Centers for Disease Control Diagnostic Criteria

Clinically evaluated, unexplained, persistent, or relapsing fatigue that is:

• Of new or definite onset
• Not a result of ongoing exertion
• Not alleviated by rest
• Results in a substantial reduction in previous levels of occupational, social, or personal activity

Four or more of the following symptoms that persist or recur during 6 or more consecutive months of illness and that do not predate the fatigue:

• Self-reported impairment of short-term memory or concentration
• Sore throat
• Tender lymph nodes
• Muscle pain
• Multijoint pain without swelling or redness
• Headaches of a new type, pattern, or severity
• Unrefreshing and/or interrupted sleep
• Postexertion malaise (a feeling of general discomfort or uneasiness) lasting more than 24 hours

Exclusion criteria:

• Active, unresolved or suspected disease that is likely to cause fatigue
• Psychotic, melancholic, or bipolar depression (but not uncomplicated major depression)
• Psychotic disorders
• Dementia
• Anorexia or bulimia nervosa
• Alcohol or other substance misuse
• Severe obesity (body mass index equal to or greater than 45)

Oxford Diagnostic Criteria

Severe, disabling fatigue of at least six months' duration that:

• affects both physical and mental functioning
• was present for more than 50% of the time

Other symptoms, particularly myalgia and sleep and mood distrubance, may be present

Exclusion criteria

• Active, unresolved, or suspected disease likely to cause fatigue
• Psychotic, melancholic or bipolar depression (but not uncomplicated major depression)
• Psychotic disorders
• Dementia
• Anorexia or bulimia nervosa