Background and purpose: There is concern that the Patient Health Questionnaire-9 (PHQ-9) depression scale may be impacted by the presence of somatic symptoms (differential item functioning [DIF]) in patients with neurological conditions. We evaluated the PHQ-9 for the presence and impact of DIF in large clinical samples of neurological patients.
Methods: We conducted a cross-sectional study of patients seen at the Cleveland Clinic Cerebrovascular, Headache, Movement Disorder, and Neuromuscular clinics who completed the PHQ-9 and patient-reported disease severity measures as part of standard care between 29 July 2008 and 21 February 2013. We evaluated PHQ-9 items for DIF with respect to disease-specific severity for each condition. Salient DIF impact was characterized as a difference between DIF-adjusted and unadjusted PHQ-9 scores.
Results: Included in the study were 2112 patients with stroke, 8221 with migraine, 440 with amyotrophic lateral sclerosis (ALS), and 5022 with Parkinson disease (PD). Several PHQ-9 items demonstrated DIF with respect to disease-specific severity, although salient DIF was present in very few patients (stroke, n = 0; migraine, n = 1; ALS, n = 13; PD, n = 1).
Conclusions: PHQ-9 items function consistently across disease severity, with salient levels of DIF impact found only for a very small proportion of people. These results suggest that the PHQ-9 provides a consistent measure of depression severity among people with neurological conditions associated with somatic symptoms that overlap with depression.
Keywords: depression; neurological disorders; psychometrics; screening.
© 2021 European Academy of Neurology.