@Jonathan Edwards, I'm struggling to interpret the purpose of this discussion re diagnostic criteria. Are we discussing research criteria, as opposed to clinical criteria? And isn't this a decision best left to individual researchers, and doesn't it depend on the type of research being carried out? Or are we discussing a general set of research criteria for ME that may be used by researchers for guidance? Or are you attempting to understand the best way to select cohorts for ME/CFS research, from your own perspective?
I get the feeling that you don't consider PEM to be a precise enough criterion for selecting patients for research? And that's central to your discussion here? i.e. you believe that it wouldn't be helpful for you in research because the idea of PEM is a bit wooly, not precise, and not objectively measurable at the molecular or cellular level, and you would need something precise or reliably objectively measurable, such as a molecular biomarker before you start to narrow-down the selection of patients for a study? You're interested in differences between patients because they can give researchers unexpected answers or clues, and you wouldn't want to prematurely exclude subsets of patterns in case they could provide you with some answers or clues?
If that's the case then we may all be talking somewhat at cross-purposes. From our perspective, a lack of recognition of the realities of our illness has been our enemy over the years. We consider PEM to be a fundamental feature of the illness that separates it from e.g. depression and stress. (Many academics and clinicians attempt to conflate the symptoms of ME with everyday stress, and they attempt to describe ME as a maladaptive psychological reaction to stress. We consider PEM to be one feature of the illness that distinguishes it from stress and deconditioning etc.) We believe that PEM can be defined and that it is measurable, and it has the potential to be precisely and objectively measurable at the molecular/cellular level, but with more research needed to define it reproducibily. (Two day CPET tests have demonstrated some value in distinguishing ME patients from controls, via a PEM reaction, but perhaps you are interested in using or seeking a molecular marker for your research purposes.)
I think we'd all like to see much more research at the cellular/molecular level looking at the mechanism of PEM in ME/CFS, as we think it might provide answers to the illness, because of the unique nature of PEM in ME. Post exertional 'fatigue' isn't unique to ME, but the specific nature of post exertional symptom exacerbation seems to be unique, as far as I've been able to discern. i.e. a delayed reaction to minimal exertion characterised by a disproportionate exacerbation of
all symptoms (i.e. the whole range of symptoms that an individual experiences) that can be a relatively mild reaction or can result in a severe relapse lasting for months or years. I've not yet come across any other illness that has the same reaction to exertion.
I understand that you would need to be able to quantify this precisely, at the molecular level, for it to be the most use in research. There has been research that has attempted to define PEM using biological markers, which has shown apparently remarkable differences from controls, but obviously more research is needed. I haven't got the research to hand but others can point you towards it if you are interested in having a quick look. (It's the Light's research papers, and was there a paper by Maureen Hanson?)
I don't know if this helps the discussion at all? I think you might be thinking in very narrow terms, in relation to your specific ideas about your own research practices, that the rest of us can't quite relate to.
Edit: Actually, reading the most recent posts, I'm not sure if I've understood the purpose of the discussion. We now seem to be discussing clinical criteria as well.
Edit 2: As you and others on this thread have said, not everyone with a 'CFS' diagnosis has PEM... But everyone with an 'ME' diagnosis must have PEM. From my (self-interested) point of view, PEM defines my illness and I would like my illness to be investigated and researched. I have absolutely no objection to other types of illnesses (or subgroups) being researched, in addition to, or alongside my illness, but I'd like my illness to be carefully defined and characterized, and researched. I don't mind what illnesses are researched, but I'd like mine not to be ignored. I can't see how it's possible to research this illness if it isn't carefully defined, and if it can't be distinguished from other fatiguing illnesses, unless carrying out very large scale studies that can distinguish subsets in the outcomes. (Perhaps this is what you have in mind for these discussions?) In any case, any answers for any section of our community will surely help the others in our community. For example, if rituximab proves to help 20 or 30% of our community, then at least we know that the others don't have a B cell mediated autoimmune illness, so that helps narrow down the places to look for answers.