Background: The nine-item depression module from the Patient Health Questionnaire (PHQ-9) is well validated and widely used as a brief diagnostic and severity measure, but its validity as an outcome measure for depression has not yet been established. Therefore, we investigated the sensitivity to change of the PHQ-9 in three groups of patients whose depression status either improved, remained unchanged, or deteriorated over time.
Methods: From three cohorts of medical outpatients, with an equal distribution of major depressive disorder, other depressive disorders, or no depressive disorder, 167 patients (82.7%) were followed up after a mean of 12.3 +/- 3.0 months. The PHQ-9 and the Structured Clinical Interview for DSM-IV (SCID) were completed at both baseline and follow-up. Depression diagnoses from the SCID were used as the criterion standard to divide patients into subgroups with (a) improved depression status, (b) unchanged depression status, and (c) deteriorated depression status.
Results: Effect sizes (ES) of PHQ-9 change scores were ES = -1.33 for the improved depression status subgroup (n = 52), ES = -0.21 for the unchanged status subgroup (n = 91), and ES = 0.47 for the deteriorated status subgroup (n = 24). PHQ-9 change scores differed significantly between the three depression outcome groups.
Limitations: The PHQ-9 and the SCID were completed in person at baseline, whereas they were completed in a telephone interview at follow-up.
Conclusions: This study demonstrates the ability of the PHQ-9 to detect depression outcome and changes over time. Data from treatment trials will help further establish the sensitivity to change of the PHQ-9 in comparison to other depression severity measures.