How SIBO Is Diagnosed: Which Test and When

In short: SIBO is diagnosed either by culturing fluid from the small intestine (more than 1,000 bacteria per milliliter) or, much more often, with a glucose or lactuloseLactulose A synthetic sugar the body cannot digest or absorb. As a breath-test substrate it travels the whole small intestine, which also makes false positives more likely. breath test¹. Testing makes most sense when you have typical symptoms, such as bloating, diarrhea or abdominal discomfort, and a risk factor such as slow gut movement or previous gut surgery⁴. Stool tests and microbiome panels cannot diagnose SIBO¹⁹.

This guide is an overview of how SIBO is diagnosed. For details on each test, see our guides to breath tests, endoscopy and fluid culture, blood tests and stool tests.

Who should be tested?

SIBO causes symptoms that overlap with many other conditions: bloating, diarrhea and abdominal pain or discomfort, and in more severe cases fatty stools (steatorrhea)². The European breath test guideline suggests testing when these symptoms occur without another explanation on endoscopy or imaging, especially when there is an underlying condition that raises the risk of SIBO⁴.

Common risk factors include⁶:

• Slow gut movement, for example from diabetes, connective tissue disease or opioid medicines
• Low stomach acid, including from long-term acid-reducing medicines (PPIsProton pump inhibitor (PPI) A class of acid-reducing medication used for reflux, such as omeprazole. Long-term use lowers stomach acid and is associated with a higher risk of SIBO.)
• Structural changes such as strictures from Crohn’s disease, small-intestinal diverticula, gastric bypass or removal of the ileocecal valveIleocecal valve The muscular valve where the small intestine meets the colon. It keeps colonic bacteria from flowing backward into the small intestine.
• Chronic pancreatitis, cystic fibrosis and immune deficiencies

Our root causes guide covers each in detail. If you are unsure whether your symptoms fit, our symptom checker is a starting point for a conversation with your doctor.

Warning signs such as blood in the stool, unintended weight loss, anemia, difficulty swallowing or symptoms that wake you at night need medical assessment first, whatever the cause.

The two main tests

The US guideline accepts both methods¹, and the European guideline recommends breath testing “until a true gold standard is established”⁴. In practice, most people are diagnosed with a breath test, and methane overgrowth can only be diagnosed this way⁵.

Choosing a breath test

Glucose is absorbed in the upper small intestine, so it gives fewer false positives but can miss overgrowth further down. Lactulose travels the whole small intestine but reaches the colon, where fast transit can create a false positive⁴.

Compared with fluid culture, pooled data show the glucose test detects about 54% of cases with 83% specificity, while the lactulose test detects about 42% with 71% specificity⁵. In a 2011 imaging study of 40 people with IBS, 5% or more of a lactulose test meal had already reached the colon when hydrogen rose in 22 of 25 positive tests⁸. For these reasons the European guideline recommends glucose for people who have not had gut surgery⁴. A test that also measures methane is now the standard⁵, and newer three-gas tests add hydrogen sulfide.

Our breath test guide covers preparation step by step.

Other tests and what they add

• Blood tests can show the effects of SIBO, such as vitamin B12 deficiency, and folate is often high because bacteria make it². They cannot diagnose SIBO itself. Antibody tests for 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. were developed as markers of post-infectious, diarrhea-predominant IBS, not SIBO¹⁰. See our blood tests guide.
• Biopsies taken during endoscopy are usually normal in SIBO. In a 2010 study, more than half of the biopsies from people with SIBO looked normal; the only finding more common than in controls was flattened villi (24% vs. 7%)¹¹.
• Stool tests reflect the colon and cannot diagnose SIBO. Calprotectin is not useful for detecting SIBO², and a 2025 international consensus found insufficient evidence to recommend microbiome panels in routine practice⁹. Targeted stool tests can rule out other causes of symptoms. See our stool tests guide.

Making sense of the result

Breath test results should be read together with symptoms and risk factors. The European guideline states that the diagnostic criteria “have not been confirmed and uniformly accepted”⁴. A 2020 meta-analysis found that lactulose tests diagnosed SIBO 3.6 times more often than glucose tests in people with IBS, and 7.6 times more often in healthy controls⁷, a sign that a positive lactulose result in someone at low risk may be a false positive.

• Positive result with typical symptoms and a risk factor: treatment is usually discussed. See our guides to antibiotics, herbal antimicrobials and the elemental diet.
• Positive result with few symptoms or risk factors: consider whether it could be a false positive, especially with lactulose.
• Negative result but strong suspicion: a different substrate, a test that measures methane or hydrogen sulfide, or fluid culture may help.

Our breath test interpreter shows how your readings compare with the published cut-offs.

Common questions

What is the most accurate test for SIBO?

Culture of small-intestinal fluid is considered the reference test, but it is invasive and imperfect, and breath tests are used far more often¹⁴. Of the breath tests, glucose has better specificity than lactulose⁵.

Can a doctor diagnose SIBO from symptoms alone?

No. The symptoms overlap with IBS, celiac disease, food intolerances and other conditions, so a test is needed to confirm SIBO¹.

Do I need to retest after treatment?

Breath tests are often repeated after treatment to check whether the overgrowth has cleared, especially before deciding on another round³.