Defining Chronic Fatigue Syndrome (ME/CFS) Resource Center



The many definitions proposed for CFS over time indicate  how difficult it has been for the research community to agree what this disorder is.

Success in the ME/CFS research field may be more a function of how good  the definition  is than anything else. A poor definition that inadvertently brings in other disease groups will result in inconsistent or mediocre study results and stagnation in the research field.  A good, tight definition, on the other hand, should allow researchers to laser in on the cause/causes of ME/CFS, lead to positive study results and advance the field.

Researchers have been haggling over how to define this disorder since the term myalgic encephalomyelitis was coined over 60 years ago…..

Myalgic Encephalomyelitis 

An early name for ME/CFS in the UK and Europe, myalgic encephalomyelitis or ‘ME’  formed the basis for several early attempts to define the disorder. Several aspects of the early definition including the focus on ‘post-exertional malaise’ informed later conceptions of CFS.  Starting in 2005 efforts to merge the older definition of ME and the conception of ‘CFS’ formed in the US., lead to the emergence of the term ‘ME/CFS’  which is in wide use today and has been adopted by the NIH.

The Fukuda (International) Definition (1994)

Produced by a CDC sponsored committee, the Fukuda definition was developed because of concerns that the first definition developed for ME/CFS, the Holmes definition, was, because of its requirement for eight symptoms, selecting  ‘CFS patients’ with more psychiatric disorders.  The Fukuda definition with its fewer symptom requirements (4 plus fatigue) has held sway over the ME/CFS research field for over two decades despite the fact that the authors readily acknowledged the definition’s limitations at the time of publication.

In its 2012 program announcement for ME/CFS the NIH, for the first time, allowed other research definitions in its grant applications. Only recently have other definitions begun to appear in research studies.

The Canadian Consensus Criteria for ME/CFS

Developed in Canada by physicians and researchers  to provide a better description for doctors, the  CCC emphasized post-exertional malaise (relapse after exertion) and cognitive problems. The definition won acclaim for its apt portrayal and description of CFS patients but has not (as of 2013) been used much in the research arena.  Several non-profit foundations (CFI, CAA, WPI), are, however, using the definition in some of their studies.

Recently questions have been raised whether, by requiring more symptoms than other definitions, the CCC is inadvertently selecting patients with higher incidences of psychiatric disorders.

The Pediatric Definition of ME/CFS

Developed  under the auspices of International Association of chronic fatigue syndrome/ ME (IACFS/ME), the Pediatric Definition describes how the disease appears in children and is very similar to the Canadian Consensus Definition

Abstract paintingEmpirical Definition (2005)

The Centers for Disease Control (CDC) created a controversial new definition of CFS. In this series of papers Cort takes the definition apart to see what’s in it, examine the controversy surrounding it and try to figure out what it holds for the future.

The International Consensus Criteria for ME

An updated, expanded and reworked version of the CCC, the ICC, published in 2011, provided the most recent conception of  ’CFS’ by a portion of the research community.


The Present

Dr. Leonard Jason’s group at DePaul University are in the midst of operationalizing ME/CFS symptoms with the idea of producing a clear and statistically validated definition.  Thus far the DePaul team has published two studies suggesting both the CCC and ICC select for more functionally disabled patients than the Fukuda definition but also more patients with psychiatric disorders. At this point Jason suggests a shorter definition focusing on post-exertional malaise and cognitive symptoms may be the most appropriate for ME/CFS. Studies, however are ongoing…


When Definitions Obscure: A Neuroinflammatory View of Chronic Fatigue Syndrome Pt. IV – Marco suggests that the search for a ‘tighter’ definition could inadvertently limit the search for broad, underlying pathologies.

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