To develop and validate a frailty index, derived from aged care eligibility assessment data.
Retrospective cohort study; analysis of the historical national cohort of the Registry of Senior Australians (ROSA).
903 996 non-Indigenous Australians aged 65 years or more, living in the community and assessed for subsidised aged care eligibility during 2003-2013.
44-item frailty index; summary statistics for frailty index score distribution; predictive validity with respect to mortality and entry into permanent residential aged care during the five years after assessment.
The mean frailty index score during 2003-2013 was 0.20 (SD, 0.07; range, 0-0.41); the proportion of assessed older people with scores exceeding 0.20 increased from 32.1% in 2003-2005 to 75.0% in 2012-2013. The risks of death and entry into permanent residential aged care at one, three and five years increased with frailty index score level (at one year, high [over 0.35] v low scores [under 0.05]: hazard ratio for death, 5.99; 95% CI, 5.69-6.31; for entry into permanent residential aged care, 8.70; 95% CI, 8.32-9.11). The predictive validity (area under the receiver operating characteristic curve) of Cox proportional hazard models including age, sex, and frailty index score was 0.64 (95% CI, 0.63-0.64) for death and 0.63 (95% CI, 0.62-0.63) for entry into permanent residential aged care within one year of assessment.
We used Australian aged care eligibility assessment program data to construct and validate a frailty index. It can be employed in aged care research in Australia, but its application to aged care planning requires further investigation.

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