What are the limitations/difficulties in arriving at a prevalence rate for ME/CFS in the UK?

Several studies do provide estimates for the prevalence of ME/CFS in the UK – and elsewhere in the world – with published figures for the UK ranging from 0.1% to 2.62%.

The most recent peer-reviewed estimate for the UK, published in April 2025, uses information recorded in hospital records – hospital episode statistics (HES) data – for 62,782,175 people in England between 1 April 1989 and 7 October 2023. Results suggested that on the 7th October 2023, 100,055 people in England – 0.16% of the 62,782,175 individuals in the study – had been diagnosed with ‘post-viral fatigue syndrome’ (ICD code G93.3) – the diagnosis with symptoms most closely matching those of ME/CFS. The authors then take the G93.3 prevalence rates for males and females facing the least barriers to diagnosis in England (0.25% for males and 0.92% for females) and extrapolate them to the UK population size for 2023 of 68.3 million people. This calculation suggests that if the prevalence rates for England apply to all people in the UK 403,922 people would have a G93.3 diagnosis in their lifetime. This would equate to a prevalence rate of approximately 0.6%.

While this study does provide an up to date estimate of the prevalence of those with a diagnostic code most closely matching the symptoms of ME/CFS in the UK, there are limitations which must be considered including:

The data are from England rather than the whole of the UK.
The medical records are for a diagnosis for G93.3 and not ME/CFS specifically.
Only those with a G93.3 diagnosis are included in the estimate; many people with ME/CFS may be undiagnosed.
The study period includes diagnoses made under both versions of the NICE guideline for ME/CFS which are not consistent.
The data are from hospital and outpatient appointments only and do not include any diagnoses made in GP practices.
The accuracy of HES data relies on how clearly and correctly information is recorded.

There are also recent estimates from elsewhere in the world, such as a report from the USA that identified a prevalence rate of 1.3% for self-reported CFS. However, these estimates are based on information from populations that are different from the UK in terms of the number of people in certain groups, such as age groups, and different ethnic groups.

This means that existing estimates of ME/CFS prevalence may not apply to the UK population today for several reasons.

1. Differences in population

There are differences in population structure – the types of people who make up a population, between countries, between different regions of the same country, and between different groups of participants in research studies. This can mean that extrapolating a prevalence estimate outside the location or population it was initially intended for may not be appropriate.

There are also natural changes in population size and structure over time – for example, in the UK the estimated population size in 2012 was 63.7 million, and in 2023 it was 68.3 million. As population size changes, the number of people with ME/CFS is also likely also change.

Although increases in population size may not necessarily change the prevalence (the proportion) of people with ME/CFS, we know that population size is affected by factors such as increased life expectancy, and migration.

These factors can mean that there are also changes in the structure or makeup of a population over time. For example, changes in the number of people of a certain age, the gender balance, and different ethnic groups.

As research suggests that ME/CFS is more common in certain age groups, in females, and in some ethnic groups than others, changes in these factors within population structure may impact the proportion of people with ME/CFS overall.

2. Differences in methods used to define or diagnose ME/CFS

There are many potential ways to identify people who have ME/CFS in a population to estimate the prevalence and each method has limitations.

Medical records

Although information such as hospital episode statistics (HES), or data from GP surgeries can be used to assess how many people have a recorded diagnosis of a disease – including ME/CFS, there are limitations:

