Assessing the construct validity of the Patient Health Questionnaire-9 in measuring severity of Major Depressive Disorder

The Patient Health Questionnaire (PHQ-9) is a 9-item self-administered instrument used for detecting depression. The objective of this study was to estimate the construct validity of PHQ-9 for detecting the severity of depression, as defined by the Diagnostic and Statistical Manual (DSM). Individual data was obtained from 1408 participants using an informed consent form followed by a 1 question survey (see appendix), which asked on what basis the participant filled out their PHQ questionnaire. Only 151 out 1408 participants (10.72%) reported that they filled out the PHQ-9 solely based on severity of symptoms; meanwhile, 855 out of 1408 participants (60.72 %) reported they filled out the PHQ-9 based on a combination of frequency and severity; 402 out of 1082 participants (28.55 %) reported that they filled out the PHQ-9 based on frequency alone. These findings indicate that, contrary to the findings of numerous studies, the construct validity of the PHQ-9 may be quite poor, owing in part to the disparity in the language used on the PHQ-9 and how clinicians interpret the scores. We propose that the language of the PHQ-9 be revised so that it asks for severity of symptoms rather than the frequency.