Factor Structure and Measurement Invariance of the Patient Health Questionnaire-9 for Female and Male Primary Care Patients with Major Depression in Germany

Background: Depression is characterized by gender-specific distinctions, with women being affected more often than men. The Patient Health Questionnaire-9 (PHQ-9) is frequently used to assess depression in primary healthcare. Previous research has yielded heterogeneous findings on the factor structure, and little is known of its measurement invariance across gender. The aim of this study was 1) to evaluate the fit of four previously hypothesized models of PHQ-9 factor structure in patients with major depression in German family practices, and 2) to test the measurement invariance of the best-fitting model across gender.

Methods: We used the baseline data from a cluster-randomized controlled trial. The diagnosis of major depression was based on the PHQ-9 and confirmed by the family physician. We calculated Confirmatory Factor Analyses (CFA) to assess which of the previously hypothesized factor structures (a one- and three different two-factor solutions) would best fit our data. We also calculated Complex Survey Analyses (CSA) and Multi Sample Analyses (MSA).

Results: We included 626 participants (75.4% women and 24.6% men). A two-factor model with five 'somatic' labeled items and four 'non-somatic' labeled items presented the best fit indices. The model measurement was invariant across gender.

Limitations: The inclusion criteria used in the main trial mean the study sample was not representative of all patients with major depression in German family practices.

Conclusions: The measurement invariance across gender revealed by this study is a precondition for the use of the PHQ-9 without gender-specific adaptation in patients with major depression in German family practices.