If you wish to establish that your exercise problems cannot be fully explained by a sedentary lifestyle (and depression / anxiety as suggested by a psychiatrist) then a single CPET should be sufficient.
This is a good point. Deconditioned healthy people will score below normal on a single CPET, but not by a lot. Whereas a moderate ME patient (mostly housebound) will score far below normal. The two-day CPET is likely to be most useful for mild ME patients who only score a bit below normal on Day 1. These are usually the patients who are able to work, shop, and socialize somewhat, but often still suffer for it afterward.
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@A.B. , my scores were far below normal. And even then were probably over-estimated a bit due to my "resting" heart rate not being a proper rested heart rate, as it was elevated due to traveling and sitting up too long. Normal VO2max for an active woman aged 35-44 (I'm on the low end of that) is 34.9, and a bit lower at 30.0 for a sedentary woman. But mine is 11.9.
A sedentary woman in my age group has a VO2max at 86% of what an active woman has, which isn't a large difference. But mine is only 34% of an active woman's score, and 40% of what a sedentary woman experiences. That's a pretty huge drop. On the American Medical Association's "scale of disability" that puts me in Class IV on the scales used to assess cardiac and pulmonary disability. On one of those scales, it's the worst possible score, and on the other, it's the 2nd worst out of 5.
My experience in using that data thus far has been overwhelmingly positive. An independent doctor at an assessment seemed to think I could be cured by CBT and GET, and accordingly advised against a waiver for language testing needed to stay in the country. But by the time of the meeting with the actual agency making the final decision a couple weeks later, I had the results of my single CPET, and that was literally all they needed to completely disregard the assessing doctor. Similarly, that data, plus the explanation of my low blood pressure if I stand around waiting for someone, resulted in an independent doctor's immediate approval for a passenger's handicapped parking permit - and those are MUCH harder to get for non-drivers.
Thus far no one who has looked at those results has had a single thing to say about deconditioning or CBT and GET. I'm hoping the trend continues, because to get my citizenship I will need a doctor from the group which initially recommended against a language waiver to now recommend a more serious waiver. I'm hoping the VO2max results will make a difference, plus we'll argue with them about Dutch psychobabblers if we have to. Last time I was trying not to offend by disagreeing outright with their base assumptions, but rather I was innocently explaining how those things had not worked for me when I tried them.