SIBO — small intestinal bacterial overgrowth

What is SIBO?

Small Intestinal Bacterial Overgrowth (SIBO) is defined as an abnormal increase in the concentration and/or an altered composition of bacteria in the small intestine. The small bowel is normally sparsely populated because of gastric acid, biliary and pancreatic secretions, intact peristalsis and the ileocaecal valve. When one or more of these defences fails, colonic bacteria colonise the small intestine and ferment carbohydrates locally — producing the hydrogen (H₂) and methane (CH₄) we measure on the breath test.

Symptoms include bloating (especially after meals), gas, abdominal pain, diarrhoea or constipation and, in more severe cases, malabsorption with weight loss and nutritional deficiencies. The picture overlaps substantially with irritable bowel syndrome (IBS): 30–60 % of IBS patients test positive for SIBO in clinical series.

Risk factors and causes

• Altered motility: post-infectious IBS, diabetes, systemic sclerosis, hypothyroidism
• Anatomical abnormalities: previous abdominal surgery, adhesions, diverticula, blind loops
• Reduced gastric acid: long-term proton pump inhibitor (PPI) therapy
• Immune deficiencies (including selective IgA deficiency)
• Chronic pancreatitis with reduced enzyme output

Diagnosis: hydrogen and methane breath test

The breath test is the standard non-invasive diagnostic modality for SIBO. The patient drinks a defined substrate (glucose, lactulose or fructose) and exhaled breath is sampled every 15–20 minutes over 2–3 hours. If bacterial overgrowth is present in the small intestine, fermentation produces H₂ and/or CH₄ that is absorbed into the blood and exhaled — the earlier and higher the rise, the more proximal the overgrowth.

Diagnostic thresholds (North American Consensus, Rezaie 2017)

• Hydrogen (H₂): rise ≥ 20 ppm above baseline within 90 minutes = positive for SIBO.
• Methane (CH₄): value ≥ 10 ppm at any time point = positive for methane-predominant intestinal overgrowth (IMO).
• Combined H₂ + CH₄: assessed together — methanogens metabolise hydrogen, so patients with high CH₄ may show a paradoxically low H₂ curve.

Substrate choice

• Glucose (75 g): absorbed proximally — high specificity, lower sensitivity. Preferred when proximal overgrowth is suspected.
• Lactulose (10 g): non-absorbable — reaches the whole small bowel. Higher sensitivity, lower specificity; risk of false positives from rapid orocaecal transit.
• Fructose (25 g): used specifically when fructose malabsorption is suspected.

Preparation

• 24 hours before: avoid fermentable carbohydrates (legumes, whole grains, cabbage, onion, dairy, complex fibre)
• 12 hours before: fast completely — only water
• 4 weeks before: no antibiotics
• 1 week before: no probiotics or laxatives
• Day of test: no smoking, no vigorous exercise, no chewing gum

Treatment

A positive test is treated in the clinic with targeted antibiotic therapy plus support of the underlying cause and diet.

• Hydrogen-predominant SIBO: rifaximin 550 mg × 3 daily for 14 days. Rifaximin is a non-absorbed antibiotic that acts locally in the small intestine — very few systemic side effects.
• Methane-predominant IMO: rifaximin 550 mg × 3 + neomycin 500 mg × 2 for 14 days.
• Diet: temporary low-FODMAP diet during and immediately after treatment; long-term dietary variety to avoid maintaining an altered microbiome.
• Address underlying causes: optimise motility (prokinetics if relevant), reassess PPI indication, treat structural or endocrine drivers.

Post-treatment retesting is considered in relapsing cases or when the initial diagnostic decision hinges on it. Recurrence is common (up to 40 % within 6–12 months) when the underlying cause is not corrected.

SIBO and IBS

There is a substantial overlap between IBS and SIBO. If your IBS has not responded to conventional treatment, or if bloating and gas dominate, a breath test can identify a treatable cause. We combine SIBO diagnostics with the full IBS assessment per Rome V — including red-flag screening, calprotectin and, when indicated, endoscopy — so the plan addresses both the underlying picture and the functional layer.

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