The development of intrinsic capacity measures for longitudinal research: The Longitudinal Aging Study Amsterdam

Background: The World Health Organization has introduced the construct of intrinsic capacity (IC) as an important component of healthy ageing and overall well-being in older adults The present study aimed to develop domain-specific and composite IC scores and to validate these scores by examining their longitudinal relation with functioning. Methods: We used prospective data on participants aged 57 to over 90 years, with a 10-year follow-up, from the Longitudinal Aging Study Amsterdam, an ongoing cohort study of older Dutch men and women Using a formative, stepwise approach, we identified indicators across the different domains of IC, i.e. vitality, sensory, cognition, psychology, and locomotion, using a combination of unidimensional factor analyses and Partial Least Squares Structural Equation Modelling (PLS-SEM). Next, domain-specific and composite IC scores were generated, and the construct validity (score across age groups) and criterion validity (relationship with change in functional limitations) were assessed. Results: The multiple unidimensional factor analyses and PLS-SEM identified a total of 18 indicators, covering the five domains of IC. The mean composite IC score was 70.9 (SD = 0.9) in men and 69.7 (0.8) in women. The domain-specific and composite IC scores all showed good construct validity, with known-group validation results indicating age-related declines. A higher composite IC score was associated with less functional limitations over time (B = 0.20, 95%CI [0.19, 0.22]). Conclusion: The developed domain-specific IC scores and the composite IC score effectively discriminated age-related declines in IC. Additionally, the composite IC score was longitudinally associated with functional limitations. By creating this comprehensive and reliable tool for tracking IC, we aim to provide valuable insights into the dynamics of ageing and support more effective strategies for promoting health and well-being throughout later life. These scores establish a foundation for future research to track longitudinal changes across various IC domains and relate these changes to key age-related outcomes.