Can a Blood Test Detect Depression? It Depends

When I was researching my book, Coming of Age on Zoloft, a number of psychiatrists pointed out to me that there is no blood test for depression or any other mental illness. But many of them said that they felt confident, and hopeful, that there would be such a test some day.

I always thought that the idea of a physical test for depression was intriguing—and also a big challenge. After all, while depression is real, it's also a construct. As often as we hear that depression is a "disease," the fact is that it's not a disease like chicken pox or strep throat. It isn't caused by a germ and we can't see it under a microscope. To a large extent, depression is what we've decided it is. So what would a blood test for depression actually measure?

Given all that, my ears pricked up at a recent headline in the Atlantic magazine's online technology section:

"Study of the Day: Blood Tests Can Accurately Diagnose Depression."

The post described a study co-authored by George Papakostas and John Bilello, published in the journal Molecular Psychiatry.

The study looked at 79 adults, 36 with a diagnosis of major depression, and 43 without. The patients' blood was screened for nine biomarkers associated with depressive symptoms, including inflammation, the development of neurons, and the interaction of various brain structures. Based on these readings, each patient was given a figure called an "MDDScore," a number between 1 and 100, indicating the likelihood that the patient has major depression.

The study successfully predicted a diagnosis of depression in 33 of the 36 depressed patients, and generated eight false positives for the group of 43 healthy patients.

So do we now have a blood test to diagnose depression?

Well, sort of. What we have a test that's capable of telling us something we already know: which patients are depressed according to the DSM-IV criteria for depression. The authors of the current work can take a person's blood and say with reasonable accuracy whether or not they would be diagnosed as depressed by a psychiatrist giving the standard diagnostic interview from the Diagnostic and Statistical Manual of Mental Disorders. That's impressive. But what this work doesn't do, it seems to me, is shed new light on what depression actually is, or support the case that depression is truly a disease, different from any other state of consciousness. Because it uses the DSM-IV diagnostic criteria as its benchmark, it's unlikely to satisfy those who already aren't happy with those criteria (for example, critics who contend that the DSM is unable to distinguish between real depression and ordinary sadness. A blood test feels more 'scientific' because it's physical, but this test can't be more scientific than the DSM-IV criteria if it's simply trying to be an analogue to them.

The study was sponsored by Ridge Diagnostics, a San Diego and North Carolina-based company that developed and owns the technology behind the test. MDDScore is a registered trademark of Ridge, and the company's chief scientific officer was one of the study's main co-authors.

It would be great to see the researchers expand on this work in a way that actually deepens our grasp on the nature of depression and recovery. As it is, it's not clear that having a blood test that closely tracks to the DSM criteria for depression is much to crow about. After all, in a clinical setting, what is the point of such a test? Will we deny treatment to people who claim to be depressed but do not display the right biomarkers? That's hard to imagine. Will we force treatment on people who say they feel fine, but test 'positive' for depression? That's equally hard to imagine, not to mention repulsive. Or—more likely—will this just be another test that insurance ends up paying for, and a company profiting from?