Blood Tests as Windows Into the Gut
Blood tests are often the first investigation in gut health assessment. While they cannot directly visualise or sample the gut, they detect systemic consequences of intestinal disease — anaemia from malabsorption, inflammation from mucosal damage, and immune responses to specific antigens.
Full Blood Count (FBC)
Haemoglobin: Low haemoglobin (anaemia) in gut disease most commonly reflects iron deficiency (from chronic blood loss in IBD or malabsorption in coeliac disease) or anaemia of chronic disease (from sustained inflammation). Microcytic anaemia (low MCV) suggests iron deficiency; normocytic anaemia suggests chronic inflammation.
White cell count: Elevated neutrophils may reflect active inflammation or infection. Eosinophilia (raised eosinophil count) may suggest allergic conditions (EoE, parasitic infection). Lymphopenia can occur with immunosuppressive therapy.
Platelet count: Thrombocytosis (elevated platelets) is a reactive marker of inflammation and is common in active IBD.
Iron Studies
Ferritin: The primary iron storage marker. Low ferritin (below 30 micrograms per litre) confirms iron deficiency. However, ferritin is also an acute-phase reactant — it rises with inflammation, meaning a 'normal' ferritin in an actively inflamed patient may mask true iron deficiency. In IBD patients with active inflammation, transferrin saturation (below 16 percent) is a more reliable indicator.
C-Reactive Protein (CRP)
As discussed in the biomarkers entry, CRP reflects systemic inflammation. It is useful for monitoring IBD activity but non-specific and insensitive in up to 30 percent of IBD patients. Serial trends are more informative than single values.
Liver Function Tests (LFTs)
ALT and AST: Elevated transaminases in a gut health context may suggest NAFLD (the most common cause of mildly raised LFTs), drug-related hepatotoxicity (from azathioprine, methotrexate), or autoimmune liver disease. ALP and GGT: Elevated alkaline phosphatase with a cholestatic pattern should prompt investigation for primary sclerosing cholangitis (PSC) in UC patients.
Coeliac Serology
Anti-tissue transglutaminase IgA (tTG-IgA) is the first-line screening test — sensitivity and specificity both exceed 95 percent. Total IgA should be measured simultaneously, as approximately 2 to 3 percent of coeliac patients have selective IgA deficiency, producing false-negative tTG results. In IgA-deficient patients, IgG-based tests (deamidated gliadin peptide IgG) are substituted.
Thyroid Function
Thyroid dysfunction (hypothyroidism causes constipation, hyperthyroidism causes diarrhoea) mimics IBS symptoms and should be excluded in patients presenting with altered bowel habits. TSH is the screening test.
Vitamin D and B12
Vitamin D deficiency is common in IBD (40-70 percent of Crohn's patients) and has immunological implications. B12 deficiency occurs in terminal ileal Crohn's disease (where B12 is absorbed) and after ileal resection. Both should be checked in IBD patients and those with malabsorption syndromes.
Putting It Together
No single blood test diagnoses a gut condition. Results are interpreted as a panel — combining FBC, iron studies, inflammatory markers, organ function, and specific antibodies to build a clinical picture that guides further investigation.