Evaluating context effects on PHQ-8 somatic item scores among people with a chronic medical condition: a scleroderma...

Assessing depression symptoms in people with a chronic illness is challenging due to possible bias from overlapping somatic symptoms associated with both depression and chronic illnesses. Previous studies, however, have found that people with a chronic illness do not report more somatic symptoms on depression measures than people without a chronic illness with similar levels of mood and cognitive symptoms. The reason for this surprising finding is unknown. Our primary objective was to evaluate differences in mean sum scores of Patient Health Questionnaire-8 (PHQ-8) somatic symptom items (sleep disturbances, fatigue, appetite changes) in people with a chronic illness when the items were administered outside the context of a depression questionnaire versus as part of the PHQ-8. Secondary objectives were to evaluate individual somatic item scores. We hypothesised that people who completed somatic items outside of a depression assessment would have significantly higher scores than those who completed items as part of a depression assessment.

We conducted a randomised controlled experiment within the Scleroderma Patient-centred Intervention Network (SPIN) Cohort, a multinational cohort of people with systemic sclerosis. SPIN Cohort participants were randomly allocated to complete the PHQ-8 with somatic items (sleep disturbances, fatigue, appetite changes) presented separately from psychological items and without any indication that they were part of a depression questionnaire (Reordered Items arm) or in standard format (Standard PHQ-8 arm). Participants were automatically randomised when they logged into the SPIN Cohort platform to complete routine research assessments. The primary outcome was the mean sum score of PHQ-8 somatic items. Secondary outcomes were the mean scores of individual somatic items. Differences were assessed using between-groups t-tests.

In total, 851 participants were included (N = 428 in Reordered Items arm, N = 423 in Standard PHQ-8 arm). Mean (SD) PHQ-8 score was 6.0 (5.3) for all participants. We found no statistically significant differences in PHQ-8 somatic item sum scores (0.05 points; 95% confidence interval [CI]: -0.29 to 0.38) or in mean scores for item 3 (sleep disturbances; 0.04 points; 95% CI: -0.09 to 0.19), item 4 (fatigue; 0.03 points; 95% CI: -0.11 to 0.16) and item 5 (appetite changes; -0.03 points; 95% CI: -0.15 to 0.10).

We did not find evidence that responses to PHQ-8 somatic items were influenced by whether participants were aware they were responding to items about depression. This finding supports the validity of self-reported questionnaires for depression symptom assessment in people with chronic medical conditions.