Somatic symptoms have negligible impact on Patient Health Questionnaire-9 depression scale scores in neurological patients.

Abstract

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. 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. 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). 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.