The 2015 definition by the National Academy of Medicine (then referred to as the "Institute of Medicine") is not a definition of exclusion (differential diagnosis is still required). "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* 3) Unrefreshing sleep*; At least one of the two following manifestations is also required: 1) Cognitive impairment* 2) Orthostatic intolerance" and notes that "*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 the time with moderate, substantial, or severe intensity. "Clinical practice guidelines are generally based on case descriptions, with the aim of improving diagnosis, management and treatment. An example is the ME/CFS guideline for the National Health Services in England and Wales, updated in 2021. Other guidance can be found at the New York Department of Health. Differential diagnosis
Certain medical conditions can cause chronic fatigue and must be ruled out before a diagnosis of CFS can be given.
Centers for Disease Control. Retrieved 20 May 2020.