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Chronic fatigue syndrome is defined as a severe disabling fatigue which lasts for several months. The fatigue is made worse by minimal physical or mental exertion, and there is no adequate medical explanation for the fatigue.

Suspect ME/CFS if:

  • the person has had all of the persistent symptoms * for a minimum of 6 weeks in adults and 4 weeks in children and young people and
  • the person's ability to engage in occupational, educational, social or personal activities is significantly reduced from pre-illness levels and
  • symptoms are not explained by another condition

*Persistent symptoms in suspected ME/CFS

All of these symptoms should be present:

  • debilitating fatigue that is worsened by activity, is not caused by excessive cognitive, physical, emotional or social exertion, and is not significantly relieved by rest.
  • post-exertional malaise after activity in which the worsening of symptoms:
    • is often delayed in onset by hours or days
    • is disproportionate to the activity
    • as a prolonged recovery time that may last hours, days, weeks or longer

  • unrefreshing sleep or sleep disturbance (or both), which may include:
    • feeling exhausted, feeling flu-like and stiff on waking
    • broken or shallow sleep, altered sleep pattern or hypersomnia

  • cognitive difficulties (sometimes described as 'brain fog'), which may include problems finding words or numbers, difficulty in speaking, slowed responsiveness, short-term memory problems, and difficulty concentrating or multitasking.


  • post-exertional malaise
    • worsening of symptoms that can follow minimal cognitive, physical, emotional or social activity, or activity that could previously be tolerated. Symptoms can typically worsen 12 to 48 hours after activity and last for days or even weeks, sometimes leading to a relapse. Post-exertional malaise may also be referred to as post-exertional symptom exacerbation
  • fatigue in ME/CFS typically has the following components:
    • feeling flu-like, especially in the early days of the illness
    • restlessness or feeling 'wired but tired'
    • low energy or a lack of physical energy to start or finish activities of daily living and the sensation of being 'physically drained'
    • cognitive fatigue that worsens existing difficulties
    • rapid loss of muscle strength or stamina after starting an activity, causing for example, sudden weakness, clumsiness, lack of coordination, and being unable to repeat physical effort consistently

If ME/CFS is suspected, carry out:

  • a medical assessment (including symptoms and history, comorbidities, overall physical and mental health)
  • a physical examination
  • an assessment of the impact of symptoms on psychological and social wellbeing
  • investigations to exclude other diagnoses, for example (but not limited to):
    • urinalysis for protein, blood and glucose
    • full blood count
    • urea and electrolytes
    • liver function
    • thyroid function
    • erythrocyte sedimentation rate or plasma viscosity
    • C-reactive protein
    • calcium and phosphate
    • HbA1c
    • serum ferritin
    • coeliac screening

    • creatine kinase

Use clinical judgement to decide on additional investigations to exclude other diagnoses (for example, vitamin D, vitamin B12 and folate levels; serological tests if there is a history of infection; and 9am cortisol for adrenal insufficiency)

Be aware that the following symptoms may also be associated with, but are not exclusive to, ME/CFS:

  • orthostatic intolerance and autonomic dysfunction, including dizziness, palpitations, fainting, nausea on standing or sitting upright from a reclining position
  • temperature hypersensitivity resulting in profuse sweating, chills, hot flushes, or feeling very cold
  • neuromuscular symptoms, including twitching and myoclonic jerks
  • flu-like symptoms, including sore throat, tender glands, nausea, chills or muscle aches
  • intolerance to alcohol, or to certain foods and chemicals
  • heightened sensory sensitivities, including to light, sound, touch, taste and smell
  • pain, including pain on touch, myalgia, headaches, eye pain, abdominal pain or joint pain without acute redness, swelling or effusion.

Primary healthcare professionals should consider seeking advice from an appropriate specialist if there is uncertainty about interpreting signs and symptoms and whether an early referral is needed. For children and young people, consider seeking advice from a paediatrician.

Diagnose ME (myalgic encephalitis)/CFS (chronic fatigue syndrome) in a child, young person or adult who has the symptoms as above that have persisted for 3 months and are not explained by another condition

Severity of ME/CFS

Definitions of severity are not clear cut because individual symptoms vary widely in severity and people may have some symptoms more severely than others. The definitions below provide a guide to the level of impact of symptoms on everyday functioning.


  • people with mild ME/CFS care for themselves and do some light domestic tasks (sometimes needing support) but may have difficulties with mobility. Most are still working or in education, but to do this they have probably stopped all leisure and social pursuits. They often have reduced hours, take days off and use the weekend to cope with the rest of the week.

Moderate ME/CFS

  • people with moderate ME/CFS have reduced mobility and are restricted in all activities of daily living, although they may have peaks and troughs in their level of symptoms and ability to do activities. They have usually stopped work or education, and need rest periods, often resting in the afternoon for 1 or 2 hours. Their sleep at night is generally poor quality and disturbed.

Severe ME/CFS

  • people with severe ME/CFS are unable to do any activity for themselves or can carry out minimal daily tasks only (such as face washing or cleaning teeth). They have severe cognitive difficulties and may depend on a wheelchair for mobility. They are often unable to leave the house or have a severe and prolonged after-effect if they do so. They may also spend most of their time in bed and are often extremely sensitive to light and sound.

Very severe ME/CFS

  • people with very severe ME/CFS are in bed all day and dependent on care. They need help with personal hygiene and eating, and are very sensitive to sensory stimuli. Some people may not be able to swallow and may need to be tube fed


  • clinical features that can be caused by other serious conditions ('red flags') should not be attributed to CFS/ME without consideration of alternative diagnoses or comorbidities. In particular, the following features should be investigated:
    • localising/focal neurological signs
    • clinical features of inflammatory arthritis or connective tissue disease
    • clinical features of cardiorespiratory disease
    • significant weight loss
    • sleep apnoea
    • clinically significant lymphadenopathy
  • orthostatic intolerance
    • a clinical condition in which symptoms such as light-headedness, near-fainting or fainting, impaired concentration, headaches, dimming or blurring of vision, forceful beating of the heart, palpitations, tremulousness and chest pain occur or worsen on standing up and are improved (although not necessarily resolved) by sitting or lying down. Orthostatic intolerance may include postural orthostatic tachycardia syndrome (POTS), which is a significant rise in pulse rate when moving from lying to standing, and postural hypotension, which is a significant fall in blood pressure when moving from lying to standing. People with severe orthostatic intolerance may find they are unable to sit up for any length of time.


  1. Prescribers' Journal (2000), 40 (2), 99-106.
  2. NICE (October 2021). Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management

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