Work Ability and Functioning Among Individuals with Persistent Indoor Environment-Related Symptoms or Chronic Fatigue: Nine-Month Follow-Up of a Web-Based Randomized Controlled Trial

PURPOSE: Persistent physical symptoms (PPS) can significantly impair work ability and daily functioning, exacerbated by comorbid illness worry (IW). This randomized controlled trial (RCT) examined the effectiveness of guided internet-based treatment on work ability and daily functioning of working-aged individuals with PPS associated with indoor environment (IE) and/or chronic fatigue (CF) with disability. The intervention was based on acceptance and commitment therapy (ACT) combined with individual case formulation, compared to treatment as usual (TAU). IW's role in moderating effects was explored. METHODS: In this RCT (Clinicaltrials.gov NCT04532827), 103 individuals with PPS linked to IE, CF or both were assigned to a video-based case conceptualization and 10-week internet-based ACT with TAU (iACT + TAU; n = 50) or TAU alone (n = 53). Self-reported work ability and daily functioning were assessed from pre-intervention to 9-month follow-up, examining the potential moderating effect of baseline IW. Analyses used linear mixed models and Type III ANOVA. RESULTS: Of the 103 participants, 62% completed the 9-month follow-up. Baseline demographics showed no significant differences between treatment groups. Significant positive interaction (treatment group and time) effects for work ability were found in the iACT + TAU group at post-measurement (β = 0.41, p = 0.041), 3-month (β = 0.58, p = 0.004), and 9-month (β = 0.47, p = 0.022) follow-ups. Daily functioning improved significantly at 3-month (β = 0.57, p = 0.007), particularly in the work domain (β = 0.67, p = 0.003). IW and PPS group did not moderate outcomes. CONCLUSION: Internet-based ACT with individual case formulation was associated with improved work ability for individuals with PPS associated with IE and CF, providing preliminary support for further examination as a potential frontline treatment.