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Small intestinal bacterial overgrowth: ACG updates

M3 India Newsdesk Jun 23, 2022

The ACG clinical guideline on Small Intestinal Bacterial Overgrowth (SIBO) provides criteria for diagnosis, defines the optimal methods for diagnostic testing, and outlines the treatment options. This article throws light on the main recommendations from the guideline.

Small intestinal bacterial overgrowth (SIBO), defined as excessive bacteria in the small intestine, remains a poorly understood disease. SIBO may be more prevalent than previously thought.

Definition of SIBO

SIBO is defined as a clinical syndrome of gastrointestinal (GI) symptoms caused by the presence of excessive numbers of bacteria within the small intestine. There must be measurable and excessive bacterial burden within the small bowel. Also, the microbial overgrowth must result in specific GI signs and/or symptoms.

For threshold cut-off of a positive culture, the North American Consensus suggests considering a bacterial colony count of ≥103 colony-forming units per millilitre (CFU/mL) in a duodenal/jejunal aspirate as a diagnostic of SIBO.

For breath testing, a rise in exhaled hydrogen of at least 20 parts per million (ppm) above baseline within 90 minutes of oral ingestion of either 75 g of glucose or 10 g of lactulose, is diagnostic of SIBO.

Most common symptoms in patients with SIBO:

  • Abdominal pain, bloating, gas, distension, flatulence, and diarrhoea
  • In severe cases, nutritional deficiencies including vitamin B12, vitamin D, and iron deficiencies can be found
  • Some patients may also experience fatigue and poor concentration

No single symptom can be specifically attributed to SIBO as the symptoms of SIBO often overlap with IBS, functional diarrhoea, functional dyspepsia, or bloating. Hence, in a patient presenting with unexplained abdominal pain, gas, bloating, diarrhoea, and/or malabsorptive symptoms, the symptoms should be evaluated along with risk factors for SIBO and any treatment history.

Diagnosis of SIBO

Recommendation 1: The guideline suggests the use of breath testing (glucose hydrogen or lactulose hydrogen) for the diagnosis of SIBO in patients with IBS.

Recommendation 2: The guideline suggests using glucose hydrogen or lactulose hydrogen breath testing for the diagnosis of SIBO in symptomatic patients with suspected motility disorders.

Recommendation 3: The guideline suggests testing for SIBO using glucose hydrogen or lactulose hydrogen breath testing in symptomatic patients (abdominal pain, gas, bloating, and/or diarrhoea) with previous luminal abdominal surgery.

Human cells are incapable of producing hydrogen and methane gases, and this fact forms the basis of breath tests. If hydrogen and methane gases can be detected in breath samples, it points out another source such as the fermentation of carbohydrates by microbes in the gut, their subsequent absorption into the bloodstream, and their expiration through the lungs.

Performing and interpreting breath test results – The North American consensus

  • Before breath testing, it is recommended that patients avoid the use of antibiotics for 4 weeks and avoid promotility agents and laxatives for at least 1 week.
  • The day before the breath test, fermentable foods (e.g., complex carbohydrates) should be avoided, and patients should fast for 8–12 hours.
  • In addition, during the breath test, patients should avoid smoking and minimize physical exertion.
  • Administering 75-g glucose or 10-g lactulose, either taken with or followed by 1 cup of water (∼250 mL) is recommended.
  • The breath samples should be measured for hydrogen and methane. An increase in hydrogen concentrations of ≥20 ppm from baseline within 90-120 minutes is recommended to be diagnostic of SIBO.

Substrates in breath testing for SIBO

  1. Glucose and lactulose are the most common substrates used in breath testing for SIBO.
  2. When using lactulose as a substrate, an initial peak from bacterial overgrowth in the small intestine followed by a second peak from colonic bacterial fermentation has been reported. However, as per the North American Consensus, a second peak is not required; but the first peak must occur within 90 minutes of substrate administration for the test to be considered positive.
  3. The sensitivity of lactulose range from 31% to 68% and specificity ranges from 44% to 100%.

Gastric acidity and proton-pump inhibitors

Recommendation 4:

  1. It is suggested to not use breath testing for the diagnosis of SIBO in asymptomatic patients on proton-pump inhibitors (PPIs).
  2. Gastric acidity prevents the overgrowth of bacteria in the upper GI tract; hence patients with hypochlorhydria or achlorhydria, secondary to autoimmune gastritis, or partial or total gastrectomy are at increased risk of SIBO.
  3. PPIs are frequently used to treat unexplained GI symptoms, gastroesophageal reflux disease, ulcers, and functional dyspepsia. Studies show that people using PPIs are at a higher risk of developing SIBO.

Methane production and IMO

Recommendation 5:

  1. The guideline suggests testing for methane using glucose or lactulose breath tests to diagnose the overgrowth of methane-producing organisms (IMO) in symptomatic patients with constipation.
  2. The North American Consensus defines a positive methane breath test as the presence of methane levels of ≥10 ppm during the breath test.
  3. Deciphering the role of methane in breath testing has been a matter of great interest. Numerous studies show that a positive methane breath test is associated with constipation, with the level of methane on the breath is proportional to the degree of constipation.
  4. As excessive methane production is rather due to archaeal overgrowth, hence the term intestinal methanogen overgrowth (IMO) has been coined for methanogens rather than SIBO. Another point which supports the use of the term “IMO” is the presence of methanogens throughout the intestinal tract and not just the small bowel.

Treatment of SIBO


Recommendations 6:

  1. We suggest the use of antibiotics in symptomatic patients with SIBO to eradicate overgrowth and resolve symptoms.
  2. Before initiating antibiotic therapy, it is suggested to objectively diagnose SIBO. As SIBO usually recurs following a course of antibiotic therapy, retreating SIBO with another course of antibiotics is a frequent practice.
  3. It should be noted that there are no universally accepted treatment approaches to therapy; the practice of antibiotic retreatment is completely based on anecdotal evidence and expert opinion.


Dietary changes may be beneficial in the treatment of SIBO. The main idea in diet manipulation for SIBO is the reduction of fermentable products.

Probiotics- The concept of using probiotics to treat a condition with excessive bacteria seems counterintuitive.

Click here to see references


Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.



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