Chronic Fatigue Syndrome (CFS/ME): Blood Tests to Rule Out

Chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), affects approximately 250,000 people in the United Kingdom. It is a debilitating condition characterised by persistent, unexplained fatigue that is not improved by rest and is worsened by physical or mental exertion — a phenomenon called post-exertional malaise. Other common symptoms include unrefreshing sleep, cognitive difficulties (often described as "brain fog"), muscle pain, joint pain, headaches, and sore throats.

CFS/ME is a diagnosis of exclusion. This means that before the diagnosis can be made, your doctor must systematically rule out other medical conditions that produce similar symptoms. Many of these conditions are common, treatable, and detectable through blood tests. The frustration reported by many CFS/ME patients is that their GP ordered an incomplete set of tests, declared everything "normal," and left them without answers.

What NICE Guidelines Recommend

The National Institute for Health and Care Excellence (NICE) published updated guidelines for CFS/ME in 2021 (NG206). These guidelines specify a comprehensive list of blood tests that should be performed before CFS/ME is diagnosed. The purpose is not to diagnose CFS/ME — there is no blood test that can do that — but to exclude conditions that mimic it.

According to NICE, CFS/ME should be suspected when fatigue has persisted for at least 4 weeks in children and young people, or at least 6 weeks in adults, and all of the following apply:

• The fatigue is not explained by another medical condition
• It is not the result of excessive physical or mental exertion
• It is not significantly relieved by rest
• It has resulted in a substantial reduction in activity levels

A provisional diagnosis of CFS/ME can be considered at 3 months if symptoms persist despite normal investigation results. The NICE-recommended blood tests are outlined below.

The Full Investigation Panel

Full Blood Count (FBC)

A full blood count screens for anaemia (iron deficiency, B12 deficiency, or chronic disease), infection (elevated white cells), and haematological conditions. Anaemia is one of the most common treatable causes of chronic fatigue, affecting approximately 14% of non-pregnant women in the UK. Even mild anaemia (haemoglobin 100-119 g/L in women) can produce significant fatigue, poor concentration, and exercise intolerance.

Thyroid Function (TSH, Free T4)

Hypothyroidism is the great mimicker. Its symptoms — fatigue, weight gain, constipation, dry skin, depression, difficulty concentrating, muscle aches — overlap almost entirely with CFS/ME. Approximately 2% of the UK population has hypothyroidism, with women affected 5-10 times more frequently than men. Subclinical hypothyroidism (mildly elevated TSH with normal free T4) is even more common, affecting up to 8% of women, and can cause fatigue in its own right.

TSH alone is insufficient. Free T4 should always be measured alongside TSH to distinguish between overt hypothyroidism (high TSH, low free T4) and subclinical hypothyroidism (mildly elevated TSH, normal free T4). If autoimmune thyroid disease is suspected, anti-TPO antibodies can confirm Hashimoto's thyroiditis, the most common cause of hypothyroidism in the UK.

Liver Function Tests (ALT, AST, GGT, ALP, Bilirubin, Albumin)

Chronic liver disease can cause profound fatigue, often as the earliest and most prominent symptom. Non-alcoholic fatty liver disease (NAFLD) affects approximately 25-30% of the UK adult population and frequently presents with nothing more than persistent tiredness. Hepatitis B and C infections, autoimmune hepatitis, and haemochromatosis (iron overload) are other liver conditions that present with fatigue and are detectable through liver function tests combined with specific additional markers.

Kidney Function (eGFR, Creatinine, Urea)

Chronic kidney disease (CKD) affects roughly 7% of the UK population. As kidney function declines, waste products accumulate in the blood (uraemia), causing fatigue, nausea, poor appetite, and cognitive impairment. CKD is frequently asymptomatic in its early stages and may be detected only through blood testing. eGFR (estimated glomerular filtration rate) is the primary marker — values below 60 mL/min/1.73m² sustained for 3 months or more confirm CKD.

HbA1c (Glycated Haemoglobin)

Type 2 diabetes and prediabetes are major causes of fatigue that are often missed for years. HbA1c reflects average blood glucose over the preceding 8-12 weeks. A result of 42-47 mmol/mol indicates prediabetes; 48 mmol/mol or above confirms diabetes. Diabetes UK estimates that 4.3 million people in the UK have diabetes, with nearly 1 million undiagnosed. Fatigue, increased thirst, and frequent urination are classic symptoms, but many people with type 2 diabetes report only tiredness.

