pem is considered a cardinal symptom by a number of the different diagnostic criteria for me/cfs, including the international consensus criteria (icc). the canadian consensus criteria (ccc) does not consider pem compulsory, rather it requires "post exertional malaise and/or [post exertional] fatigue". the national academy of medicine's (nam) systemic exertion intolerance disease (seid) criteria (also called iom 2015 diagnostic criteria) requires pem. the centers for disease control and prevention (cdc) use the seid/iom 2015 diagnostic criteria on its website's me/cfs page and list pem as a primary symptom.