In addition to the official researchers' definition discussed below, patien=
ts and
experienced clinicians have noticed symptom patterns that seem prominent in=
 CFS.
These are described in question 1.01 above, and also include the observatio=
ns that
cognitive dysfunction often increases over time (over several years), and t=
hat brain
scans often show that blood flow to the brain is decreased.

CFS is defined somewhat differently by various medical groups in different =
countries.
The 1994 research definition published by the U.S. Centers for Disease Cont=
rol and
Prevention recommends a step-wise approach for identifying CFS cases. The f=
irst step
is to clinically evaluate the presence of chronic fatigue, i.e. "self-repor=
ted persistent or
relapsing fatigue lasting 6 or more consecutive months".

Conditions that explain chronic fatigue should exclude a diagnosis of CFS. =
These are:=20

- "any active medical condition that may explain the presence of chronic
fatigue ..." - any previous condition which might explain fatigue and which
has not documentably come to an end; - "any past or current diagnosis of
a 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"; - substance abuse within 2 years prior to onset; - severe obesity=
=2E

The following should not exclude a diagnosis of chronic fatigue:=20

- conditions which cannot be confirmed by lab tests, "including
fibromyalgia, anxiety disorders, somatoform disorders, nonpsychotic or
nonmelancholic depression, neurasthenia, and multiple chemical sensitivity
disorder"; - any condition which might produce chronic fatigue but which
is being sufficiently treated; - any condition which might produce chronic
fatigue but whose treatment has already been completed; - any finding
which on its own is not sufficient to strongly suggest one of the
exclusionary conditions.=20

After the above criteria are met, the following core criteria for CFS are a=
pplied: "A
case of the chronic fatigue syndrome is defined by the presence of the foll=
owing:

1) clinically evaluated, unexplained persistent or relapsing chronic fatigu=
e that is
of new or definite onset (has not been lifelong); is not the result of ongo=
ing
exertion; is not substantially alleviated by rest; and results in substanti=
al reduction
in previous levels of occupational, educational, social or personal activit=
ies; and

2) the concurrent occurrence of four or more of the following symptoms, all=
 of
which must have persisted or recurred during 6 or more consecutive months o=
f
illness and must not have predated the fatigue:

- self-reported impairment in short term memory or concentration
severe enough to cause substantial reduction in previous levels of
occupational, educational, social or personal activities;

- sore throat;

- tender cervical or axillary lymph nodes;

- muscle pain;

- multi-joint pain without joint swelling or redness;

- headaches of a new type, pattern or severity;

- unrefreshing sleep;

- and post exertional malaise lasting more than 24 hours."

The journal citation for the CDC definition article is: Keiji Fukuda, Steph=
en Straus, Ian
Hickie, Michael Sharpe, James Dobbins, Anthony Komaroff, and the Internatio=
nal
CFS Study Group. "The Chronic Fatigue Syndrome: A Comprehensive Approach to
Its Definition and Study". Ann Intern Med. 1994;121:953-959.