Psychometric properties of the fatigue severity scale and the modified fatigue impact scale

Objective Fatigue is one of the most common, debilitating and life altering symptoms experienced by those with multiple sclerosis (MS) and has become the focus of therapeutic interventions and clinical rehabilitation. There is limited evidence regarding the psychometric properties and clinical relevance of fatigue outcomes for interpreting the effectiveness of intervention and rehabilitation strategies. This study determined the reliability, precision and clinically important change of the uni-dimensional Fatigue Severity Scale (FSS) and the multi-dimensional Modified Fatigue Impact Scale (MFIS). Methods The FSS and MFIS along with physical, psychological and cognitive clinical outcomes were administered to a sample of 82 persons with MS in a clinical research setting on two time points, separated by six months. Intraclass correlation coefficient (ICC) analyses established reliability; standard error of measurement (SEM) and coefficient of variation (CV) determined precision; minimal detectable change (MDC) defined clinically important change. Results Participants varied in type of MS and disability status, with 77% of participants classified as having substantial fatigue, based on the criteria of a mean FSS score ≥ 4. The MFIS (ICC = 0.863) and the FSS (ICC = 0.751) had acceptable reliability over six months. Precision was reasonable for both scales (based on SEM and CV estimates) but better for the FSS. MDC estimates were established and were lower for the FSS. Conclusion Reliability of the FSS and MFIS falls within acceptable ranges, and precision and clinically important change estimates provide guidelines for interpreting change in scores from these outcomes in clinical research of intervention and rehabilitation approaches for managing fatigue