Validity of the Derby Scale of Intrinsic Capacity and its association with Pain: a usable measurement of intrinsic capacity based on the English Longitudinal Study of Ageing

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Abstract
Introduction: Intrinsic capacity (IC) is the composite of an individual’s physical and mental capacities which maintain functional ability. We aimed to develop and validate an IC scale that was ready for clinical use, and to explore the interaction between pain, IC, and functional outcomes. Methods: Confirmatory factor analysis was used to test the construct validity of the model drawing on items from the English Longitudinal Study of Ageing (ELSA). The final Derby Scale of Intrinsic Capacity (DSIC) included mobility (Short Physical Performance Battery), cognition (orientation and 10-word recall), vitality (body mass index), sensory (self-reported vision and hearing), and psychology (Satisfaction with Life). Logistic regression, repeated-measures ANOVA, and structural equation modeling were used to explore interactions between pain, IC, and outcomes. Results: 2486 (wave 6) out of 4052 (wave 4) people aged ≥ 60 years had complete data for analysis. A higher (better) baseline DSIC was associated with decreased risk of basic and instrumental activities of daily living [OR 0.73, 95% CI 0.67–0.80], informal help [OR 0.69, 95% CI 0.62–0.78], formal help [OR 0.66, 95% CI 0.53–0.82], and falls [OR 0.90, 95% CI 0.83–0.97]. People with pain at baseline and follow-up had a lower (poorer) DSIC than those with no pain. The model we created accounted for 36% of variability of ADL difficulties. Pain reduced DSIC by 12% and in turn ADL difficulties by a further 12%. Conclusion: The DSIC is a validated, clinically user-ready tool for measuring IC in older people. Pain was associated with poorer IC and mediated the relationship between IC and ADL difficulties.

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