Reliability, Precision and Clinically Meaningful Change Of Fatigue Outcomes In MS

Background: Fatigue is one of the most common, debilitating, and life altering symptoms experienced by those with multiple sclerosis (MS). Accordingly, this symptom has become the focus of management in therapeutic interventions and clinical rehabilitation. The interpretation of intervention effects on fatigue scores in MS is hampered by the limited understanding of the reliability, precision and clinically important change of fatigue outcomes.

Objectives: This study determined the reliability, precision and clinically important change of scores from two common measures of fatigue, namely the uni-dimensional Fatigue Severity Scale (FSS) and the multi-dimensional Modified Fatigue Impact Scale (MFIS).

Methods: The FSS and MFIS were administered to 82 persons with MS in a clinical research setting on two-time points, separated by six months. Analyses were undertaken for the whole sample for both outcomes, the physical, cognitive and psychosocial dimensions of the MSIF were analysed separately. Intraclass correlation coefficient (ICC) analyses established reliability; standard error of measurement (SEM) and coefficient of variation (CV) determined precision; and minimal detectable change (MDC) defined clinically important change.

Results: Participants varied in type of MS (Relapsing Remitting; n=65, Secondary Progressive; n=10 and Primary Progressive; n=7) and disability status (Expanded Disability Status Scale range = 0-6.5, median = 3), and 77% of participants were classified as having substantial fatigue, based on the criteria of a mean FSS item score ≥ 4. All outcome measures were reliable with precision and MDC varying between the two outcome measures: FSS - ICC=0.751, SEM=0.7 points, CV=10.3%, MDC=1.9 points (38% of the overall FSS mean score); MFIS - ICC=0.863, SEM=7.3 points, CV=21.4%, MDC=20.2 points (49% of the overall MFIS mean score). The individual dimensions of the MFIS each were reliable, with varying precision and MDC values: physical MFIS - ICC=0.860, SEM=3.6 points, CV=24%, MDC= 9.9 points; cognitive MFIS - ICC=0.837, SEM=3.8 points, CV=27.2%, MDC = 10.6 points; psychosocial MFIS - ICC = 0.755, SEM = 0.8 point, CV = 33.5%, MDC = 2.3 points.

Conclusions: Reliability of the FSS and MFIS is good; falling within acceptable ranges. Precision and clinically important change estimates provide guidelines for interpreting changes in scores from these outcomes in clinical and research settings.