Bloating after eating, stomach cramps, an unsettled gut. Many people experience these daily and blame stress or "eating the wrong things". But when symptoms persist, a blood test can provide surprising clarity. Not every cause shows up in blood work, but several important conditions do leave traces.
When is a blood test worthwhile?
See your doctor or consider testing if symptoms persist for more than a few weeks, keep returning, or come with other complaints such as unexplained weight loss, fatigue or blood in the stool.
1. CRP: detecting inflammation
CRP (C-reactive protein) rises in response to inflammation. In someone with gut complaints, elevated CRP may indicate inflammatory bowel disease (IBD) such as Crohn's disease or ulcerative colitis, or a bacterial infection. In irritable bowel syndrome (IBS), CRP is typically normal, which helps distinguish functional from inflammatory conditions.
2. Ferritin: a sign of malabsorption
Low ferritin in someone with gut symptoms is an important signal. It may mean your intestines are not absorbing nutrients properly (malabsorption). Common causes include coeliac disease, IBD and chronic diarrhoea. If your ferritin is low despite adequate dietary iron intake, your gut deserves closer investigation.
3. Coeliac markers: tTG-IgA
Coeliac disease is an autoimmune condition where gluten damages the intestinal lining. It affects roughly 1 in 100 people, but the majority are undiagnosed. The screening blood test is tTG-IgA. Important: you must still be eating gluten at the time of testing for reliable results. Do not start a gluten-free diet before being tested.
Symptoms that may point to coeliac disease: chronic bloating, diarrhoea, fatigue, unexplained iron deficiency, skin rashes and joint pain.
4. White blood cells: infection and inflammation
White blood cells (leukocytes) indicate whether your body is actively fighting infection or inflammation. Elevated levels may suggest bacterial infection or IBD. Combined with CRP, they give a clear picture of whether an active inflammatory process is occurring.
5. TSH: the thyroid-gut connection
The link between your thyroid and digestive system is often overlooked. TSH can reveal thyroid dysfunction that directly affects gut motility. An underactive thyroid slows digestion (causing constipation and bloating), while an overactive thyroid speeds it up (causing diarrhoea).
Food intolerance vs. food allergy
A food allergy involves the immune system (IgE-mediated) and can be severe. A food intolerance is a digestive problem (e.g., lactose intolerance). Coeliac disease is neither: it is an autoimmune condition. Commercial IgG food tolerance tests sold online are not considered reliable by medical science.
Frequently asked questions
Can a blood test explain all digestive complaints?
No. It is a valuable first step, but conditions like IBS or food intolerances are not diagnosed through blood work alone. Further investigation may be needed.
I have been diagnosed with IBS. Is a blood test still useful?
Yes. IBS is a diagnosis of exclusion. If you have never been tested for coeliac disease or inflammatory markers, it is worth doing so.
When should I see a doctor immediately?
Blood in the stool, unexplained weight loss, persistent vomiting, severe abdominal pain, or new symptoms after age 50 are alarm signs that require prompt evaluation.
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