Development and Validation of an Intrinsic Capacity Score in the UK Biobank Study

Background: In 2015, the World Health Organization introduced the concept of intrinsic capacity (IC) to define the individual-level characteristics that enable an older person to be and do the things they value. This study developed an IC score for UK Biobank (UKB) study participants and validated its use as a tool for health outcome prediction, understanding healthy aging trajectories, and genetic research. Methods: Our analysis included data from 45,208 UKB participants who had a complete record of the ten variables included in the analysis. Factor adequacy was tested using Kaiser Meyer Olkin (KMO), Barthelt, and the determinant of matrix tests, and the number of factors was determined by the parallel analysis method. Exploratory and confirmatory factor analyses were employed to determine the structure and dimensionality of indicators. Finally, the IC score was generated, and its construct and predictive validities as well as reliability were assessed. Results: The factor analysis identified a multidimensional construct comprising, one general factor (IC) and five specific factors (locomotor, vitality, cognitive, psychological, and sensory). The bifactor structure showed a better fit (comparative fit index = 0.995, Tucker Lewis index = 0.976, root mean square error of approximation = 0.025, root mean square residual = 0.009) than the conventional five-factor structure. The IC score, generated using the bifactor confirmatory factor analysis has good construct validity, as demonstrated by an inverse association with age (lower IC in older age; beta = -0.035 (95%CI: -0.036, -0.034)), frailty (lower IC score in prefrail, beta = -0.104 (95%CI: (-0.114, -0.094) and frail, beta = -0.227 (95%CI: -0.267, -0.186) than the robust), and Charlson comorbidity index (CCI) for incident cases (a lower IC score associated with increased CCI, beta, = -0.019 (95%CI: -0.022, -0.015). The IC score also predicted incident CCI (a one-unit increase in baseline IC score led to lower CCI, beta = 0.147 (95%CI: -0.173, -0.121)). Conclusion: The bifactor structure showed a better fit in all goodness of fit tests. The IC construct has strong structural, construct, and predictive validities and it is a promising tool for monitoring aging trajectories. Keywords: Intrinsic capacity, healthy aging, functional ability, physical functioning, mental functioning