The Frailty Index (FI) is a popular operationalization of frailty. FI cut-off points have been proposed to define, regardless of age, frailty categories with increasing risk. Here, an alternative method is described that takes age into account.

29,905 participants aged 50 years or more from the first wave of the Survey of Health, Ageing and Retirement in Europe. The mean follow-up for mortality was 2.4 years. Curve estimation procedures were carried out between age and a FI, and 50% Confidence Intervals (CI) for the regression mean were derived. As opposed to the usual method (FI≤0.08: non-frail; FI≥0.25: frail; rest: pre-frail), the alternative method defines as ‘fit for their age’ those with a FI below the lower 50% CI; ‘frail for their age’ those with a FI above the upper 50% CI; the rest as ‘average for their age’. Using both methods, the prevalence of the frailty categories and their associated mortality rates were compared for each age group.

The best fit between age and the FI was by cubic regression (R²=0.174, P<0.001). Among those in their 50s, 5% were frail by the usual method (mortality: 5%) and 14% by the alternative (mortality: 2%). Among those in their 90s, 64% were frail by the usual method (mortality: 43%) and 41% by the alternative (mortality: 48%).

The alternative method may be more sensitive in younger ages and more specific in older ages. This may have implications for population screening.