While considerable attention has focused on improving the detection of depression, assessment of severity is also important in guiding treatment decisions. Therefore, we examined the validity of a brief, new measure of depression severity.
The Patient Health Questionnaire (PHQ) is a self-administered version of the PRIME-MD diagnostic instrument for common mental disorders. The PHQ-9 is the depression module, which scores each of the 9 DSM-IV criteria as “0” (not at all) to “3” (nearly every day). The PHQ-9 was completed by 6,000 patients in 8 primary care clinics and 7 obstetrics-gynecology clinics. Construct validity was assessed using the 20-item Short-Form General Health Survey, self-reported sick days and clinic visits, and symptom-related difficulty. Criterion validity was assessed against an independent structured mental health professional (MHP) interview in a sample of 580 patients.
As PHQ-9 depression severity increased, there was a substantial decrease in functional status on all 6 SF-20 subscales. Also, symptom-related difficulty, sick days, and health care utilization increased. Using the MHP reinterview as the criterion standard, a PHQ-9 score ≥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. Results were similar in the primary care and obstetrics-gynecology samples.
In addition to making criteria-based diagnoses of depressive disorders, the PHQ-9 is also a reliable and valid measure of depression severity. These characteristics plus its brevity make the PHQ-9 a useful clinical and research tool.
Depression is one of the most prevalent and treatable mental disorders and is regularly seen by a wide spectrum of health care providers, including mental health specialists, medical and surgical subspecialists, and primary care clinicians. There are a number of case-finding instruments for detecting depression in primary care, ranging from 2 to 28 items in length.12 Typically, these can be scored as continuous measures of depression severity and also have established cut points above which the probability of major depression is substantially increased. Scores on these various measures tend to be highly correlated,3 and it is not evident that any one measure is superior to the others.124
The Patient Health Questionnaire (PHQ) is a new instrument for making criteria-based diagnoses of depressive and other mental disorders commonly encountered in primary care. The diagnostic validity of the PHQ has recently been established in 2 studies involving 3,000 patients in 8 primary care clinics and 3,000 patients in 7 obstetrics-gynecology clinics.56 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 9 criteria upon which the diagnosis of DSM-IV depressive disorders is based. The latter feature distinguishes the PHQ-9 from other “2-step” depression measures for which, when scores are high, additional questions must be asked to establish DSM-IV depressive diagnoses. The PHQ-9 has the potential of being a dual-purpose instrument that, with the same 9 items, can establish depressive disorder diagnoses as well as grade depressive symptom severity. In this paper, we analyze data regarding the PHQ-9 to address 3 major questions:
What is the reliability and efficiency of the PHQ-9 in clinical practice?
What are the operating characteristics (sensitivity and specificity) of the PHQ-9 as a diagnostic instrument for depressive disorders?
What is the construct validity of the PHQ-9 as a depression severity measure in relation to functional status, disability days, and health care utilization?
Most pairwise comparisons of mean SF-20 scores between each PHQ-9 level within each scale are significant at P < 0.05 using Bonferroni's correction for multiple comparisons. Only those pairwise comparisons that share a common superscript letter (a, b, or a,b) are not significant.
Most pairwise comparisons between each PHQ-9 severity level for a given variable are significant at P < 0.05 using Bonferroni's correction for multiple comparisons. Only those pairwise comparisons that share a common superscript letter (a, b, or a,b) are not significant.
The development of the PHQ-9 was underwritten by an educational grant from Pfizer US Pharmaceuticals, New York, NY. PRIME-MD is a trademark of Pfizer Copyright held by Pfizer.
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