The exact notes made in medical records relating to ME/CFS may differ between hospitals and GP practices, by region, and by clinician.
- In England, all health care providers use SNOMED CT UK codes to capture clinical information. Under this coding system, Myalgic Encephalomyelitis, Myalgic Encephalopathy, and Chronic fatigue syndrome (amongst other names) are classified under ‘Chronic fatigue syndrome (disorder) SCTID: 52702003 ‘ but the preferred term of Chronic fatigue Syndrom is currently under review with ‘ME/CFS ‘ME/CFS – myalgic encephalomyelitis/chronic fatigue syndrome’’ being considered.
  - However, it must be remembered that use of SNOMED CT UK is not currently standard practice throughout the UK. NHS Scotland, for example, announced only in February 2024 a programme aiming to implement SNOMED CT as the standard clinical terminology used throughout the NHS Scotland digital realm – work is presently ongoing.
- Additionally, in secondary care data – a type of healthcare that occurs when a primary care provider (such as a GP) refers a patient to a specialist with more specific expertise in a health issue, the medical coding system ICD-10 5^th edition is used – in this, there is no diagnostic code specifically for “ME/CFS” – rather, as seen in the paper by Samms and Ponting, the ICD-10 code G93.3 for “post-viral fatigue” which includes “benign myalgic encephalomyelitis” is used. This means that the medical codes used in clinical practice may not accurately capture “ME/CFS” – or at the very least, the terminology used is inconsistent.
  - Although there is a newer version of the coding system – ICD-11, which has the code 8E49 for “Post viral fatigue syndrome” – including both “chronic fatigue syndrome and myalgic encephalomyelitis”, this is not yet being used in practice. In June 2024 NHS Digital stated that “It will become a mandated information standard for the NHS in England to replace ICD-10 in the next 5 years”.
    - While this code may better reflect a diagnosis of “ME/CFS” it may further complicate comparisons of prevalence over time as those diagnosed with code G93.3 may not be directly comparable with those who are diagnosed with 8E49.
  - It is also important to remember that the devolved nations – England, Scotland, Wales, and Northern Ireland, have their own NHS structures – as such, coding practices for ME/CFS may differ between them.

There have been changes in guidelines for the diagnosis and management of ME/CFS in the UK – importantly, the symptoms required for a diagnosis of ME/CFS to be made before 2021 are different from those that are now required under the 2021 NICE guidelines.
- Notably, in comparison with the 2021 guidelines, the 2007 version did not require either cognitive difficulties or unrefreshing sleep for a diagnosis of ME/CFS to be made. The 2007 guidelines were also less clear about the requirement for the cardinal symptom of ME/CFS – post-exertional malaise (PEM).
- As it is unlikely that those who received a diagnosis of ME/CFS before 2021 have been reassessed under the new criteria, those with a diagnosis of ME/CFS before 2021 may not be comparable with those who received a diagnosis under the new guidelines.

Click to show more: Table detailing the differences between the 2007 Guidelines and 2021 Guidelines.

2007 Guidelines

2021 Guidelines

“Healthcare professionals should consider the possibility of CFS/ME if a person has:

Fatigue with all of the following features:
– new or had a specific onset (that is, it is not lifelong)
– persistent and/or recurrent
– unexplained by other conditions
– has resulted in a substantial reduction in activity level
– characterised by post-exertional malaise and/or fatigue (typically delayed, for example by at least 24 hours, with slow recovery over several days)

AND

One or more of the following symptoms:
– difficulty with sleeping, such as insomnia, hypersomnia, unrefreshing sleep, a disturbed sleep–wake cycle
– muscle and/or joint pain that is multi-site and without evidence of inflammation
– headaches
– painful lymph nodes without pathological enlargement
– sore throat
– cognitive dysfunction, such as difficulty thinking, inability to concentrate, impairment of short-term memory, and difficulties with word-finding, planning/organising thoughts and information processing
– physical or mental exertion makes symptoms worse
– general malaise or ‘flu-like’ symptoms
– dizziness and/or nausea
– palpitations in the absence of identified cardiac pathology.”

“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
– has 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.”

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Medical records may also be limited due to the general hesitancy in approaching GPs due to the stigma associated with ME/CFS, the perceived disbelief amongst healthcare professionals, and the fact that to date there is no cure for the disease – in fact, these were all barriers to seeking a diagnosis identified in an Australian study published in 2022.

There also appear to be some subgroups of the population, such as men with ME/CFS and ethnic minority groups who may be more hesitant to seek a diagnosis for the disease – or to experience greater delays in diagnosis and higher rates of misdiagnosis.

Application of ME/CFS diagnostic criteria

Research studies can also look at the number of people meeting specific ME/CFS diagnostic criteria in a population.

Several different criteria have been – and continue to be, used to diagnose “ME/CFS” including:

Criteria	Year	Disease	Purpose
Fukuda criteria	1994	CFS	Research
International Consensus Criteria (ICC)	2011	ME	Research and Clinical
Canadian Consensus Criteria (CCC) and Revised CCC	2003	ME/CFS	Research and Clinical
Institute of Medicine criteria (IoM; now National Academy of Medicine)	2015	ME/CFS (recommended a name change to “systemic exertion intolerance disease”)	Clinical
NICE ME/CFS Guideline	2021	ME/CFS	Clinical

This means that even when considering the same population, estimates of ME/CFS prevalence differ depending on the criteria used. It is also the case that not all consider that certain diagnostic criteria actually diagnose the illness they purport to.

