International Consensus Criteria (ICC)

Published in 2011, the International Consensus Criteria (ICC) is a set of diagnostic criteria used for identifying cases of myalgic encephalomyelitis (ME) for research and clinical purposesWhilst based on the Canadian Consensus Criteria (CCC), significant changes were made to create a far more selective set of criteria. One interesting difference is the removal of the 6-month waiting period before diagnosis; instead, a diagnosis can be made if the required symptoms are present and clinical investigation to rule out other causes has been completed.

Summary of the International Consensus Criteria

Diagnosis should be made when the clinician is satisfied that the patient has ME, rather than being constrained by a specific timeframe. To be diagnosed, a patient must have:

  1. Post-exertional neuroimmune exhaustion according to the criteria described;
  2. At least 1 symptom from 3 neurological impairment categories;
  3. At least 1 symptom from 3 immune/gastrointestinal/genitourinary impairment categories;
  4. At least 1 symptom from energy metabolism/transport impairments categories.

Thorough history-taking, physical examination and tests are needed to exclude other illnesses. Having more than one disease is a possibility but it is important that each one is identified and treated.


  • Post-exertional neuroimmune exhaustion (PENE)
    • Marked, rapid physical and/or cognitive fatigability in response to exertion which can be debilitating and cause a relapse. Exertion may be minimal; e.g. activities of daily living or simple mental tasks.
    • Low threshold of physical and mental fatiguability; i.e. lack of stamina, resulting in a substantial reduction in pre-illness activity level.
    • Post-exertional symptom exacerbation; e.g. acute flu-like symptoms, pain and worsening of other symptoms.
    • Post-exertional exhaustion which may occur immediately after activity or be delayed by hours or days.
    • Prolonged recovery period which is usually 24 hours or longer. A relapse can last days, weeks or longer.

At least 1 symptom from 3 of the 4 following neurological impairments categories must be present:

  • Neurocognitive impairments
    • Difficulty processing information: slowed thought, impaired concentration
      (e.g. confusion, disorientation, difficulty with decision-making, slowed speech)
    • Short-term memory loss
      (e.g. difficulty remembering what one wanted to say, difficulty retrieving words, difficulty recalling information, poor working memory)
  • Pain
    • Headaches
    • Significant pain which can be experienced in muscles, muscle-tendon junctions, joints, abdomen or chest; it is non-inflammatory in nature and often migrates
  • Sleep disturbance
    • Disturbed sleep patterns
      (e.g. insomnia, prolonged sleep, sleeping most of the day rather than at night, frequent awakenings, waking up much earlier than pre-illness, nightmares)
    • Unrefreshing sleep
      (e.g. waking up feeling exhausted regardless of sleep duration, daytime sleepiness)
  • Neurosensory, perceptual and motor disturbances
    • Neurosensory and perceptual
      (e.g. inability to focus vision, hypersensitivity [light, noise, vibration, odour, taste and touch], impaired depth perception)
    • Motor
      (e.g. muscle weakness, twitching, poor coordination, unsteady on feet)

At least 1 symptom from 3 of the 5 following immune/gastrointestinal/genitourinary impairment categories must be present:

  • Flu-like symptoms
    • May be recurrent or chronic and typically activate or worsen with exertion
      (e.g. sore throat, sinusitis, enlarged or tender cervical and/or axillary lymph nodes)
      Note: Whilst sore throat, tender lymph nodes and flu-like symptoms are not specific to ME, their activation in response to exertion is abnormal.
  • Susceptibility to viral infections with prolonged recovery periods
  • Gastrointestinal tract
    (e.g. nausea, abdominal pain, bloating irritable bowel syndrome)
  • Genitourinary
    (e.g. urinary urgency or frequency, nocturia)
  • Sensitivities to food, medications, odours or chemicals

At least 1 symptom from energy production/transportation impairment categories must be present:

  • Cardiovascular
    (e.g. light-headedness/dizziness, neurally-mediated hypotension, orthostatic intolerance [inability to tolerate an upright position], postural orthostatic tachycardia syndrome, palpitations with or without cardiac arrhythmias)
  • Respiratory
    (e.g. air hunger, laboured breathing, fatigue of chest wall muscles)
  • Loss of thermostatic stability
    (e.g. subnormal body temperature, marked diurnal fluctuations, sweating episodes, recurrent feelings of feverishness with or without low grade fever, cold extremities)
  • Intolerance of extremes of temperature

Additional details:

For a diagnosis of ME, the severity of symptoms must result in a significant reduction in activity level compared with before the illness. Categories include mild (approximately 50% reduction in pre-illness activity level), moderate (mostly housebound), severe (mostly bedbound), or very severe (completely bedbound, requiring help with basic functions).

Atypical myalgic encephalomyelitis may be diagnosed if an individual meets criteria for post-exertional neuroimmune exhaustion but falls short by a maximum of 2 of the remaining criterial symptoms. In rare cases, pain or sleep disturbance may be absent.


There are several factors to consider when utilising the ICC.

  • Strictness and flexibility of criteria: The rationale behind the many requirements of the ICC, making it seem stringent, was to enhance homogeneity (alikeness) of patient cohorts. They mention that “patient sets that include people who do not have the disease lead to biased research findings, inappropriate treatments and waste scarce research funds”. Yet despite the perceived rigidity, the ICC allows exceptions to the rule as can be seen by the inclusion of atypical myalgic encephalomyelitis.
  • Omission of post-exertional malaise: The exclusion of the commonly used term ‘post-exertional malaise’ (PEM) from the ICC might raise questions. The authors contend that ‘malaise’ is vague and does not capture the complexity of the low-threshold fatiguability and post-exertional symptom flare. Instead the term ‘post-exertional neuroimmune exhaustion’ is proposed as a more precise representation of underlying pathological mechanisms.
  • Relapse: Like the CCC, the ICC briefly mentions ‘relapse’, but does not provide a clear definition. Whilst the criteria provide a comprehensive framework for diagnosis, the absence of a relapse definition leaves room for interpretation and could warrant further clarification to maintain consistency.
  • Exclusion of fatigue: Notably, ‘fatigue’ is not included as a separate category within the ICC, a departure from previous frameworks which encompass ME/CFS and CFS. The criteria aims to distance itself from the broad use of ‘fatigue’ and its potential to undermine a diagnosis of ME. Nevertheless, fatigue is incorporated into the post-exertional neuroimmune exhaustion category which mentions physical and mental fatiguability.


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