Functional health index of intrinsic capacity: multi-domain operationalisation and validation in the Singapore Longitudinal Ageing Study (SLAS2)

Background ad hoc approaches are used to create composite indexes of intrinsic capacity (IC) based on five domains recommended by the World Health Organization for healthy ageing. We examined how combinations of domain-specific measures determine measurement performances of composite IC indexes. Methods in this population-based prospective cohort study, community-dwelling older persons (N = 2,906) aged 55 years and above were recruited. We used 12 domain-specific measures: cognition (Mini-Mental State Examination, MMSE), psychological (Geriatric Depression Scale, GDS), locomotion (Timed Up-and Go [TUG], GV, Knee Extension Strength, Performance Orientated Mobility Assessment), sensory (logarithm of the Minimum Angle of Resolution [LogMAR] vision and Whisper Test hearing) and vitality (forced expiratory volume in 1 second pulmonary function, Elderly Nutritional Indicators for Geriatric Malnutrition Assessment [ENIGMA], Nutritional Screening Initiative) to derive 144 composite 2- to 5-domain functional health indexes (FHI), and evaluated their abilities to predict 9-year mortality and their associations with health determinants. Results with 5-domain FHI, TUG, logMAR and MMSE showed the largest factor loadings (0.65-0.75). All single-domain FHI were significantly associated with mortality risks. Area under the receiver operating characteristic curve (AUC) values of mortality prediction increased with the number of domains (from mean 0.615 for single-domain FHI to mean 0.705 for 5-domain FHI), but the difference between 3-domain versus 4-domain FHI (P = 0.082) or versus 5-domain FHI (P = 0.109) was not statistically significant. Highest AUCs (P < 0.001) of best performing FHI were single-domain TUG: 0.735; 2-domain TUG + ENIGMA: 0.743; 3-domain TUG + LogMAR + ENIGMA: 0.762; 4-domain TUG + MMSE + LogMAR + ENIGMA: 0.757; 5-domain TUG + MMSE + GDS + LogMAR + ENIGMA: 0.751. FHI showed excellent construct validity based on correlations with known health determinants. Conclusions among Singaporean older adults, cognition, sensory and locomotion are predominant IC domains. A multi-domain IC index performs better with more domain measures, but a minimalist 3-domain index performs just as robustly as a 4- or 5-domain index.