Influence of Somatic Symptoms on Patient Health Questionnaire-9 Depression Scores Among Patients With Systemic Sclerosis Compared to a Healthy General Population Sample

Objective. Depression symptom measures that include somatic symptoms may inflate severity estimates among medically ill patients, including patients with systemic sclerosis (SSc; scleroderma). The 9-item Patient Health Questionnaire (PHQ-9) is increasingly used to assess depressive symptoms in medical settings, but it is not known whether PHQ-9 scores are influenced by somatic symptoms common in medical illness. The objective was to assess whether SSc patients had higher somatic symptom scores on the PHQ-9 than non-medically ill respondents from the general population matched on cognitive/affective scores. Methods. SSc patients from the Canadian Scleroderma Research Group Registry were matched with respondents from a random population survey of Alberta, Canada residents who were without chronic disease on total PHQ-9 cognitive/affective scores (5 items), sex, and, as close as possible, age. PHQ-9 somatic scores (4 items) were compared between SSc patients and healthy Alberta survey respondents using t-tests for unadjusted analyses and analysis of covariance to adjust for age differences that remained after matching. Results. Somatic symptoms accounted for 64% of the total PHQ-9 scores for 762 matched SSc patients (n = 837 total) compared to 56% for 762 matched Alberta population survey respondents (n = 3,304 total), a mean difference of 1.0 point, or 19% of the total scores for the SSc patients (Hedges's g = 0.38). After adjusting for age, the mean difference increased to 1.4 points, reflecting 25% of the SSc patients' total scores (Hedges's g = 0.55). Conclusion. PHQ-9 scores among patients with SSc may include a small to moderate amount of variance from somatic symptoms that are not necessarily related to depression.