In the quest of a Standard Index of Intrinsic Capacity. A Critical Literature Review.

Objectives: Intrinsic capacity is a composite of five domains that summarizes the physical and mental capacities of an individual. Intrinsic capacity is increasing in relevance for adapting health systems to population ageing. Therefore, our objective was to analyse how intrinsic capacity has been assessed in older adults and if these measurements have been validated, as an initial step towards the construction of a standard intrinsic capacity index.

Design: Narrative review with electronic searches performed in PubMed and Cochrane databases, including the studies which used the term "intrinsic capacity" in the context of human ageing and health. The full text was then accessed to select studies with at least one operationalised domain of intrinsic capacity. We also looked for information on the validity and reliability of the reported measures of intrinsic capacity.

Results: We included ten articles reporting a quantitative measurement of intrinsic capacity. There were two intrinsic capacity scores which combined retrospective data on the intrinsic capacity domains sub-scores, with low concordance among tests chosen to measure each domain. Two studies reported on reliability and validity of the IC scores. The main gaps in the construction and validation process were a) analysis undertaken with each domain separately rather than for the construct of intrinsic capacity, b) lack of a clear conceptual and operational definition of the vitality domain, c) summary score that depends upon the distribution of the study sample.

Conclusion: Further validation of the intrinsic capacity concept is needed, together with more robust approaches to measure it. A standard index of IC has not been validated for translation into clinical or research purposes.