Description
The PHQ-9 is the nine item depression subscale of the Patient Health Questionnaire, and is a widely used tool for assisting primary care clinicians in diagnosing depression as well as monitoring treatment. The PHQ-9 is based directly on the diagnostic criteria for major depressive disorder in the Diagnostic and Statistical Manual Fourth Edition (DSM-IV).

Validity
To establish reliability and validity the PHQ-9 was administered to 6,000 patients in 8 primary care clinics and 7 obstetrics-gynecology clinics, and construct validity and criterion validity were assessed against independent measures (Kroenke, Spitzer and Williams 2001). Criterion validity was assessed against an independent structured mental health professional interview in a sample of 580 patients. The mean PHQ-9 score was 17.1 (SD, 6.1) in the 41 patients diagnosed as having major depression; 10.4 (SD, 5.4) in the 65 patients diagnosed as other depressive disorder; and 3.3 (SD, 3.8) in the 474 patients with no depressive disorder.A PHQ-9 score of more than 10 had a sensitivity of 88% and a specificity of 88% for major depression. PHQ-9 scores of 5, 10, 15, and 20 represented mild, moderate, moderately severe, and severe depression, respectively.

Interpretation
A raw score (from 0 to 27) is presented, as well as a percentile based on sample of non-depressed individuals, a sample of those diagnosed with major depression, and sample diagnosed as having other depressive disorders.An individual with a percentile of around 50 compared to the major depression (MD) sample has a typical level of depression when compared to others diagnosed with major depression. The non-depressed percentile and the other-depressed percentile will be substantially higher. Conversely, an individual who scores in the 50th percentile compared to the non-depressed sample will likely score very low on the two depressed sample subscales.

Developer
Drs. Robert L. Spitzer, Janet B.W. Williams, Kurt Kroenke and colleagues.