I’m after some information about scales which measure severity of disease in ME/CFS, in terms of functional impairment (mild vs severe). There seem to be several which measure symptom severity and, whilst there will likely be a correlation between symptom and disease severity, they’re not the same. I’m aware of Bell’s CFS Disability Scale, but I’m not sure how much (if any?) work has been done on validating it? Any tips would be most appreciated!