Blood Test for Bloating: What Your Blood Can Reveal About Digestive Problems

Bloating is one of the most common digestive complaints in the UK, with studies suggesting that up to 30% of the general population experiences it regularly. For many people, bloating is dismissed as a minor inconvenience — something to manage with dietary adjustments and over-the-counter remedies. But persistent bloating can be a signal that something more significant is happening beneath the surface, and a blood test is often the fastest route to finding out what.

Why Blood Testing Matters for Digestive Symptoms

Bloating, abdominal discomfort and changes in bowel habits are frustratingly non-specific. The same symptoms can be produced by food intolerances, irritable bowel syndrome (IBS), coeliac disease, thyroid dysfunction, small intestinal bacterial overgrowth (SIBO), or nutrient malabsorption. A well-chosen blood panel can narrow the diagnostic field considerably, either identifying a specific cause or providing evidence that guides further investigation.

Key Biomarkers to Investigate

Tissue Transglutaminase IgA (tTG IgA) — Coeliac Screening

Coeliac disease affects approximately 1 in 100 people in the UK, but an estimated 75% remain undiagnosed. This means roughly 500,000 people in the UK have coeliac disease without knowing it. The condition is an autoimmune response to gluten that damages the lining of the small intestine, causing bloating, diarrhoea, abdominal pain, fatigue and nutrient malabsorption.

The tTG IgA blood test is the recommended first-line screening test for coeliac disease. A result above 10 U/mL is considered positive and should prompt referral for a confirmatory intestinal biopsy. Critically, you must be eating gluten regularly (at least one meal containing gluten daily for six weeks) for the test to be accurate. Stopping gluten before testing can produce a false negative.

Thyroid Function (TSH, Free T4, Free T3)

Hypothyroidism — an underactive thyroid — slows gut motility, the muscular contractions that move food through the digestive tract. The result is constipation, bloating and a feeling of fullness. Hypothyroidism affects approximately 2–5% of the UK population and is particularly common in women over 40. A TSH above 4.0 mIU/L with a low free T4 (below 12 pmol/L) indicates an underactive thyroid that could be contributing to your digestive symptoms.

Iron Studies (Ferritin, Serum Iron, Transferrin Saturation)

Iron deficiency in the context of bloating is a particularly important finding because it can indicate malabsorption — the body's failure to properly absorb nutrients from food. This is a hallmark of coeliac disease, but also occurs in inflammatory bowel disease (Crohn's and ulcerative colitis) and other conditions affecting the small intestine. Ferritin below 30 µg/L in someone with digestive symptoms should prompt investigation into the cause of the deficiency, not simply iron supplementation.

Vitamin B12 and Folate

Like iron, B12 and folate are absorbed in the small intestine. Deficiency of either — B12 below 200 ng/L, folate below 3.9 nmol/L — in the presence of bloating and digestive symptoms raises the possibility of malabsorption. B12 deficiency is particularly associated with conditions affecting the terminal ileum, including Crohn's disease and pernicious anaemia.

Full Blood Count (FBC)

A full blood count can reveal anaemia (low haemoglobin), which may point to blood loss from the GI tract or malabsorption. It also shows the mean cell volume (MCV) — large red blood cells (macrocytic anaemia) suggest B12 or folate deficiency, while small red blood cells (microcytic anaemia) suggest iron deficiency. This pattern-matching helps clinicians determine where in the gut the problem may lie.

CRP (C-Reactive Protein)

CRP is a marker of systemic inflammation. While it is not specific to the gut, an elevated CRP (above 5 mg/L) in someone with persistent bloating and altered bowel habits raises the possibility of inflammatory bowel disease rather than IBS. IBS does not typically elevate CRP, so this marker can help distinguish between functional and inflammatory conditions.

When Bloating Points to Something Deeper

Certain patterns in blood work should prompt further investigation:

• Iron deficiency + positive tTG IgA: Strongly suggestive of coeliac disease
• Iron deficiency + elevated CRP: Consider inflammatory bowel disease
• Multiple nutrient deficiencies + normal CRP: Possible coeliac disease or SIBO
• Elevated TSH + constipation-predominant bloating: Likely thyroid contribution
• Normal blood work + persistent bloating: IBS or food sensitivities — consider microbiome testing

The Microbiome Dimension

When blood tests come back normal but bloating persists, the gut microbiome may hold the answer. An imbalanced microbiome — dysbiosis — can drive bloating through excessive gas production, altered gut motility and increased intestinal permeability. This is where a dedicated microbiome test can add significant diagnostic value, particularly for IBS-type symptoms where standard blood work is unremarkable.

Recommended Tests

The Core Health 45 blood test (£120) covers a full blood count, iron studies, ferritin, B12, folate, CRP, thyroid function, liver and kidney function — the essential panel for investigating bloating. tTG IgA for coeliac screening is available as an add-on. For persistent IBS-type bloating where blood work is normal, the GutID CGI (Core Gut Insights) test (£300) provides a detailed microbiome analysis that can identify dysbiosis, pathogenic bacteria and markers of gut barrier function.

The Bottom Line

Bloating is common, but "common" does not mean "normal." A targeted blood test can identify coeliac disease, thyroid dysfunction, nutrient malabsorption and inflammatory markers that would otherwise go undetected. If your bloating has persisted for more than a few weeks, testing is a practical first step before resigning yourself to a life of food diaries and elimination protocols.