Comparing self-reported symptom scores across time requires longitudinal measurement invariance (LMI), a psychometric property that means the measure is functioning identically across all time points. Despite its prominence as a measure of depression symptom severity in both research and health care, LMI has yet to be firmly established for the Patient Health Questionnaire-9 depression module (PHQ-9), particularly over the course of antidepressant pharmacotherapy. Accordingly, the objective of this study was to assess for LMI of the PHQ-9 during pharmacotherapy for major depressive disorder. This was a secondary analysis of data collected during a randomized controlled trial. A total of 1,944 veterans began antidepressant monotherapy and completed the PHQ-9 six times over 24 weeks of treatment. LMI was assessed using a series of four confirmatory factor analysis models that included all six time points, with estimated parameters increasingly constrained across models to test for different aspects of invariance. Root-mean-square error of approximation of the chi-square difference test values below 0.06 indicated the presence of LMI. Exploratory LMI analyses were also performed for separate sex, age, and race subgroups. Root-mean-square error of approximation of the chi-square difference test showed minimal change in model fits during invariance testing (≤ 0.06 for all steps), supporting full LMI for the PHQ-9. LMI was also supported for all tested veteran subgroups. As such, PHQ-9 sum scores can be compared across extended pharmacotherapy treatment durations. (PsycInfo Database Record (c) 2024 APA, all rights reserved).
Trial registration: ClinicalTrials.gov NCT03170362.