Today, courtesy of Talla Trialogue on Twitter, I have read an article by Shari Roan called Blood Test for Depression Proves It’s Not All In Your Head. You can see it here.
The article appears to be an interview with Lonna Williams, the CEO of Ridge Diagnostics. This company is reportedly introducing a blood test for depression. The test is called MDDScore and is expected to cost $745.
The article tells us that: “You get a numerical score that suggests how likely it is that you have depression. Studies show that MDD Score is about as accurate at making a diagnosis as the most rigorous evaluations.”
If we remember that the kappa score for major depression in the DSM-5 field trial was only .32, this doesn’t seem terribly inspiring.
The author also tells us that: “MDDScore is the first-ever blood test for depression that screens biological indicators of depression, like inflammation and neurochemicals (the company that makes it won’t reveal which ones it tests for, exactly).” For decades, the biological marker for depression has been something of a holy grail in this business, and we’ve seen a range of candidates come and go. It’s difficult to evaluate Ridge Diagnostic’s claim because they are apparently keeping the details secret. The neurochemical theory of depression, however, has been around for a long time. The basic idea is that people become depressed because of a neurotransmitter deficiency in the brain. The specific neurotransmitters usually cited in this regard are serotonin and norepinephrine, and it is widely claimed in psychiatric circles that a neural deficiency in one or both of these chemicals is the cause of depression.
So presumably the MDDScore test will measure the levels of these chemicals, (or possibly their metabolites) in the blood, and if the score is high – voila – a “diagnosis” of depression.
But it’s not that simple. Firstly blood serotonin/norepinephrine levels do not necessarily reflect brain serotonin/norepinephrine levels. Secondly, only about 25% of depressed people have low blood levels of these neurotransmitters or their metabolites. Thirdly, some depressed people have high blood levels. (Elliot Valenstein, Blaming the Brain, 1998, p 101)
So Ridge Diagnostics are probably correct: MDDScore will be about as accurate a measure of depression as a “rigorous” psychiatric evaluation!
In my view, the quest for a biological test for depression is intrinsically problematic, primarily because what we call depression is not a unified phenomenon. There are numerous ways that one could come to be seen as depressed. Here’s a short list.
1. A person might have a genuine brain malfunction that effectively prevents him/her from experiencing joy. I have no idea what the dynamics of such a neurological malfunction would be, but I think it’s reasonable to believe that such a malfunction could occur. I think that the prevalence of this hypothetical condition is probably low to miniscule.
2. People who have experienced great losses, e.g. bereavement, loss of health, loss of careers, etc. Sadness is the normal human response to loss.
3. People whose lifestyle promotes feelings of depression.
4. People who grew up in homes where joy was not expected or pursued, and where subdued, gloomy behavior was the norm.
5. People who grew up in rigid, puritanical households where fun was considered sinful and for whom joy of any kind continues to be strongly associated with guilt.
6. People who find that their chosen careers/lifestyles are no longer fulfilling.
7. People who have profound regrets about their pasts.
8. People who live in substandard housing without the economic means to improve their lot.
9. People who have just retired and can’t think of anything fulfilling to do with their time.
10. Empty nesters.
11. People who live their lives in conditions of bleak hopelessness.
12. People who have experienced a long string of failures and disappointments.
13. People who consume a lot of alcohol or other depressant drugs.
Etc., etc., etc…
The notion that all these people are suffering from the same “illness” and have a specific neurotransmitter deficiency is, to my mind, as silly as the assertion that all small bright objects in the night sky are essentially the same. And the notion that there is a blood test that would embrace all these people is hard to credit.