There is, as yet, no definite test or marker that conclusively identifies ME/CFS, so diagnosis depends on symptom-based case definitions devised by various expert groups. This is less satisfactory than making a diagnosis on the basis of underlying pathology, but is, pragmatically, the only approach possible in the present state of knowledge. The definitions differ in the number and types of case they identify, and have advantages and disadvantages. Some current ones are detailed below.

All the main definitions have in common the fact that:

• Symptoms are of a new or definite onset

• Symptoms have persisted for more than six month

• Main symptom is persistent Fatigue, which is not substantially alleviated by rest.

But then add varying further conditions.

The ‘Canadian Criteria’ Definition

This is a clinical definition, promulgated in 2003, designed primarily to assist clinical care, which can also be used as a base for research. Findings from the study by Leonard A. Jason PhD (Comparing the Fukuda et al. Criteria and the Canadian Case Definition for Chronic Fatigue Syndrome) indicated that the Canadian criteria captured many of the cardiopulmonary and neurological abnormalities, which were not currently assessed by the Fukuda criteria. The Canadian criteria also selected cases with 'less psychiatric co-morbidity, more physical functional impairment, and more fatigue/weakness, neuropsychiatric, and neurological symptoms' and individuals selected by these criteria were significantly different from psychiatric controls with CFS.

A short summary of the Canadian Criteria can be viewed or a fuller version accessed via http://www.theoneclickgroup.co.uk/documents/ME-CFS_docs/Canadian%20Definition%20of%20ME-CFS.pdf

The CDC (Fukuda) Definition (1994)

Developed by the USA Centre for Disease Control and Prevention this was an attempt to identify the many aspects of ME/CFS.

A short summary of the Fukuda criteria can be viewed below a fuller version accessed via http://www.mecfsforums.com/wiki/Fukuda_definition

The Oxford Definition

A set of criteria created with considerable psychiatric input - these criteria are widely-drawn, and may include patients with fatigue as their only symptom. As such it allows far too many possibilities of inclusion of non-ME patients.

A short summary of the Oxford criteria can be viewed below.

Conclusion

The findings suggest that both the Canadian and Fukuda et al. case definitions select individuals who are significantly different from those selected using psychiatric controls (such as Oxford). The Canadian criteria selects cases with less psychiatric co-morbidity, more physical functional impairment, and more fatigue/weakness, neuropsychiatric, and neurologic symptoms. Research findings suggest that the Canadian clinical criteria appear to select a group of individuals with more severe illness than the CDC (1994) criteria, and in fact those individuals conforming to the Canadian definition are a subset of those identified by the CDC (1994) definition. According to Dr. Leonard Jason, individuals conforming to the Canadian definition demonstrate less current and lifetime psychiatric impairment than those selected according to Fukuda.

We believe the Oxford Criteria are seriously flawed and the findings from studies, based on such samples, should be viewed with caution.

We believe the Canadian Criteria is the best available and does take into account the breadth of and combinations of ME/CFS which is certainly multi-faceted. The hallmark of ME/CFS is that increased physical or mental exertion results in worsening symptoms, often with a delayed impact (i.e. it is felt later the same day or next day), and lasting for more than 24 hours. Recovery from such relapses may take days, weeks or even months.

Indeed it is the persistence of fatigue (physical and mental) after quite minor exertion is, we believe, what is so characteristic and is underplayed by other definitions.

The Disease Register is using both the CDC (1994) and Canadian definitions, in order to take advantage of the sensitivity of the former and the specificity of the latter.

Short Summary

Canadian

The patient must exhibit all of the following

1. Fatigue (mental and physical)

2. Post-Exertional Malaise/Fatigue (an inappropriate loss of physical and mental stamina)

3. Sleep Dysfunction (unrefreshing or disturbed sleep)

4. Pain (significant degree of pain in muscles and/or joints)

5. Cognitive/Neurological manifestations (must exhibit two or more from the list)

6. One symptom from each of two categories (Autonomic manifestations; Neuroendocrinal manifestations; Immune manifestations)

These must be new onset and have persisted for more than six months.

Fukuda

The concurrent occurrence of four or more of the following symptoms:

1. Substantial impairment in short-term memory or concentration;

2. Sore throat;

3. Tender lymph nodes;

4. Muscle pain;

5. Multi-joint pain without swelling or redness;

6. Headaches of a new type, pattern, or severity;

7. Unrefreshing sleep;

8. Post-exertional malaise lasting more than 24 hours.

These symptoms must have persisted or recurred during 6 or more consecutive months of illness and must not have predated the fatigue.

Oxford

1. There is a definite onset (i.e. it is not lifelong)

2. Fatigue is the main symptom

3. The fatigue is severe, disabling and affects both physical and mental functioning

4. The fatigue has been present for at least six months, during which time it has been present more than 50 per cent of the time

5. Other symptoms may be present, particularly myalgia, mood and sleep disturbance