Why are we still using the PHQ-9? A Historical Review and Psychometric Evaluation of Measurement Invariance

The Patient Health Questionnaire-9 (PHQ-9) is a widely used tool for assessing depressive symptom severity and as a screening tool in the diagnosis of major depression. Designed as both a diagnostic instrument and a severity index, it is commonly used in primary care and research. However, findings regarding its reliability and validity for these dual purposes have been mixed. This study aimed to review the history of the PHQ-9, evaluate its factorial validity, and temporal measurement invariance using the dynamic fit index cutoffs framework for model fit evaluations. In a clinical sample of 3384 participants, strong correlations were found between items 1 and 2 (i.e., anhedonia and feeling depressed), indicating a substantial overlap in their coverage. Although a unidimensional factor structure was obtained, the model fit was suboptimal when contrasted with the sample dependent dynamic fit indices. Furthermore, measurement invariance between different treatment groups could not be established, strongly indicating that not all respondents use the PHQ-9 in the same manner as the only differentiating factor between groups was their randomized treatment group allocation. Moreover, temporal measurement invariance could not be convincingly established, in turn raising concerns about its comparability across time points. This suggests that observed changes in PHQ-9 scores over treatment weeks may, at least partly, reflect shifts in how participants engage with the scale rather than true changes in depressive symptomatology. In conclusion, our results raise questions about the validity of using the PHQ-9 to index depressive symptom severity and to monitor treatment outcomes over time.