BACKGROUND

Consistent and reproducible estimates of the underlying true level of frailty are essential for risk stratification and monitoring of health changes. The purpose of this study is to examine the reliability of the frailty index (FI).

METHODS

A total of 426 community-dwelling older adults from the FRequent health Assessment In Later life (FRAIL70+) study in Austria were interviewed biweekly up to 7 times. Two versions of the FI, one with 49 deficits (baseline), and another with 44 (follow-up) were created. Internal consistency was assessed using confirmatory factor analysis and coefficient omega. Test-retest reliability was assessed with Pearson correlation coefficients and the intraclass correlation coefficient. Measurement error was assessed with the standard error of measurement, limits of agreement, and smallest detectable change.

RESULTS

Participants (64.6% women) were on average 77.2 (±5.4) years old with mean FI49 at a baseline of 0.19 (±0.14). Internal consistency (coefficient omega) was 0.81. Correlations between biweekly FI44 assessments ranged between 0.86 and 0.94 and reliability (intraclass correlation coefficient) was 0.88. The standard error of measurement was 0.05, and the smallest detectable change and upper limits of agreement were 0.13; the latter is larger than previously reported minimal clinically meaningful changes.

CONCLUSIONS

Both internal consistency and reliability of the FI were good, that is, the FI differentiates well between community-dwelling older adults, which is an important requirement for risk stratification for both group-level oriented research and patient-level clinical purposes. Measurement error, however, was large, suggesting that individual health deteriorations or improvements, cannot be reliably detected for FI changes smaller than 0.13.