribution, with frailty prevalence of 16.3% (CHARLS), 28.5% (HRS), and 15.5% (SHARE). The FI-IRT showed a high level of agreement with the FI-CD, both on a continuous scale and a hierarchical scale. A higher FI-IRT was associated with an increased risk of all-cause mortality across all 3 cohorts (hazard ratios: 1.81-2.38, p < .001). When domain-specific variables were excluded, the FI-IRT continued to offer relatively stable estimations (intraclass correlation coefficient: 0.934; 95% CI, 0.933-0.934; p < .001). A high degree of consistency was observed between the FI-IRTs calculated from models constructed based on different subpopulations (Spearman's rank correlation coefficients: 0.971-0.999, p < .001; intraclass correlation coefficient: 0.945; 95% CI, 0.850-0.972; p < .001). CONCLUSIONS: The FI-IRT provides a useful, robust, and cross-study comparable measure of frailty.