Blood tests for small intestinal bacterial overgrowth
On this page
In short: no blood test diagnoses SIBO; that requires a breath test or small-intestinal fluid culture1. Blood tests are still useful in two ways. Standard tests can show the effects of SIBO, such as low vitamin B12 and, often, high folate2. And the anti-CdtBCytolethal distending toxin B (CdtB) A toxin released by food-poisoning bacteria such as Campylobacter jejuni, E. coli and Salmonella. Antibodies against it can cross-react with vinculin and damage the gut nerves that run the migrating motor complex. and anti-vinculinVinculin A protein found in gut nerve cells. Anti-vinculin antibodies, which can form after food poisoning, are a marker of post-infectious IBS. antibody tests (sold as IBS-Smart) are markers of post-infectious, diarrhea-predominant IBS, a common background to SIBO. Their results have been mixed across studies35.
This guide covers which blood tests are useful for people with SIBO or suspected SIBO, and what they can and cannot tell you. For an overview of all tests, see our diagnosis guide.
Anti-CdtB and anti-vinculin antibodies
What they measure
Food-poisoning bacteria such as Campylobacter, E. coli and Salmonella make a toxin called cytolethal distending toxin B (CdtB). In animal studies, antibodies the body makes against CdtB cross-react with vinculin, a protein in gut nerve cells, producing an IBS-like condition3. The blood test measures antibodies against both. It is a marker of post-infectious IBSPost-infectious IBS Irritable bowel syndrome that develops after a bout of food poisoning. It is linked to anti-CdtB and anti-vinculin antibodies that impair gut motility., not a test for SIBO itself. See our IBS guide for how food poisoning links IBS and SIBO.
What the studies show
| Study | Who took part | Result |
|---|---|---|
| 2015 validation study3 | 2,375 people with IBS-D, compared with IBD, celiac disease and healthy controls | Both antibodies were higher in IBS-D. A positive anti-CdtB test was 92% specific but only 44% sensitive; anti-vinculin was 84% specific and 33% sensitive. Both were less able to tell IBS-D from celiac disease |
| 2019 second-generation test4 | 100 people with IBS-D and 31 with IBD | The updated test was over 90% specific for telling IBS-D from IBD, but detected only 43–52% of IBS-D |
| 2019 independent study5 | Community and hospital populations in Australia | The antibodies did not distinguish IBS with diarrhea from organic gut disease; anti-CdtB was higher in functional dyspepsia |
What this means: a positive result makes post-infectious IBS more likely, especially when the question is IBS versus inflammatory bowel disease. A negative result does not rule it out, because most people with IBS-D test negative34. The test does not tell you whether you currently have SIBO.
Nutrient blood tests
When SIBO is more severe, it can interfere with absorption26:
| Test | Typical finding in SIBO | Why |
|---|---|---|
| Vitamin B12 | Low (less common than raised folate) | Bacteria use up B12 before it is absorbed2 |
| Folate | Often high | Bacteria make folate26 |
| Vitamins A, D and E | Can be low | Bacteria break down bile acids needed to absorb fat6 |
| Vitamin K | Can be normal or high | Some gut bacteria make vitamin K6 |
| Full blood count, iron studies | Anemia in some people | From B12 or other deficiencies6 |
Vitamin B12 blood levels can be borderline. Methylmalonic acid and homocysteine rise when the body lacks B12 and can help confirm a deficiency7. A low B12 level alone does not prove SIBO; other causes such as pernicious anemia, diet and acid-reducing medicines need to be considered7. Our symptoms guide covers the symptoms of these deficiencies.
Tests to rule out other causes
Because SIBO symptoms overlap with other conditions, the US IBS guideline suggests a celiac blood test in people with IBS-type diarrhea8. Depending on your history, a doctor may also check thyroid function and blood sugar; an underactive thyroid, for example, is linked to SIBO9. Our causes guide covers these risk factors.
Tests with little evidence
Some clinics offer blood tests for “leaky gut”, such as zonulinZonulin A protein that loosens the tight junctions between intestinal cells. High levels are associated with increased intestinal permeability. or antibodies to bacterial toxins (LPSLipopolysaccharide (LPS) A molecule in the outer membrane of Gram-negative bacteria, also called endotoxin. When it crosses the gut barrier it triggers a strong inflammatory response.). A 2018 study found that a widely used commercial zonulin test does not actually detect zonulin, but other, related proteins10. These tests are not used to diagnose SIBO. See our leaky gut guide.
Common questions
Can a blood test diagnose SIBO?
No. SIBO is diagnosed with a breath test or small-intestinal fluid culture1. Blood tests show its effects or point to underlying causes.
Is the IBS-Smart test worth doing?
It can help when the question is whether diarrhea-predominant symptoms are post-infectious IBS rather than inflammatory bowel disease, where a positive result is fairly specific4. It misses most cases3, and one independent study could not reproduce its ability to separate IBS from organic disease5.
Why is my folate high with SIBO?
Bacteria in the small intestine make folate, which is then absorbed2. High folate together with low B12 is a pattern that can suggest SIBO, but it is not diagnostic.
Works Cited & Scientific References
10
▼
- ACG clinical guideline: small intestinal bacterial overgrowth - Am J Gastroenterol 2020 (PubMed)
- AGA clinical practice update on small intestinal bacterial overgrowth: expert review - Gastroenterology 2020 (PubMed)
- Development and validation of a biomarker for diarrhea-predominant irritable bowel syndrome in human subjects - PLoS One 2015 (PMC)
- Second-generation biomarker testing for irritable bowel syndrome using plasma anti-CdtB and anti-vinculin levels - Dig Dis Sci 2019 (PubMed)
- Circulating anti-cytolethal distending toxin B and anti-vinculin antibodies as biomarkers in community and healthcare populations with functional dyspepsia and irritable bowel syndrome - Clin Transl Gastroenterol 2019 (PubMed)
- Diagnosis and treatment of small intestinal bacterial overgrowth: an official position paper from the Brazilian Federation of Gastroenterology - Arq Gastroenterol 2025 (PMC)
- Vitamin B12 fact sheet for health professionals - NIH Office of Dietary Supplements
- ACG clinical guideline: management of irritable bowel syndrome - Am J Gastroenterol 2021 (PubMed)
- Association between hypothyroidism and small intestinal bacterial overgrowth - J Clin Endocrinol Metab 2007 (PubMed)
- Widely used commercial ELISA does not detect precursor of haptoglobin2, but recognizes properdin as a potential second member of the zonulin family - Front Endocrinol 2018 (PubMed)
Topics & Tags:
