SIBO

What is SIBO?

Small Intestinal Bacterial Overgrowth (SIBO) describes a condition where the small intestine has an abnormally elevated number and/or an altered composition of bacteria compared with the normal, relatively bacteria-sparse environment.^[1],[2]

SIBO is most often investigated with a breath test. This is an indirect investigation that measures microbially produced hydrogen and methane in exhaled breath. The result may support an assessment of SIBO or intestinal methanogen overgrowth (IMO), but cannot be interpreted in isolation — it always forms part of an overall medical assessment.^[3],[4]

Symptoms of suspected SIBO — bloating, flatulence, irregular bowel habits, abdominal pain and early satiety — are non-specific and occur in many other conditions. Symptoms alone therefore cannot be used to make or exclude a diagnosis of SIBO.^[1],[5]

What can the breath test — and what can it not — show?

Professional societies do not agree on how much diagnostic value the breath test has. The ACG Clinical Guideline, the AGA Clinical Practice Update and the North American Consensus describe the test as a useful, non-invasive tool when used in patients with relevant clinical suspicion.^[1],[2],[3] Established consensus

Conversely, more recent critical reviews and the European guideline point out that the test has significant limitations: both false positive and false negative results occur, and the test's value depends heavily on how likely the diagnosis is beforehand (pre-test probability) and on the clinical context.^[5],[4] Debated

The time it takes for the test substrate to reach the caecum from the stomach (oral-caecal transit time) especially affects the lactulose test, because lactulose passes through the entire small intestine and on to the colon. The test can therefore be significantly affected by transit time and by fermentation in the colon, which can produce false positive results.^[3],[4]

Glucose is mainly absorbed proximally in the small intestine and so may be less sensitive to overgrowth further down the small bowel. On the other hand, the glucose test may have higher specificity than the lactulose test in certain populations. The two substrates each have different strengths and weaknesses, and the choice depends on the clinical question.^[3],[1]

How can overgrowth arise? A multifactorial picture

SIBO is not simply a matter of colonic bacteria migrating up into the small intestine. It is thought to reflect an interplay between several systems: gut motility (including the migrating motor complex), anatomical factors, secretory barriers (gastric acid, bile, pancreatic enzymes), immune function and changes in the local microbiota.^[1],[2]

• Motility: Reduced function of the migrating motor complex (MMC), which normally sweeps food residue and bacteria down into the colon between meals, can contribute to overgrowth.
• Anatomy: Structural factors such as diverticula, strictures or previous gastrointestinal surgery (e.g. Roux-en-Y) can create stasis.
• Secretion: Low gastric acid (e.g. with long-term PPI use), reduced biliary secretion or exocrine pancreatic insufficiency can weaken the chemical barriers against bacteria.
• Immune function and microbiota: Changes in gut immune defences and in the composition of the wider microbiota may also play a role.

With prolonged, marked overgrowth, bacteria may deconjugate bile salts, which can impair fat absorption and contribute to reduced absorption of the fat-soluble vitamins A, D, E and K. The bacteria may also consume some of the patient's vitamin B12, while bacterial synthesis can simultaneously produce elevated folate levels. Such findings should always be assessed together with the overall clinical picture.

PPI use has been reported in observational studies as an associated factor for SIBO, but the association is not necessarily causal, and long-term PPI use is not on its own an indication for a breath test. The need for investigation always depends on the overall symptom picture and clinical assessment.^[1],[5]

SIBO and IBS — a debated overlap

Studies have reported a higher rate of positive breath tests in some IBS populations, but estimates vary considerably depending on patient selection, substrate and diagnostic criteria. A positive breath test does not by itself demonstrate that SIBO is the cause of the IBS symptoms.^[5],[6],[8] Debated

IBS is defined in the Rome criteria as a condition with recurrent abdominal pain linked to altered bowel habit, without requiring a positive breath test.^[7]

Gas types: what they can tell us — and what they cannot

It is not possible to determine which gas predominates from symptoms alone. Symptom-based classification into "hydrogen type", "methane type" and "H₂S type" is not used as a diagnostic method — only the breath test's own gas measurements can say anything about which gases are present.^[3],[5]

• Methane is associated in studies with slower gut transit and constipation, but a symptom picture of constipation cannot on its own be used to diagnose methane overgrowth.^[1] Conditional recommendation
• Hydrogen sulphide (H₂S): Hydrogen sulphide measurement is a newer method. There is not yet a universally accepted diagnostic threshold or standardised treatment algorithm.^[2] New or non-standardised method

The ACG Clinical Guideline primarily covers hydrogen-based SIBO and methane/IMO. This is not a matter of three equally established microbiological phenotypes.^[1]

When is a breath test relevant?

A breath test is relevant in selected symptomatic patients — particularly with documented or suspected dysmotility, relevant anatomical changes, previous gastrointestinal surgery, systemic sclerosis, significant malabsorption or other concrete clinical suspicion. The test is not a routine investigationand is not recommended without an independent clinical suspicion — the result is always interpreted together with the overall clinical picture.^{[1],[2],[4],[5]}

Schematic overview

The breath test in practice

The patient drinks a standardised amount of substrate (glucose or lactulose) dissolved in water after fasting and a preparatory diet. Exhaled breath is then measured continuously over 2–3 hours.^[3]

• Hydrogen (H₂): A rise of at least 20 ppm above baseline within 90 minutes is used in the North American Consensus to support a positive result. The threshold is consensus-based and has known limitations.^[3] Established consensus
• Methane (CH₄ / IMO): A methane level of at least 10 ppm at any point may support a diagnosis of intestinal methanogen overgrowth in a relevantly symptomatic patient.^[3],[1] Conditional recommendation
• Lactulose: Lactulose passes through the small intestine and on to the colon. The test can therefore be significantly affected by oral-caecal transit time and fermentation in the colon.^[4]
• Glucose: Glucose is mainly absorbed proximally and so may be less sensitive to more distal overgrowth. The test may have higher specificity than lactulose in certain populations.^[3]

None of these thresholds alone proves or diagnoses SIBO or IMO — they may support a clinical assessment, which is always made by the treating doctor.

Preparing for the breath test

When you book, you receive the clinic's written, standardised preparation guide. It generally follows these principles:

• Avoid antibiotics for the period stated in the clinic's instructions before the test.
• Avoid fermentable and slow-digesting foods the day before the test.
• Fast overnight before the test.
• No smoking, no vigorous exercise and no chewing gum on the morning of the test.
• Brush your teeth before the test.

Important: Prescribed medication — including PPIs and antibiotics — must never be stopped without prior agreement with the treating doctor. Any adjustment of medication as part of preparing for the test must always be agreed individually.

Testing for fructose malabsorption

Fructose is not part of the SIBO breath test. Suspected fructose malabsorption is investigated as a separate question with a fructose breath test, where there is clinically relevant suspicion of it.

Duodenal aspiration with culture

A more invasive method is aspiration of small-intestinal content during gastroscopy, with culture of bacterial counts. The method is used less often in clinical practice because of its invasiveness and variation in cut-off values between laboratories.^[1]

Next steps after the breath test

A breath-test result is assessed together with symptoms, risk factors, previous surgery, medication and other possible explanations. A positive result does not automatically lead to antibiotic treatment. The treating doctor assesses whether there is sufficient clinical indication and whether an underlying cause should be investigated or treated.^[1],[2]

A low-FODMAP diet may give some patients temporary symptom relief, but the diet does not eradicate any bacterial overgrowth and does not prevent relapse.^[6] Low certainty evidence

Prognosis

Relapse occurs in some patients, particularly if the underlying cause has not been clarified or corrected. With irreversible anatomical changes or severe dysmotility, SIBO is more often seen as a condition that requires ongoing follow-up and an individually tailored plan, determined by the treating doctor.^[1],[2]

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