SIBO

What is SIBO?

The small intestine normally holds far fewer bacteria than the colon. Gastric acid, peristalsis, bile, pancreatic secretions and the ileocaecal valve all keep numbers low. When one of these defences fails, bacteria can multiply in a segment of gut where they compete for nutrients, deconjugate bile acids and ferment carbohydrates before they are absorbed.

The American College of Gastroenterology guideline (Pimentel et al., Am J Gastroenterol, 2020) defines SIBO as excessive numbers of bacteria in the small bowel causing gastrointestinal symptoms. It is a problem of location and quantity rather than a global shift in the microbiota, so it is not the same thing as colonic dysbiosis, although the two ideas overlap. The guideline also separates out intestinal methanogen overgrowth (IMO), driven by methane-producing archaea.

Risk factors and symptoms

There is usually something that predisposes to it:

• impaired gut motility, for instance diabetic neuropathy or systemic sclerosis;
• surgery that alters the anatomy, such as blind loops or resection of the ileocaecal valve;
• reduced gastric acid, including long-term proton pump inhibitor use;
• small bowel diverticula, Crohn’s disease or chronic pancreatitis.

Typical symptoms overlap heavily with irritable bowel syndrome and with sugar malabsorption, such as fructose intolerance, so symptoms alone cannot establish the diagnosis.

How it is tested

1. Duodenal or jejunal aspirate culture. Regarded as the reference method; the ACG guideline proposes a threshold of ≥10³ CFU/mL. It requires endoscopy and is not standardised between centres.
2. Breath test with glucose or lactulose. Non-invasive. Under the 2017 North American Consensus (Rezaie et al., Am J Gastroenterol), a hydrogen rise of ≥20 ppm above baseline within 90 minutes is positive, and methane ≥10 ppm at any point is methane-positive.

Breath testing has known pitfalls. With lactulose, fast transit can produce false positives; glucose is absorbed early and may miss distal overgrowth. Results are always read alongside the clinical history.

Example

A gastroenterologist sees a woman with long-standing systemic sclerosis, daily bloating and loose stools. He requests a glucose breath test measuring both hydrogen and methane. The report shows hydrogen climbing 26 ppm above baseline at 45 minutes, with low methane throughout. Taken with the clinical picture, this is consistent with hydrogen-predominant SIBO; treatment choices and management of the underlying motility disorder rest with the specialist.

SIBO at Xenogene

Xenogene runs a SIBO breath test that quantifies hydrogen and methane by gas chromatography, using CO₂ to correct the quality of each sample. The substrate is lactulose (10 g) or glucose (75 g) depending on the clinical indication, and samples are collected over 2-3 hours after dietary preparation. The report shows both gas curves; within an integrated panel alongside microbiota analysis, the AI assistant Sophia helps the clinician relate the gas pattern to the other markers.

The test is available on prescription from a healthcare professional, who interprets the result. For more on preparation and reading the curves, see the blog guide to the SIBO breath test.