Fatigue is a common symptom of chronic hepatitis C virus (cHCV) infection and a common side effect of interferon-based treatment for cHCV. This study provides confirmatory evidence of the reliability and validity of the Fatigue Severity Scale (FSS) to document fatigue in cHCV research and identifies values that indicate clinically important differences in FSS to aid in interpreting fatigue in cHCV clinical trials.
The study used data from two double-blind, randomized, placebo-controlled, Phase IIb trials evaluating the efficacy and safety of simeprevir plus peginterferon-alpha/ribavirin in treatment-naive (PILLAR, n = 386) and treatment-experienced patients (ASPIRE, n = 462) with cHCV infection. Patients completed the FSS and EuroQoL 5 dimension questionnaire (EQ-5D) at baseline and at regular intervals throughout both trials. Reliability was assessed using Cronbach's coefficient alpha at Week 24 (internal consistency reliability) and intraclass correlation (ICC) between FSS at Weeks 12 and 24 in stable patients (<0.5 g/dL hemoglobin [Hb] change between Weeks 12/24). Correlation with the EQ-5D visual analog scale (VAS) and Usual Activity domain score was used to assess concurrent validity. Clinical validity was evaluated using a case-control method to link spontaneously reported fatigue and anemia adverse events (AEs) during the study to FSS scores.
FSS total scores demonstrated good reliability (Cronbach's alpha: 0.95, 0.96; ICC: 0.74, 0.86 for PILLAR and ASPIRE, respectively) and concurrent validity (correlation with EQ-5D VAS: -0.63, -0.66) with a monotonic relationship between the EQ-5D "usual activities" item response and FSS. Clinical validity was confirmed by a significant difference between cases and controls for fatigue AEs (p < 0.05); however, anemia defined by AE or Hb abnormalities was only weakly related to FSS score. Analyses indicate that a change of 0.33-0.82 in mean FSS scores represents a meaningful improvement in fatigue, and a one-point change is a conservative indicator of an important change in individual FSS scores.
A difference of >=0.7 in mean FSS scores can be considered a clinically important difference within groups over time or between groups. A 1-point change or less in individual FSS scores indicates a clinically relevant change in fatigue.