Quinolinic acid – marker of suicidal thoughts or just depression severity?

A friend of mine sent me links to this news story last week, thinking that it sounded a little dodgy and it does (but it’s still interesting). One article claims that scientists are trying to develop a blood test to see if people are suicidal:


The second article (http://www.huffingtonpost.com/2012/12/18/quinolinic-acid-suicide-spinal-fluid_n_2325556.html) doesn’t talk about a blood test, but both articles point out that the focus of the new research is a chemical called quinolinic acid (which I don’t believe is related to quinoa, which is delicious). Levels of this chemical appear to be higher in people that are suicidal compared to those that are not.

The problem I have with the report that a “blood test” for suicide might be on the way, is that firstly, a mental disorder like depression cannot be diagnosed in the same way as something purely physical, like an infectious disease. Although we still know very little about depression, many people are beginning to suspect that not all types of depression are equal – people can become depressed for many different reasons and it is a disorder that can manifest itself in many different ways, with some people experiencing some symptoms while others do not – and sometime the opposite symptoms. For example, both weight gain and weight loss can be symptoms of depression. If there are different types of depression, then there are likely many different causes and mechanisms – and that means that it needs to be treated in different ways, and it also means that, unfortunately, a simple blood test is not always going to be able to tell if someone is depressed, or certainly if they are suicidal.

That brings me to my other point, which is that although suicidal thoughts and attempts are often a sign of depression, they can also be a sign of another mental disorder. And certainly not everyone who is depressed is going to be suicidal. As my friend pointed out, the chemical is likely to be an indication of depression severity rather than suicidality. Luckily, the second article does bring this up as an aside: “The researchers did not compare the suicidal patients with severely depressed but not suicidal individuals, so it’s not clear whether quinolinic acid is linked only to suicide or to difficult-to-treat depression more broadly.” But I think this is actually a major flaw in the study, and in no way can they claim yet that it’s specific to suicide.

It would be nice if we could diagnose mental disorders with a simple blood test, but biomarkers are usually not that specific.

It would be nice if we could diagnose mental disorders with a simple blood test, but biomarkers are usually not that specific.
Photo: © Motorolka | Dreamstime Stock Photos

I think what is more interesting and worth highlighting in this research is the proposed mechanism for quinolinic acid, which may be the bridge between inflammation and the brain. Although I don’t necessarily agree that “We still were missing the link between inflammation and what was actually happening to the brain cells,” as the author was quoted as saying (have a look at any of Brian Dean’s stuff), it is an interesting link between inflammation and a more direct effect on the brain.
But creating a blood test to screen for quinolinic acid? And then assuming someone is suicidal if they have high levels? I don’t think that’s a good idea. After all, why wouldn’t we have already developed blood tests for many of the other abnormalities in biomarkers associated with depression (serotonin, C-reactive protein, cortisol… the list goes on).

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