Validation of the Flinders Fatigue Scale as a measure of daytime fatigue.

Study Objectives: To clinically validate the Flinders Fatigue Scale (FFS) as a brief measure of daytime fatigue, and to derive cut-off scores to classify fatigue severity.

Method: The FFS was administered to 439 adult volunteers from the general population, 292 adults with insomnia, 132 adults with Obstructive Sleep Apnoea (OSA) and 66 adults with Chronic Fatigue Syndrome/Myalgic Encephalomyelitis (CFS/ME), together with the Fatigue Severity Scale (FSS) and the Epworth Sleepiness Scale (ESS).

Results: A factor analysis revealed a single factor solution for the seven-item scale (67% of total variance), although a better fit was obtained for a modified six-item version (75% of total variance). Group FFS scores varied in accordance with theorised fatigue levels, with CFS/ME and insomnia samples reporting significantly higher fatigue than OSA and volunteer samples. Good convergent validity was established with the FSS for volunteer (r = 0.67) and CFS/ME samples (r = 0.61). Excellent discriminant validity with the ESS was observed for the insomnia (r = -0.08) and CFS/ME groups (r = 0.03), while a small-to-moderate correlation was found within the volunteer sample (r = 0.29). Cut-off scores were identified to categorise borderline (13-15), moderate (16-20) and severe (≥21) fatigue.

Conclusions: The FFS is a reliable and valid instrument to quantify subjective daytime fatigue. Sensitivity and specificity analyses indicate scores that best discriminate insomniacs and CFS/ME populations from a non-clinical population. However, it is proposed that the data can also be used to indicate the severity of fatigue by reference to these first two groups.