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 (DSM-IV). The scale is appropriate to use with adults and asks about symptoms in the previous two weeks.
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. This data can be used to produce percentiles.
A raw score (from 0 to 27) is presented where higher scores indicate more depressive symptoms. A PHQ-9 score of more than 10 has a sensitivity of 88% and a specificity of 88% for major depression. Severity categories are as follows:
As well as raw scores, three percentiles are presented to contextualise scores compared with normative samples:
– a sample of people without depression (non-depressed sample)
– a sample of those diagnosed with major depression
– a 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 is not depressed and will score very low on the two depressed sample subscales.
If the scale is administered more than once graphs are produced over time to track the progress of symptoms.
Kroenke, K., Spitzer, R. L., & Williams, J. B. W. (2001). The PHQ‐9. Journal of general internal medicine, 16(9), 606-613.