Beyond the total score: Factorial validity of the PHQ-9 in high-symptom contexts

Background: Depression is heterogeneous, with symptoms that organise differently across populations and clinical contexts. How the Patient Health Questionnaire-9 (PHQ-9) performs in clinically elevated populations, where measurement accuracy matters most, remains insufficiently understood. Methods: We examined the factor structure and measurement invariance of the PHQ-9 in two independent samples restricted to PHQ-9 (Formula presented) : the PROACTIVE trial of older Brazilian adults ((Formula presented) ) and the U.S. National Health and Nutrition Examination Survey (NHANES, 2007 to 2023; (Formula presented) ). Confirmatory factor analysis compared unidimensional, two-factor, and bifactor models, with invariance assessed across sociodemographic groups using multi-group analysis. Results: In both samples, a two-factor model fit better than a single-factor model, but the structure differed by context. In the older Brazilian cohort, fatigue grouped with anhedonia and depressed mood in a cognitive-affective domain; the domains were only moderately correlated ((Formula presented) ), and a bifactor analysis found no dominant general factor. In NHANES, the conventional somatic and cognitive-affective split emerged, but the domains were strongly correlated ((Formula presented) ), with a dominant general factor explaining most shared variance. Invariance was broadly supported, with minor item-level departures in PROACTIVE and a marginal race/ethnicity decrement in NHANES. Conclusions: Rather than being uniformly multidimensional, the PHQ-9’s structure is context-dependent: genuinely multidimensional in the high-burden older cohort, but dominated by a general factor in the broad community sample. The total score remains an appropriate severity summary in both settings, whereas attention to somatic versus cognitive-affective profiles is most useful in high-burden clinical populations.