> This is approximately the same phenomenon that causes medical students to self-diagnose themselves with every disease they learn about
Could you say more about this? It seems pretty counter-intuitive that medical students could look at some diagnostic criteria—which I assume often contain some fairly straightforward things like, "green spots on armpits" or whatever—and come out convinced they have the disease.
When you say diagnostic criteria function as a "tool," what is the purpose of the tool? My only guess so far is that it's about leaving things up to the doctor's case-by-case judgement by leaving the criteria overly broad. Maybe this ties into legal concerns as well as the fuzzy nature of real instances of diseases (i.e. it's rare to be able to exactly specify the symptoms in a way that works for all cases)?
I think the idea is that an implied symptom in diagnostic criteria is that the person was distressed enough to seek treatment. That bit of info is important for determining likelihood of having the disease.
Say you have a test for someone having a heart attack that is 99.9% accurate. If someone comes in complaining of chest pains, and the test says they are having a heart attack, then it is pretty certain they are having a heart attack.
Now, if you gave that same test to everyone in the United States, and you see that you have been diagnosed with a heart attack, most likely you AREN'T having a heart attack.
If you give this test to 300 million people, and 1 in 1000 tests give the wrong result, you are going to get a lot of false results.... 300,000 or so, in fact.
Given that the vast majority of people are not currently having a heart attack, the vast majority of those false results will be false positives.
This is why knowing the prior base rates is so important for diagnostic tests.
Hmm. So that looks like two separate issues to me, but maybe I'm missing something. I see:
1. Additional always-present criterion of distress
2. Knowledge of prior base rates as a means of estimating a proportion of false positives.
The first one makes sense to me in connection with the earlier comment. We aren't great at definitively reading our own states: it's easy for us to imagine symptoms, especially when prompted. But if something is actually wrong, an element of distress is typically present.
The second one seems like a separate issues though, or at least I don't see the connection. It seems like there are two important bits of information for correcting false positives. The first is that you have a known accuracy which suggests a 1 in 1000 chance of any diagnosis being incorrect; the second is that through historical record you can estimate how many people in e.g. the US are having a hard attack at any given moment on average.
I don't see the connection between either of those pieces of information and loose diagnostic criteria though.
The base rate part is just explaining why the diagnostic doesn't work when applied on a healthy population (like med students)
The instruction would be something like "if the patient complains of x, check for symptom y... if the have symptom y, they are likely suffering from z"
But the med student then checks themselves for symptom y, and thinks they have z, but that is only the case if they are suffering from x. The suffering from x part is what changes the base rate, and can't be ignored.
But in the patient's complaint about X, wouldn't X just be another symptom?
For instance, let's say a patient complains of a headache. The doctor uses whatever other knowledge they have of the patient to come up with some candidate diagnoses, and proceeds checking for symptoms from each of the candidates.
Presumably 'headache' is just another symptom on the diagnostic criteria for each of the other candidates. So why is it treated specially?
(Btw, thanks for your response—not sure why you were downvoted, but that was not from me!)
Right, my point is that the symptom X is often an implied one.... it isn't expressed in the diagnostic test explicitly, because the diagnostic test is run only when someone has symptom x.
Could you say more about this? It seems pretty counter-intuitive that medical students could look at some diagnostic criteria—which I assume often contain some fairly straightforward things like, "green spots on armpits" or whatever—and come out convinced they have the disease.
When you say diagnostic criteria function as a "tool," what is the purpose of the tool? My only guess so far is that it's about leaving things up to the doctor's case-by-case judgement by leaving the criteria overly broad. Maybe this ties into legal concerns as well as the fuzzy nature of real instances of diseases (i.e. it's rare to be able to exactly specify the symptoms in a way that works for all cases)?