PHQ-9 as a Diagnostic Tool

Seeing as I will start my patient interviews this week, it seems fitting that I should first tie up any loose ends relating to my survey. As I mentioned in my previous blog post, "Nine Questions," I will be using the Patient Health Questionnaire-9 (PHQ-9). Unlike HAM-D which is usually administered after a patient is diagnosed with depression, PHQ-9 is often used to diagnose depression.

Earlier, I said that I would only use my survey to rank the severity of depression symptoms. But after finding that PHQ-9 is well-validated as a diagnostic tool for depression (as opposed to merely a depression scale), I figure that I may as well take full advantage of the PHQ-9 scores. So what exactly does that entail?

Figure 4. Severity scoring of the Patient Health Questionnaire-9 (PHQ-9).

Figure 4 shows the corresponding provisional diagnosis and recommended treatment for a given PHQ-9 score. DSM-IV defines criteria for two depressive disorders: dysthymia and major depressive disorder. Though there are many other depressive disorders that have been defined within the scientific community with their own set of criteria, for the purposes of my research, dysthymia and major depression are the only ones I am concerned with. Scores equal to or above 10 are indicative of some depressive disorder, or, in the least, depressive symptoms that should not go ignored. 

So...
I can use this survey to answer the question, "Is there a higher incidence of depression (a PHQ-9 score greater than or equal to 10) in diabetics than in the general population on average?" So what is the incidence of depression in the general population on average? The National Institute of Mental Health puts this number at

6.7%* for Major Depressive Disorder

and

1.5%* for Dysthymic Disorder.

*of the U.S. adult population

You might ask why I am not going to retrieve my own version of these numbers by surveying the non-diabetics here as well. Since I am working at a specialist's office (an endocrinology practice), almost all patients that are seen here were referred by a primary care physician who felt it was necessary due to the patient's specific case. In other words, just because I can survey patients here who aren't diabetic, this doesn't mean that the non-diabetic patients here are very representative of the general non-diabetic population. This is because they're being seen for other endocrinology-related complaints (e.g. thyroid disorders) that could — for all I know — affect their PHQ-9 scores. It wouldn't be a good idea to trust their scores because there could be many extraneous variables at work other than my main independent variable, the presence of diabetes mellitus.

So keep these two numbers in mind! They will remain in the "largest"-sized font on my blog for the rest of my project. At the end of my last sixty days of high school, I will hopefully be able to compare my results against these numbers from the NIHM.

Lalani.