Screening for depression in arthritis populations: an assessment of differential item functioning in three self-reported questionnaires.

PURPOSE: To determine if persons with arthritis differ systematically from persons without arthritis in how they respond to questions on three depression questionnaires, which include somatic items such as fatigue and sleep disturbance.
METHODS: We extracted data on the Centers for Epidemiological Studies Depression (CES-D) scale, the Patient Health Questionnaire-9 (PHQ-9), and the Kessler-6 (K-6) scale from three large population-based national surveys. We assessed items on these questionnaires for differential item functioning (DIF) between persons with and without self-reported physician-diagnosed arthritis using multiple indicator multiple cause models, which controlled for the underlying level of depression and important confounders. We also examined if DIF by arthritis status was similar between women and men.
RESULTS: Although five items of the CES-D, one item of the PHQ-9, and five items of the K-6 scale had evidence of DIF based on statistical comparisons, the magnitude of each difference was less than the threshold of a small effect. The statistical differences were a function of the very large sample sizes in the surveys. Effect sizes for DIF were similar between women and men except for two items on the Patient Health Questionnaire-9. For each questionnaire, DIF accounted for 8% or less of the arthritis-depression association, and excluding items with DIF did not reduce the difference in depression scores between those with and without arthritis.
CONCLUSIONS: Persons with arthritis respond to items on the CES-D, PHQ-9, and K-6 depression scales similarly to persons without arthritis, despite the inclusion of somatic items in these scales.