Towards a global scale for functional ability: what gets measured and gets done-but are we measuring the right thing?

Functional ability-'the health-related attributes that enable people to be and do what they have reason to value'-is the core outcome indicator of the UN Decade of Healthy Ageing (2021-30). Despite its centrality, there is still no globally standardised tool to measure functional ability across five domains defined by WHO: meeting basic needs; learning, growing, and making decisions; mobility; building and maintaining relationships; and contributing to society. Current approaches remain fragmented, limiting efforts to monitor progress and support equitable policy action. This commentary outlines three strategic priorities for addressing this gap. First, a globally validated functional ability scale should be developed, grounded in the WHO's five-domain framework. This effort must begin with clarifying the concept of functional ability-moving beyond narrow biomedical definitions-to ensure conceptual consistency. Early efforts in China and Japan demonstrate the feasibility of operationalising domain-level constructs, though challenges remain in empirical alignment. Second, this scale must be embedded in WHO global ageing population surveysto ensure cross-country comparability and data coverage in high-low- and middle-income contexts. Third, the field must move towards personalised functional ability profiles, reflecting older adults' own values and goals. Innovations in AI and machine learning may support this transition. With only five years remaining in the Decade, finalising and implementing a standardised, person-centred measurement approach is urgent. Doing so would not only advance international monitoring but also help realise the vision of enabling older people everywhere to achieve healthy ageing.