Abstract
BACKGROUND: People with multiple sclerosis (MS) are at increased risk for depression and anxiety. The symptoms of MS are often similar to the somatic or physical symptoms of depression and anxiety (fatigue, trouble concentrating). This study examined whether MS symptoms and effects biased the assessment of somatic symptoms of anxiety and depression. METHODS: People with MS (n = 513) completed a survey about MS symptoms, treatments, and distress. The Patient Health Questionnaire-9 assessed depression, and the patient-report version of the Primary Care Evaluation of Mental Disorders assessed anxiety. Participants were grouped into low versus high MS symptoms based on self-reported symptoms and as high versus low disability by the Expanded Disability Status Scale (EDSS). Groups were compared using differential item functioning analysis. RESULTS: No bias was found on somatic symptoms of depression comparing high versus low MS symptom groups (P > .15) or comparing groups based on EDSS scores (P > .29). Two anxiety symptoms (fatigue and muscle tension) showed bias comparing high versus low MS symptom groups (P < .01) and comparing high versus low groups based on EDSS scores (P ≤ .01). Intraclass correlations suggested a small effect due to bias in the somatic symptoms of anxiety. CONCLUSIONS: Somatic symptoms of depression are unlikely to be biased by MS symptoms. However, the use of certain somatic symptoms to assess anxiety may be biased for those with high MS symptoms.