Interpretation of the Patient Health Questionnaire 9 in High-Impact Chronic Pain: Do We Measure Depressive Symptoms the Way We Think?

Objectives: Depression is prevalent among patients with chronic pain and may impact pain management. An accurate assessment is, however, complicated by overlapping symptoms. This study investigated how patients with high-impact chronic pain interpreted and responded to the Patient Health Questionnaire 9 (PHQ-9) to identify problematic items and causes hereof.

Materials and methods: Cognitive interviews using the Three-Step Test-Interview procedure were conducted during the completion of the PHQ-9 in 33 patients with high-impact chronic pain referred to interdisciplinary treatment. Responses were analyzed using 4 coding categories: (1) "congruent" (response consistent with intention); (2) "incongruent" (response not consistent intention); (3) "ambiguous" (response both congruent and incongruent or insufficient to evaluate congruency); and (4) "confused" (response with confused or misunderstood statements). Next, the content of responses to problematic items was analyzed to identify causes for noncongruency, and encountered response difficulties were identified across all items.

Results: Three items (items 2, 6, and 9) performed as intended (>97% congruent responses), while 7 items (items 1, 3, 4, 5, 7, 8, and 10) were identified as problematic (<50% congruent responses). Problematic items had 1 or more issues: Responses were based on (1) pain-related issues or (2) other (non-pain) factors unrelated to depression, or item structure caused response difficulties due to wordings, reversion, or having 2 questions in 1.

Discussion: Problematic items limit the construct validity of the PHQ-9, leaving an increased risk of inflated depression scores in high-impact chronic pain. Identified problems should guide future revisions to enhance validity and screening accuracy for the benefit of both research and clinical practice.