The multiplicity of criteria – both diagnostic and research – also affects comparisons between prevalence studies both in terms of the criteria used but also when studied as criteria evolve over time. For example, the aforementioned differences between the 2007 and 2021 NICE guidelines for the diagnosis and management of ME/CFS.

Self-reported ME/CFS

Studies can also consider the number of self-reported cases of ME/CFS. Whilst people who self-report may indeed have the disease, it is also important to remember that:

People may report that they have the disease, but they may not meet diagnostic criteria if it were to be applied.
People may report that they have been diagnosed with the disease in the past but they may not meet current diagnostic criteria.
People may report not to have the disease, when if tested, they would meet diagnostic criteria.

It is important to note that research has suggested that prevalence estimated based on self-reported “CFS” is likely to be higher than estimates based on the number of people with ME/CFS diagnosed using detailed symptom-based criteria.

3. The Impact of COVID-19 and long COVID

Research suggests that the symptoms of long COVID and ME/CFS overlap – and evidence from the USA has suggested that the number of people self-reporting symptoms of ME/CFS has increased since the COVID-19 pandemic. Nevertheless, not everyone with long COVID meets the diagnostic criteria for ME/CFS.

One study – which considered medical records in the USA – found that although the rate of developing ME/CFS following a COVID-19 infection was no higher than for other acute infectious illnesses at 3% to 4%, “given the millions of people infected with SARS-CoV-2” this would “still have a high societal burden”.

Another paper, using information from electronic health records in the USA between January 2020 and December 2023, found that in more than 3 million people, compared to people who had no record of COVID-19 infection, those with a record of COVID-19 had a 62% higher chance of receiving a diagnosis of “CFS” after a one-year follow-up period.

This means that it is likely that the prevalence of those whose symptoms meet ME/CFS diagnostic criteria – or who have a diagnosis of ME/CFS from a health professional, has increased due to the COVID-19 pandemic – as suggested by research in the USA by Mirin and Jason.

However, updating the estimate for ME/CFS prevalence is not as simple as just adding those with long COVID who have a diagnosis of ME/CFS – or who in theory meet ME/CFS diagnostic criteria, to existing (pre-COVID) ME/CFS estimates – there is much more to think about.

For example, although ME/CFS symptoms can develop following a COVID-19 infection, without a validated diagnostic biomarker for ME/CFS – or indeed for long COVID, it is impossible to assess whether ME/CFS triggered by something other than long COVID (or the COVID-19 virus) is the same as ME/CFS which had a different onset.

Additionally, it is crucial to consider those whose symptoms of ME/CFS started during the COVID-19 pandemic but the onset was not the COVID-19 virus – it was, for example, another infection – such as the common cold, or indeed a stressful life event, which led to the reactivation of a latent virus they were exposed to earlier in life.

In the USA, the National Academies of Sciences, Engineering, and Medicine (NASEM) long COVID definition, does not require a positive COVID test for a diagnosis of the condition to be made – additionally, this definition does not differentiate between long COVID and ME/CFS – rather, it states that ME/CFS is a “diagnosable condition” within long COVID.

Under the NASEM definition, since the COVID-19 pandemic anyone who has post-viral fatigue could be given a diagnosis of long COVID – which for many, the symptoms mimic those of ME/CFS, would be given.

In short, differentiating between ME/CFS and long COVID is complex, and at present it is unclear how clinicians are separating the two in practice – especially now the rates of testing to confirm a COVID-19 infection have decreased.

Summary

While estimates of the prevalence of ME/CFS exist, these are either not up to date – and/or are based on previous samples of the UK population, or they are more recent but reflect the prevalence in another country – such as the USA. As many factors change over time – and vary between countries (and different research studies), it is not always appropriate to directly compare estimates or to extrapolate figures to populations for which they were not intended/beyond that for which they were initially calculated.

In conclusion, without a validated diagnostic biomarker for ME/CFS, the diagnostic process for the disease will remain complex, and accurately capturing ME/CFS prevalence will remain an ongoing challenge for researchers.