C-Reactive Protein (CRP) and ESR

CRP and erythrocyte sedimentation rate (ESR) are non-specific markers of inflammation and infection. Elevated levels indicate that something is driving an inflammatory response — whether an infection, autoimmune condition, or malignancy. Persistent low-grade CRP elevation (above 3-5 mg/L) in a fatigued patient warrants further investigation rather than dismissal. Conditions such as polymyalgia rheumatica, giant cell arteritis, and systemic lupus erythematosus can present primarily with fatigue and raised inflammatory markers.

Coeliac Screen (tTG IgA)

Coeliac disease is a grossly underdiagnosed autoimmune condition triggered by gluten. It affects approximately 1 in 100 people in the UK, but only 30% are currently diagnosed. Fatigue is the most commonly reported symptom — often in the absence of the "classic" gastrointestinal symptoms. Tissue transglutaminase IgA (tTG IgA) is the recommended screening test, with a sensitivity and specificity both exceeding 95%. Total IgA should be measured simultaneously, as IgA deficiency (present in 2-3% of coeliac patients) can produce a false-negative tTG result.

Calcium (Corrected Calcium)

Hypercalcaemia (elevated calcium) causes fatigue, low mood, constipation, and confusion — a cluster sometimes summarised as "bones, stones, abdominal groans, and psychic moans." Primary hyperparathyroidism is the most common cause in outpatients and is treatable, usually with surgery. Hypocalcaemia can also cause fatigue alongside muscle cramps and tingling sensations.

Ferritin

Ferritin below 30 µg/L indicates iron depletion, which can cause fatigue, poor concentration, hair loss, and restless legs even when haemoglobin remains within the normal range. This is one of the most frequently missed causes of fatigue in women of reproductive age. NICE guidelines specifically include ferritin in the CFS/ME investigation panel for this reason.

Vitamin B12 and Folate

B12 deficiency causes fatigue, cognitive impairment, mood changes, and neurological symptoms (tingling, numbness, balance problems). Pernicious anaemia — the autoimmune cause of B12 deficiency — affects approximately 1 in 1,000 people in the UK. Folate deficiency produces similar fatigue and can coexist with B12 deficiency. Both should be measured in any fatigue investigation.

Vitamin D (25-Hydroxyvitamin D)

Vitamin D deficiency is extremely common in the UK, particularly during winter months (October to March), when UVB radiation is insufficient for cutaneous synthesis at UK latitudes. Symptoms of deficiency include fatigue, muscle weakness, bone pain, and low mood. Levels below 25 nmol/L are classified as deficient; 25-50 nmol/L as insufficient. Supplementation can produce noticeable improvements in energy within 4-8 weeks when levels are genuinely low.

What If Everything Comes Back Normal?

If all of these tests return within normal ranges and fatigue has persisted for 3 months or more with the characteristic features described above (post-exertional malaise, unrefreshing sleep, cognitive difficulties), then CFS/ME becomes the working diagnosis. This is not a diagnosis of defeat — it is a recognised medical condition with specific management strategies outlined in the NICE guidelines, including activity management (pacing), psychological support, and specialist referral where available.

However, the key point is that "normal" blood tests should be genuinely normal — not just within the laboratory reference range but clinically optimal. A TSH of 4.5 mIU/L is technically within range at most labs but may indicate subclinical hypothyroidism. A ferritin of 18 µg/L is often reported as "normal" but represents significant iron depletion. A vitamin D of 30 nmol/L is technically "insufficient" rather than "deficient" but may still contribute to symptoms. Context-aware interpretation of results matters.

How to Test With Lola Health

Our Core Health 45 blood test (£120) covers the vast majority of the NICE-recommended investigation panel for CFS/ME: full blood count, thyroid function (TSH, free T4), liver function, kidney function, HbA1c, CRP, ferritin, vitamin B12, folate, vitamin D, and calcium. For the most comprehensive screen — adding markers such as anti-TPO antibodies, immunoglobulins, and additional metabolic markers — our Peak Insights 70 test (£185) provides an extensive panel. tTG IgA (coeliac screen) is available as an add-on biomarker.

All results are clinician-reviewed and returned with detailed commentary, so you receive not just numbers but an interpretation of what they mean in the context of your symptoms. If treatable conditions are identified, our team provides clear next-step recommendations.

The Bottom Line

If you have been living with persistent fatigue that does not improve with rest, comprehensive blood testing is the single most productive step you can take. In a significant proportion of cases, blood tests reveal a treatable condition — hypothyroidism, iron depletion, B12 deficiency, coeliac disease, or diabetes — that explains the symptoms entirely. In the remaining cases where no cause is found, having a thorough set of normal results provides the foundation for a CFS/ME diagnosis and access to appropriate specialist support.

Either way, testing moves you forward. Uncertainty does not.