Assessment of differential item functioning of the PHQ-9, HADS-D and PROMIS-depression scales in persons with and without multiple sclerosis

Objective: We tested for the presence of differential item functioning (DIF) in commonly used measures of depressive symptoms, in people with multiple sclerosis (MS) versus people with a psychiatric disorder without MS. Methods: Participants included individuals with MS, or with a lifetime history of a depressive or anxiety disorder (Dep/Anx) but no immune-mediated inflammatory disease. Participants completed the Patient Health Questionnaire (PHQ-9), Hospital Anxiety and Depression Scale (HADS), and the Patient Reported Outcome Measurement Information System (PROMIS)-Depression. We assessed unidimensionality of the measures using factor analysis. We evaluated DIF using logistic regression, with and without adjustment for age, gender and body mass index (BMI). Results: We included 555 participants (MS: 252, Dep/Anx: 303). Factor analysis showed that each depression symptom measure had acceptable evidence of unidimensionality. In unadjusted analyses comparing the MS versus Dep/Anx groups we identified multiple items with evidence of DIF, but few items showed DIF effects that were large enough to be clinically meaningful. We observed non-uniform DIF for one PHQ-9 item, and three HADS-D items. We also observed DIF with respect to gender (one HADS-D item), and BMI (one PHQ-9 item). For the MS versus Dep/Anx groups, we no longer observed DIF post-adjustment for age, gender and BMI. On unadjusted and adjusted analyses, we did not observe DIF for any PROMIS-D item. Conclusion: Our findings suggest that DIF exists for the PHQ-9 and HADS-D with respect to gender and BMI in clinical samples that include people with MS whereas DIF was not observed for the PROMIS-Depression scale.