" A significant complicating factor in understanding the dynamics of this illness is that there are probably different types of illnesses now contained within the CFS construct...
We believe that it is crucial for CFS research to move beyond fuzzy recapitulations of the neurasthenia concept and clearly delineate precise criteria for diagnosing pure CFS and
CFS that is comorbid with psychiatric disorders. It is also necessary to better differentiate
CFS from other disorders which share some CFS symptoms but are not true CFS cases."
1.
Jason LA, et al. Politics, Science, and the Emergence of a New Disease: The Case of Chronic Fatigue Syndrome. American Psychologist 1997; 52(9): 973-83. http://www.meresearch.org.uk/research/sponsored/comparison.html
When comparing scientific studies, it is important to bear in mind that different definitions of ME/CFS may have been used, and this complicates interpretation and comparison of data. It can also be seen from the Figure below that there have been several attempts in the past decade to define diagnostic criteria for the illness. Each definition has been problematic reflecting, in part, the special interest of the author, and taking little account of the extensive literature, prior to 1988 (see Figure), that made the case for myalgic encephalomyelitis as a distinct clinical entity based on reports of epidemic and endemic cases.

Oxford criteria for the diagnosis of chronic fatigue syn.
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 fatigue as the main symptom
 definite onset and not lifelong fatigue is severe, disabling and affects physical and mental function
 fatigue should persist for 6 months or more and be present 50% of the time
 other symptoms, especially myalgia, sleep and mood disturbance http://www.gpnotebook.co.uk/cache/-476446699.htm