Substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities

A substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities as a result of the symptoms is a required for myalgic encephalomyelitis in the International Consensus Criteria.

In the earlier Canadian Consensus Criteria for ME/CFS, a similarly substantial level of impairment is needed, which must be the result of the fatigue caused by ME/CFS. It states that:
 * "The fatigue should be severe enough to substantially reduce the patient’s activity level, usually by approximately 50%. When considering the severity of the fatigue, it is important to compare the patient’s activity level to their premorbid activity level. For example, a former world class athlete could have a substantially reduced activity level and still exceed the norms for sedentary persons. Some patients may be able to do some work, but in order to do that they have had to eliminate or severely reduce other aspects of their life activities. Such interactive effects should be considered in the assessment of whether activity reduction is substantial."