The 1994 Case Definition

Introduction

The 1994 CFS case definition reflects the consensus viewpoint from many of the leading CFS researchers and clinicians (including input from patient group representatives). It was developed primarily for research studies of CFS in adults, but has also been used clinically. It was informed by earlier case definitions and has been the definition most widely used since 1994. Recently additional case definitions have been proposed, including one adapted for children and adolescents. These case definitions share many features and differ in the emphasis given to specific symptoms and severity. While information is being gathered on the usefulness of each of these case definitions, CDC uses the 1994 case definition as the basis for study and clinical education.

Guidelines for the Evaluation and Study of CFS

A thorough medical history, physical examination, mental status examination, and laboratory tests (diagram) are recommended to identify underlying or contributing conditions that require treatment. Diagnosis or classification cannot be made without such an evaluation. Clinically evaluated, unexplained chronic fatigue cases can be classified as CFS if the patient meets both of the following criteria:

1. The individual has severe chronic fatigue for 6 or more consecutive months that is not due to ongoing exertion or other medical conditions associated with fatigue (these other conditions need to be ruled out by a doctor after diagnostic tests have been conducted)
2. The fatigue significantly interferes with daily activities and work
3. The individual concurrently has four or more of the following symptoms:
   • post-exertion malaise lasting more than 24 hours
   • unrefreshing sleep
   • significant impairment of short-term memory or concentration
   • muscle pain
   • pain in the joints without swelling or redness
   • headaches of a new type, pattern, or severity
   • tender lymph nodes in the neck or armpit
   • a sore throat that is frequent or recurring

These symptoms persisted or recurred during 6 or more consecutive months of illness and they cannot have first appeared before the fatigue.

Conditions that Exclude a Diagnosis of CFS

1. Any active medical condition that may explain the presence of chronic fatigue, such as untreated hypothyroidism, sleep apnea, and narcolepsy, and iatrogenic conditions such as side effects of medication.
2. Some diagnosable illnesses may relapse or may not have completely resolved during treatment. If the persistence of such a condition could explain the presence of chronic fatigue, and if it cannot be clearly established that the original condition has completely resolved with treatment, then such patients should not be classified as having CFS. Examples of illnesses that can present such a picture include some types of cancers and chronic cases of hepatitis B or C virus infection.
3. Any past or current diagnosis of:
   • major depressive disorder with psychotic or melancholic features
   • bipolar affective disorders
   • schizophrenia of any subtype
   • delusional disorders of any subtype
   • dementias of any subtype
   • anorexia nervosa
   • or bulimia nervosa
4. Alcohol or other substance abuse, occurring within 2 years of the onset of chronic fatigue and any time afterwards.
5. Severe obesity is defined as having a body mass index equal to or greater than 45. [Body mass index = weight in kilograms ÷ (height in meters)^2. Body mass index values vary considerably among different age groups and populations. No "normal" or "average" range of values can be suggested in a fashion that is meaningful. The range of 45 or greater was selected because it clearly falls within the range of severe obesity.]

Any unexplained abnormality detected on examination or other testing that strongly suggests an exclusionary condition needs to be resolved before attempting further classification. Considerations of exclusionary conditions are important in research studies attempting to identify causes or evaluate therapies specific for CFS. Exclusionary conditions are important clinically because they are often treatable. However, once all exclusionary conditions have been fully treated, if the patient still meets criteria for CFS, they would be managed clinically as a CFS patient.

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Conditions that Do Not Exclude a Diagnosis of CFS

1. Any condition defined primarily by symptoms that cannot be confirmed by diagnostic laboratory tests, including fibromyalgia, anxiety disorders, somatoform disorders, nonpsychotic or melancholic depression, neurasthenia, and multiple chemical sensitivity disorder.
2. Any condition under specific treatment sufficient to alleviate all symptoms related to that condition and for which the adequacy of treatment has been documented. Such conditions include hypothyroidism for which the adequacy of replacement hormone has been verified by normal thyroid-stimulating hormone levels, or asthma in which the adequacy of treatment has been determined by pulmonary function and other testing.
3. Any condition, such as Lyme disease or syphilis that was treated with definitive therapy before development of chronic symptoms.
4. Any isolated and unexplained physical examination finding, or laboratory or imaging test abnormality that is not enough to strongly suggest the existence of an exclusionary condition. Such conditions include an elevated antinuclear antibody titer that is inadequate, without additional laboratory or clinical evidence, to strongly support a diagnosis of a discrete connective tissue disorder.

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