Comparative responsiveness and minimally important difference of Fatigue Symptom Inventory (FSI) scales and the FSI-3 in trials with cancer survivors

Background Fatigue is a highly prevalent and disabling symptom in cancer survivors. Although many measures have been developed to assess survivors’ fatigue, their ability to accurately capture change following intervention has rarely been assessed in post-treatment survivors. Ultra-brief fatigue measures are preferable in clinical practice but have limited evidence supporting their use with cancer survivors. We examined the psychometric properties of four Fatigue Symptom Inventory (FSI) measures, including the new FSI-3, in cancer survivors. Examined properties included responsiveness to change and minimally important differences (MIDs). Methods We analyzed data from three randomized controlled trials with post-treatment cancer survivors (N = 328). Responsiveness to change was evaluated by comparing standardized response means for survivors who reported their fatigue as being better, the same, or worse at 2–3 months. Responsiveness to intervention was assessed via effect sizes, and MIDs were estimated by using several methods. We also computed area under the curve (AUC) values to assess FSI measures’ discriminative accuracy compared to an established cut-point. Results All FSI measures differentiated survivors who reported improvement at 2–3 months from those with stable fatigue, but did not uniformly differentiate worsening fatigue from stable fatigue. Measures showed similar levels of responsiveness to intervention, and MIDs ranged from 0.29 to 2.20 across FSI measures. AUC analyses supported the measures’ ability to detect significant fatigue. Conclusions Four FSI scales show similar responsiveness to change, and estimated MIDs can inform assessment of meaningful change in fatigue. The FSI-3 shows promise as an ultra-brief fatigue measure for survivors