Background Intrinsic capacity (IC) is a relatively new concept in the field of healthy ageing. The World Health Organisation originally proposed IC in 2015 as a central part of their framework for monitoring healthy ageing in older populations. IC is defined as an individual’s total physical and mental capacities and is comprised of five domains: cognition, locomotion, sensory, vitality and psychological. There have been many operationalised definitions of IC using different methods and data sources but no definitive measurement model has been agreed on. In this study, we operationalised IC using item response theory in the English Longitudinal Study of Ageing (ELSA) and tested the predictive value of the scale against both objective and subjective health outcomes: subsequent functional ability, mortality, and hospital admission.

Methods IC was measured at baseline (2004, Wave 2) using 14 indicators: word recall, orientation in time, balance, chair rises, walking speed, upper mobility, lower mobility, eyesight, hearing, grip strength, BMI, waist circumference, depressive symptoms, life satisfaction. Performance on each indicator was dichotomised based on previously defined cut-offs where possible. A two-parameter item response theory model was used to generate a scale of intrinsic capacity at baseline. Subsequent functional ability was measured by difficulties with ≥1 activities of daily living (ADLs) and ≥1 instrumental activities of daily living (IADLs) at 4 and 8 years after baseline. Hospital admission and mortality were measured over a 14-year follow-up. Logistic regression was used for the prediction of subsequent difficulties with ADLs and IADLs and competing risk and cox regressions were used for hospital admission and mortality, respectively. All analyses were adjusted for sociodemographic and health-related factors.

Results Intrinsic capacity scores were generated for 4,537 individuals with complete information on covariates at baseline, with mean age 70.6 years (SD 7.91). Higher baseline IC scores were associated with lower risk of difficulties with ADLs and IADLs, reduced risk of hospital admission (SHR=0.99, 95%CI 0.98–0.99) and mortality (HR=0.98, 95%CI 0.98–0.99), when adjusted for sociodemographic and health-related covariates.

Conclusion Results find the IC score predictive of subsequent functional ability, hospital admission and mortality, over a long follow-up period. This suggests the utility of this IC score as a measure of risk for future adverse outcomes in older people, potentially above that indicated by other sociodemographic and health-related factors.