Perhaps, we can conclude that reduced plasma and blood volumes are features of POTS, but not of ME/CFS in general. This appears consistent with the conclusion reached by van Campen (2019) that blood volume is significantly reduced in ME/CFS patients with orthostatic intolerance.
There may be cfs patients who do not have orthostatic intolerance, but in my mind it’s an important feature of the disease and should be part of criteria.
So Fukuda criteria may be capturing patients that are either not cfs patients or just atypical. There’s also a dynamic aspect to this illness. People may get progressively worse and meet different criteria. So maybe the Fukuda patients—I’m not saying they aren’t “real cfs” patients but maybe early on , .or partially recovered?