One point though is that the diagnosis often references the exclusion of other causes of fatigue. Of course that is probably unnecessary if an adequate history of defining features, such as PEM, is obtained. In practice though I think nearly all practitioners would submit a new patient to a barrage of clinical tests, all of which come back normal. That's expensive and wasteful, with an implicit opportunity cost (including to the funding of research on the actual mechanisms). There will also be occasional false positives and iatrogenic harm from incidental findings.
My understanding of the whole discussion is that is precisely
not how things work. My understanding is that the diagnostic process is more complicated than that and always includes signs of exclusions and inclusions. Additionally we have once again seen in the intramural study why exclusion is important (my understanding is that both the patient that had cancer and MS were seen as having an adequate history of defining features). From what I gather signs of exclusions and inclusions are part of every medical diagnosis so there is no specific reason to why it should be any different for ME/CFS. And similar to how there is no “biomarker” for MS, people still get diagnosed with it and receive treatment for it and of course there are also wrong diagnoses there, with some post-mortem studies suggesting that at least 1/20 MS patients actually don’t have MS but some other rarer forms of demyelinating illnesses.
If there were some theoretical blood test with 100% accuracy as above, that was similar to something like thyroid function tests, then you could do that first and save all the irrelevant tests.
But if your ME/CFS test is 100% accurate that just means it is 100% accurate when tested against the CCC (or against a different criteria), rather than anything else. That still means that a patient with MS can in rare cases meet the CCC and not having been discovered as having MS or a comparable situation can occur. That means a 100% accurate test will have to diagnose such a person that has MS with ME/CFS. Is such a test still 100% accurate in the true meaning? The point as I understand it is that those questions don’t matter, because it is not the accuracy of diagnosis that is crucial rather than something that signals a pathology (and does so accurately).
This is obviously the case for other illnesses or diseases as well, including those mentioned by others that don’t need a “biomarker”, neither to be diagnosed nor to be treated. I view signs of exclusion and inclusions signs of any diagnosis, naturally also as part of ME/CFS.
I think thyroid function test is a good example of this. It accurately measures a value that can be tied to pathology but it is not a biomarker of a certain illness in itself. If a patient visits a GP he will never only get a thyroid function test, he will always have a whole barrage of tests done. Testing TSH is of value precisely because it can gives clues to specific pathologies. Once a TSH test returns an abnormal value one can then be sent to a specialist where additional tests are then run to help untangle said pathology and hopefully come up with a treatment. It is of use precisely because it helps untangle pathology. My understanding is that if TSH was just a test that would divide the population into higher and lower values without clues to pathology it would not be used in the first place at all. I don’t see why it would be any different for ME/CFS.
If your “test” just tells you this person has ME/CFS it would not be different to an TSH test that separates the population into high and low values without providing the crucial information that it actually carries. This value seems to largely come from it giving pathological clues. That is to say in a patient with sympoms that match ME/CFS you'll be running your complex machinery of diagnostics just like before, but at some point are now also looking at whether this newly discovered clue for pathology is occurring. This will certainly in many cases shorten the barrage of tests a patient has to undergo, but you don’t need a “diagnostic test” for this. Of course once pathology is discovered you don't test for ME/CFS anymore as it likely stops existing in its current form.
What happens in practice for patients in ME/CFS I think can be mixed, at least in terms of patients picked up in studies used for an accuracy debate. There are those, including those in BPS land, that use ME/CFS (in that case usually CFS) as a dumping ground without performing a single test and then there are the majority of patients that have gone through a whole barrage of tests. The large studies unfortunately often tend to include a lot of patients from the first category, with smaller pathological studies tending to be looking more at the second set of patients. In both cases you will have to define your accuracy against a certain predetermined set of people which creates the whole circular argument of accuracy.