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Comment by aidenn0 | Hacker News Reader

Just some quick math:

8% False-negative rate 19% False-positive rate In a given year ~7% of all people will have major depressive disorder at some point so if we gave this test to random people 75% of the people who test positive are healthy.

So this clearly isn't a useful test as a first screening. If, however, someone is showing symptoms of MDD, it could be useful as a test there, as they are in a population with fewer healthy people.

In any event the article in the Atlantic (I don't have a subscription to nature) talks about using it for a completely different reason: Given someone already diagnosed with MDD, administer the test. With 92% chance it will come back positive, and you can say "See, there is something physically wrong with you, and you need treatment." That does seem like a good use for it.

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I know this isn't your intent, but...

>"See, there is something physically wrong with you, and you need treatment."

...could be a pretty stigmatising way to talk about mental health problems. I know you're not saying that people with minor depression are "making it up"; but I feel the need to say that an illness doesn't need to be 'physical' to be real.

Sorry for the slight de-rail.

In order to be treated via medication there certainly must be something physically wrong with the patient. I don't believe that the parent is referring to all categories of mental illness, only ones which can be treated by medication. If you're talking to a physician and there's nothing physically wrong with you, you're in the wrong room.

See, for example, the serotonin hypothesis, and the fact that good quality evidence shows that for mild to moderate depression CBT is more effective than SSRIs.

I was careful to put a bunch of qualifiers in my post, but i'm keen to make sure people realise that mental health problems can be very serious even if they're not "physical".

This is important because people with chronic fatigue are often sent to see CBT practitioners. Lots of those people feel that they're being trivialised or fobbed off; they say things like "I have a real illness".

It's a shame, because CBT is used to reduce pain in cancer patients, and there's reasonable evidence that it's useful in CFS/ME type illnesses.

Interesting, I interpreted it the opposite way. One of the big problems with depression, bipolar and other physically manifesting mental health issues is that seems to be a large portion of society which says "you just need to snap out of it" (or variations on that theme). With an actual blood test to say "look, this is a physical thing" it becomes easier to ignore those who say such crap -- they can be classified with those who say "oh you have cancer, just pray your way out of it, it's not a real problem". Basically, and objectively measurable way of proving a problem gives the sufferer a chance to not blame themselves (potentially making the problem worse, or at least recovery harder).

> If we gave this test to random people 75% of the people who test positive are healthy. So this clearly isn't a useful test as a first screening

Unless I'm misreading what you're saying, I think you have Bayes's rule confused in that statement. If 75% of positive results are false, that's actually right in line with what you might want for a first screening.

Of course, you want to know not just P(X|Y), but also P(Y|X) -- you really need the joint distribution to determine how useful the test is.

For a rapid screening, you generally want a low false negative, and you're willing to compromise by having a high false positive rate as a result. The real thing you care about is P(test positive | have MDD), not P(have MDD | test positive). You don't care about the latter (within reason - obviously you could just have everyone test positive, but you're also trying to maximize the former probability subject to some constraint - usually time/money).

Psychiatric conditions are notoriously difficult to test/evaluate/treat because they are oftentimes heterogeneous, so I don't know what kind of numbers we should be looking for in this case, but I just wanted to clarify that having a majority of people who test positive ending up being healthy actually doesn't necessarily rule a test out as being a useful rapid-response screening, in the general case.

One issue of concern is the possible cost of these false positives. If doctors are very careful to communicate to patients that 75% of positive results will only require a second test, then that's OK. Sadly, that doesn't seem to be what happens in many cases. The communication given to the patient is 'this is a test for disease X', and the patient rightfully interprets a positive result as 'I have X'.

The problem comes in because 'I have depression' could lead the patient to take risky or dangerous action. Sadly, misunderstanding on the side of doctors (who, at least the ones I know well, aren't given a strong education in statistics at university) might also lead them to make poor decisions. As long as everybody is honest about the meaning of the test, and understands the differences between P(Y), P(X), P(Y|X) and P(X|Y) then that's great. Sadly, there is good evidence that this is not always the case.

For an example more along HN's normal lines, consider hard drive monitoring data from SMART. Several public studies of SMART attributes in modern drives have shown that positive predictive value of SMART data as a drive failure indicator is very poor. Many of the drives which are replaced because they are failing SMART are actually just fine, but a lot of money, labor and downtime is spent on the incorrect assumption that P(healthy|SMART failure) is much smaller than P(unhealthy|SMART failure).

The bigger issue to me is that there isn't a second test. You can't just send the samples off to the lab and let them culture for a few days. What's the point of making someone take this blood test if they still have to fill out the mental health questionnaire anyways?

That's why the low false negative rate is important. If you score a negative you don't have depression.

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