I don't know if it's useful for the general population but I really like Dr. Bell's description of CFS. It describes me very well.
"1) Characteristic symptom pattern (fatigue, headache, joint pain, muscle pain, abdominal pain, cognitive difficulties, sensitivities, sore throat, lymph node pain, etc.)
2) Activity limitation
3) Orthostatic intolerance
4) Chronicity
5) Symptom fluctuation and the presence of relapses and remissions
1. The pattern of symptoms is well known to those familiar with CFS, and I will not go into them here. For me, the most important diagnostic feature of this illness is the activity limitation. The patient with CFS is not able to do a normal amount of activity in a day because of "fatigue", whatever that is. It could be described as weakness, or sleepiness, or rubbery muscles or brain fog, or tiredness, but it limits the degree of overall activities. The other symptoms may be present but are not limiting the activity. For example, it is not the joint pain which prevents the CFS patient from activity, it is the fatigue.
2. And what is normal activity? In another study our office did with the help of friends in the CFIDS community, we asked healthy persons what they did in a normal day. All persons who considered themselves entirely well had at least 12 hours a day of upright activity. In other words they were "up and around" for half of the day, either work or some other activity. Patients with CFS have a limitation of this activity. Mild cases can only be "up and around" for 10 hours a day. This is a limitation because they used to be up and around from 7AM till 10PM, or 15 hours a day. Severe cases are "up and around" for 1 hour a day and that only with rest periods.
3. The next most important factor is orthostatic intolerance. This aspect of CFS was unknown to me until meeting with Dr. David Streeten, but now I see it as an essential for the diagnosis. Essentially orthostatic intolerance means that all symptoms are worse on being in the upright position. Walking, standing and even sitting are all upright activities. When lying down, most persons with CFS feel better, despite being bored out of their minds.
4. The chronicity of the symptoms is an equally important feature. Of the many patients I have seen, only a handful can say that they have a single day in the past year where they felt entirely well. Many persons say they have "good days", but they define a good day not by being well but by being improved over the average days. More severe patients may not even have single hours during the past year where they felt entirely well. The day to day persistence of the "sick" feeling is essential to the diagnosis.
5. And lastly I would emphasize the symptom severity fluctuation along with the variation of the worst symptom. The activity fluctuation may go from two hours a day in one month to four or five hours in another. But during this time different symptoms take the lead in being the most troublesome. Severe headaches for three weeks, followed by muscle pain for the next three. A month later the sore throat is there every day, and another month later the irritable bowel is acting up. Overall, all the symptoms seem to hang around, but they take turns leading the pack. It is the overall pattern of these symptoms that is important in making the diagnosis, not the most prominent symptom on a given day."