Diagnosis requires that the patient have the following three symptoms:
1) A substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities, that persists for more than 6 months and is accompanied by fatigue, which is often profound, is of new or definite onset (not lifelong), is not the result of ongoing excessive exertion, and is not substantially alleviated by rest, and
2) Post-exertional malaise,* and
3) Unrefreshing sleep*
At least one of the two following manifestations is also required:
1) Cognitive impairment* or
2) Orthostatic intolerance
*Frequency and severity of symptoms should be assessed. The diagnosis of ME/CFS should be questioned if patients do not have these symptoms at least half of the time with moderate, substantial, or severe intensity.
Excerpt from the IOM Report "Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome" 2015.