Course and predictors of somatic symptom disorder in irritable bowel syndrome and ulcerative colitis: A longitudinal analysis from the SOMA.GUT-RCT

Longitudinal data on somatic symptom disorder (SSD) in ulcerative colitis (UC) and irritable bowel syndrome (IBS) are lacking. Understanding SSD trajectories and predictors in these conditions may clarify clinical relevance and guide psychological treatment decisions. This study examined the 12-month course and biopsychosocial predictors of interview-based SSD in patients with UC or IBS participating in a randomised controlled trial (RCT). Longitudinal data from a RCT targeting gastrointestinal symptom persistence was analysed. SSD was assessed using DSM-5-based structured interviews at baseline and 12 months. SSD Criteria A (presence of distressing somatic symptom(s)) and B (excessive symptom-related distress) were operationalised with the Patient Health Questionnaire-15 (PHQ-15) and the Somatic Symptom Disorder – B Criteria Scale-12 (SSD-12), respectively. Further variables included gastrointestinal symptom severity, inflammatory markers, depression and gender, illness perceptions, and neuroticism. Logistic and linear regression models identified baseline predictors of SSD diagnosis and Criteria A and B at 12 months. The sample included 213 patients (73.7% female; M age = 40.5, SD = 13.98) with UC ( n = 110) or IBS ( n = 103). SSD frequency was 42.3% (95%CI: 35.2–49.3) at baseline and 15.5% (95%CI, 11.3–20.7) at follow-up. Interview-based SSD diagnosis at 12 months was predicted by baseline symptom-related distress, whereas gastrointestinal symptom severity, and depression and gender showed no independent predictive value. Dimensionally assessed Criterion A was predicted by somatic symptom severity, and Criterion B by symptom-related distress, somatic symptom severity, and negative illness perceptions, while gastrointestinal symptom severity and inflammatory markers added no predictive value. Structured interview-based SSD was prevalent in patients with UC or IBS at baseline and declined over time. Psychosocial rather than disease-related variables predicted SSD, highlighting modifiable targets for early detection and tailored interventions. Results should be interpreted cautiously due to the interventional study context, which may have influenced both baseline SSD frequency and change over time. • Interview-based SSD in UC and IBS declined from 42.3% at baseline to 15.5% after 12 months. • Symptom-related distress predicted SSD diagnosis at follow-up. • Gastrointestinal symptom severity was not predictive of SSD diagnosis at follow-up. • Findings support tailored psychological care beyond symptom burden or inflammation. • Results should be interpreted in light of the study's interventional context.