Discrepancy in frailty identification: Move beyond predictive validity

Background: To evaluate the discordance in frailty classification between the frailty index (FI) and the physical frailty phenotype (PFP) and identify factors discriminating those with discordant frailty classification from each other and from those for whom the assessments agree. Methods: A prospective observational study of older adults aged 65 and older selected from Medicare eligibility lists in four U.S. communities (n = 5,362). The PFP was measured by the Cardiovascular Health Study PFP. Participants meeting three or more of the five criteria were deemed frail. The FI was calculated as the proportion of deficits in an a priori selected set of 48 measures, and participants were classified as frail if FI is greater than 0.35. Results: The prevalence of frailty was 7.0% by the PFP and 8.3% by the FI. Of the 730 deemed frail by either instrument, only 12% were in agreement, whereas 39% were classified as frail by the PFP, but not the FI, and 48% were classified as frail by the FI, but not the PFP. Participants aged 65–72 years or with greater disease burden were most likely to be characterized as being FI-frail, but not PFP-frail. The associations of frailty with age and mortality were stronger when frailty was measured by the PFP rather than the FI. Conclusions: Despite comparable frailty prevalence between the PFP and the FI, there was substantial discordance in individual-level classification, with highest agreement existing only in the most vulnerable subset. These findings suggest that there are clinically important contexts in which the PFP and the FI cannot be used interchangeably.