Finding salvage in standardization? Structural and longitudinal evaluation of the patient health questionnaire anxiety and depression scale in a UK representative sample

OBJECTIVES: Some research funders mandate the use of the Patient Health Questionnaire-9 (PHQ-9) and Generalized Anxiety Disorder-7 (GAD-7) as independent measures of depression and anxiety. This study examined their independence in a UK representative sample by testing the structural validity, external validity, longitudinal measurement invariance and growth trajectories of the Patient Health Questionnaire Anxiety and Depression Scale (PHQ-ADS). METHODS: Data from Waves 1 (N = 2025), 2 (N = 1406) and 3 (N = 1166) of the COVID-19 Psychological Research Consortium Study were analysed. Confirmatory factor analysis and exploratory structural equation modelling were used to compare PHQ-ADS models. Structural equation modelling examined associations with Cybernetic Big Five Theory traits, intolerance of uncertainty and mental health treatment status. Longitudinal measurement invariance was tested across waves, and curve-of-factors models examined change over time. RESULTS: A bifactor model with a correlated residual produced interpretable factors. Auxiliary bifactor statistics supported a composite PHQ-ADS score, whereas PHQ-9 and GAD-7 summed scores showed little reliable unique variance after accounting for the general factor. Stability and Neuroticism were associated with the general factor (p < .001), Conscientiousness with the PHQ-9 group-factor (p < .001) and Agreeableness with the GAD-7 group-factor (p = .003). Intolerance of uncertainty was associated with the general and GAD-7 group-factors (p < .001). Mental health treatment status was primarily associated with the general factor. Residual invariance was supported after inclusion of a trait-factor, and growth trajectories differed across factors. CONCLUSIONS: Although the PHQ-9 and GAD-7 showed some tentative uniqueness within latent models, their summed scores would largely reflect general distress. These findings suggest caution when interpreting PHQ-9 and GAD-7 total scores as distinct indicators, warranting reconsideration of measurement standardization.