Why We Don't Need a Blood Test to Diagnose Depression

09/21/2014 01:02 EDT | Updated 06/16/2017 01:02 EDT

One of my favourite stories from the history of psychology is a well-known tale involving Gordon Allport and Sigmund Freud. Allport was a pioneer in the study of human personality, and is a well-known figure in the field of psychology.

After graduating from university, Allport took a trip to Vienna where he arranged a visit with the famous Sigmund Freud. Allport recalled standing nervously across from the founder of psychoanalysis, while the latter just sat in silence. Finally, Allport broke the ice by telling Freud about a little boy he saw on the train earlier in the day. The little boy had been upset about having to sit on a dirty seat and appeared to have a domineering mother.

Freud's response to Allport's story was this -- "and was that little boy you?"

Reading this story as an undergraduate, I was left with two distinct impressions. First, Freud must have been the most annoying conversationalists of the 20th century. Second, there is something to be said for simplicity and parsimony -- both in science and in psychology.

Bearing in mind my appreciation for simplicity, I couldn't help but feel puzzled at the hooplah surrounding the recent report of a potential blood test for depression.

For those who are unfamiliar with this latest science story, researchers in the U.S. claim they can diagnose depression using a blood sample. Indeed, the researchers found that 9 biomarkers in the blood of those who were clinically depressed were different from those not clinically depressed.

After hearing the news story, I couldn't help but think "Why?" and "How?"

Let's start with "Why?" I was confused because I didn't understand the incremental benefits of a blood diagnosis over and above the current process of talking to a person. After all, we're dealing with a mental illness that, for the most part, is fairly straightforward in assessing.

To give readers an idea of how depression is typically diagnosed in everyday practice, registered health care professionals (ex: psychologists and psychiatrists) interview patients about their symptoms and decide whether the person's symptom profile meets criteria. The criteria are outlined in the Diagnostic and Statistical Manual (5th Edition), which is psychiatry's guide to diagnosing mental illness. To further assist in the decision making process, health care workers may also give questionnaires that help determine the severity of symptoms.

Which brings us back to my question -- why would we need a blood test to do something professionals can already accomplish on their own in a fairly short period of time?

The most touted benefit of this test seemed to be that it would offer the first "objective" measurement of depression.

The pursuit of "objective" measures is now common in mental health research, particularly in neuropsychology where researchers are often trying to "see" the mental health issues described by patients (ex: through MRI scans of the brain).

Whereas there can certainly be value to such research, I can't help but wonder whether some of these researchers are motivated by "physics envy."

Physics envy refers to how those in the "soft sciences" (ex: psychology and sociology) are supposed to be feel toward those in the "hard" sciences (ex: physics and biology). You see, hard science researchers can "see" the things they study and measure them objectively, whereas psychologists are left floundering in a sea of variables that are unseen and therefore immeasurable -- things like thoughts, feelings and disorders.

Of course, this is silly. Physicists can't see and directly measure everything they study. In fact, the most popular physics news stories, such as those about exoplanets and subatomic particles like the Higgs boson, involve things that physicists cannot see -- they are often inferred and measured indirectly by various means. Psychologists also infer and measure indirectly unseen variables, also using the scientific method, so I am not sure there is much rationale for envy.

Nevertheless, I can't help but think that some researchers feel compelled to move mental health research in the direction of the hard sciences' goal line. Unfortunately, this is not always necessary, and it's sometimes inappropriate.

In the present case, the problem with trying to find an "objective" method of diagnosing depression is that "depression" and "diagnosis of depression" are two separate things. Depression is a physical and mental experience that exists in nature. The diagnosis of depression refers to the man made act of determining if someone's symptoms are significant enough to be considered abnormal or dysfunctional.

A diagnosis of depression requires that a person have 5 of 9 symptoms over a two-week period. Why 5 of 9? Why not 6 of 12? Or 3 of 11? The point is that a depression diagnosis is determined by people, and at some point in the past a committee agreed that 5 of 9 symptoms over two weeks tended to best indicate the presence of a mental illness.

How do you define mental illness? It is not simply the presence of symptoms -- you can have all 9 symptoms and technically not receive a diagnosis of depression. The symptoms must cause significant distress and/or impair a person's ability to function.

This brings me to my second question -- how would a blood test diagnose? Even if someone had all 9 biomarkers of depression, you would still have to talk with the patient and ask questions about how the depression is affecting their life. The invasive process of drawing blood and finding biomarkers alone leaves this key piece of diagnostic criteria unanswered.

This story makes me sympathize even more with Gordon Allport, who must have thought something along the lines of "Freud, why use a complicated theory to understand a simple conversation piece?"

Because I can't help but wonder: "why use blood tests and other complex technology to understand how a person is feeling?"