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Depression is one of the most prevalent and treatable mental disorders presenting in general medical as well as specialty settings. There are a number of case-finding instruments for detecting depression in primary care, ranging from 2 to 28 items.1'2 Typically these can be scored as continuous measures of depression severity and also have established cutpoints above which the probability of major depression is substantially increased. Scores on these various measures tend to be highly correlated3, with little evidence that one measure is superior to any other.1,2'4
PHQ AND PHQ-9
The primary care evaluation of mental disorders (PRIME-MD®) is a novel instrument developed a decade ago to assist primary care clinicians in making criteria-based diagnoses of five types of DSM-IV disorders commonly encountered in medical patients: mood, anxiety, somatoform, alcohol, and eating.5,6 The patient health questionnaire (PHQ) is a 3-page self-administered version of the PRIME-MD® that has been well validated in two large studies involving 3,000 patients in 8 primary care clinics and 3,000 patients in 7 obstetrics-gynecology clinics.7,8 Because it is entirely self-administered and has diagnostic validity comparable to the clinician-administered PRIME-MD®, the PHQ is now the most commonly used version in both clinical and research settings.
At 9 items, the PHQ depression scale (which we call the PHQ-9) is half the length of many other depression measures, has comparable sensitivity and specificity, and consists of the actual nine criteria on which the diagnosis of DSM-TV depressive disorders is based.9 The latter feature distinguishes the PHQ-9 from other two-step depression measures for which, when scores are high, additional questions must be asked to establish DSM-IV depressive diagnoses. The PHQ-9 is thus a dual-purpose instrument that, with the same nine items, can establish provisional depressive disorder diagnoses as well as grade depressive symptom severity.
An item was also added to the end of the diagnostic portion of the PHQ-9 asking patients who checked off any problems on the questionnaire: "How difficult have these problems made it for you to do your work, take care of things at home, or get along with other people?" This single item is an excellent global rating of functional impairment and has been shown to correlate strongly with a number of quality of life, functional status, and health care usage measures.