The core symptoms
Diagnostic criteria differ slightly around the world, but they agree on a core set. The widely used US Institute of Medicine (IOM) criteria from 2015 require:
- A substantial drop in your ability to do things you could do before (work, study, social life, personal activities). It must last more than six months and come with profound fatigue that is new, not caused by ongoing exertion, and not relieved by rest.
- Post-exertional malaise (PEM): symptoms get worse after physical, mental or emotional effort, often after a delay.
- Unrefreshing sleep.
Plus at least one of:
- Cognitive impairment: problems with thinking, memory, concentration or processing speed (“brain fog”)
- Orthostatic intolerance: symptoms get worse when upright and improve when lying down
In Australia and Canada, doctors often use the Canadian Consensus Criteria (CCC), which also require pain and symptoms across several body systems. Many research trials require the CCC.
Post-exertional malaise in detail
PEM is the hallmark of ME/CFS. It’s worth understanding well because it shapes everything else.
- Trigger: any effort beyond your current limit, whether physical (walking, showering, housework), cognitive (reading, screens, paperwork, conversation) or emotional (stress, excitement, conflict). Sensory overload counts too.
- Timing: often 12–72 hours after the trigger.
- What it feels like: a flare of all your usual symptoms, plus flu-like malaise, heavy limbs, pain, worse brain fog, sore throat, poor sleep.
- Duration: hours to weeks. Big or repeated crashes can lower your baseline long term.
Other common symptoms
| Area | Symptoms |
|---|---|
| Energy | Profound exhaustion, weakness, “wired but tired”, feeling poisoned or hungover |
| Brain & thinking | Brain fog, word-finding trouble, slow processing, poor short-term memory, difficulty following conversations or reading |
| Sleep | Unrefreshing sleep, insomnia, sleeping too much, broken sleep, vivid dreams |
| Upright / circulation | Dizziness, lightheadedness, racing heart on standing, needing to sit or lie down, blood pooling in the legs, fainting. See POTS |
| Pain | Muscle pain, joint pain without swelling, new or worse headaches |
| Immune | Sore throat, tender lymph nodes in the neck or armpits, flu-like feelings, new sensitivities to food or medications |
| Senses | Sensitivity to light, noise, smell, touch or temperature |
| Body temperature | Feeling feverish, chills, cold hands and feet, sweating, poor tolerance of heat or cold |
| Gut | Nausea, irritable bowel symptoms, new food intolerances |
Not everyone has every symptom, and they can change over time.
Levels of severity
ME/CFS varies hugely between people. The UK NICE guideline (2021) describes four levels:
Mild
Can look after themselves and may still work or study, but usually by cutting out social life and hobbies. Often need to rest on days off.
Moderate
Reduced mobility, limited in all daily activities. Usually stopped work or study. Need rest periods, often sleep in the afternoon.
Severe
Can do only minimal daily tasks (e.g. washing their face, cleaning teeth). Severe cognitive problems. May be wheelchair users, and are mostly housebound.
Very severe
Bedbound and dependent on care, often around the clock. May be unable to tolerate light or noise, and may need help eating or be tube-fed.
Around one in four people with ME/CFS are housebound or bedbound at some point.
Symptoms fluctuate
Symptoms go up and down, sometimes within a single day. A good day, or even a good week, can happen whether or not anything has changed. That’s why a good day isn’t a sign you can do more. See the good-day trap.
Getting a diagnosis
There’s no single blood test for ME/CFS yet, although several are in development (see The science). Diagnosis is based on:
- Your symptom history, especially PEM
- Symptoms lasting at least six months (though doctors can and should give pacing advice much earlier)
- Tests that rule out other conditions with similar symptoms, such as thyroid problems, anaemia, coeliac disease, sleep apnoea, autoimmune disease and others
See Tests to ask your GP for for the full list and a script you can take to your appointment.