Severe disabling fatigue lasting >6/12 - not medically explained
also musculo-skeletal pain, sleep disturbance, impaired concentration, headaches

prevalence 0.2-2.6%

Criteria - as above but ...
UK (Oxford) - stresses mental fatigue
US (CDC) - stresses physical fatigue (implying underlying infective cause) and unrelieved by rest
Exclusions - other active / suspected disease, psychosis, complicated depression (not just major) anorexia, dementia, substance abuse, 

DD
Depression - low mood most of the time>2/52 - check mental state exam
Physical - check for anaemia, thyroid, diabetes
Coeliac - tired, diarrhoea, wt loss, 10% rash - dermatitis herpetiformis
Addisons - tired, wt loss, hypoT, Na, glc, hyperK, neutropenia

Tests
Urinalysis, U/E, Cr
FBC ferritin
LFT
T4 TSH
Ca, Creatinine Kinase
Glc
ESR / CRP
Coeliac autoantibodies, Gluten screening - if bowel Sx

Good prognosis - esp in primary care
Unlikely to develop other problems by 12/12

Mx - bio-psycho-social approach
antidepressants - don't usually help (unless also depressed - but start carefully as may be more sensitive) - nor do vitamins or evening primrose oil
prolonged rest - adverse long term outcome
graded exercsize - 30 mins per day walk / swim - improves general functioning 
CBT - often helps