The gold standard diagnostic process for SIBO involves microbial examination of intestinal aspirates.4 However, hydrogen-breath tests (HBT) (which measure hydrogen and methane production by gut microbiota) are more frequently used as it is a non-invasive, cheaper diagnostic tool.3
The HBT involves drinking a solution of lactulose or glucose and providing breath samples every 20 minutes over a 3-hour period. The bacterial fermentation of carbohydrates in the intestine produces hydrogen. In approximately 15%-30% of the population who have a specific strain of gut bacteria (Methanobrevibacter smithii), the hydrogen is converted to methane.5 A positive SIBO case, diagnosed by a glucose HBT, will show a rise in breath hydrogen. A positive SIBO case, diagnosed by a lactulose HBT, will show a double peak of hydrogen and methane breath production.6
There are significant limitations in the diagnosis of SIBO. For example, confounding factors need to be tightly controlled to ensure they do not influence test results. HBT requires the patient to adhere to strict guidelines prior to the testing. This may include the avoidance of low glycaemic index (GI) carbohydrates and fibre-rich foods for 24-hours prior to the test as well as no cigarette smoking or exercise for two hours prior to the test.
Medications that can increase the risk of developing SIBO, including proton pump inhibitors, should be discontinued during the days leading up to the test. If these guidelines are not followed correctly, the hydrogen content and excretion in the breath can be affected, which may negatively impact results, thus leading to inaccuracies in interpretation of results and diagnosis.6
Additional diagnostic limitations are outlined below:
- HBT is not a validated diagnostic tool
- The ability to accurately interpret results and define SIBO is limited1
- HBT assumes that the patient has a normal gut transit time; it assumes that mouth-to-cecum transit time is always greater than 90 minutes, however studies have identified that there is great individual variation6
- Functional symptoms associated with SIBO are similar to IBS and other GI disorders, which could lead to mis-diagnosis
- There are a subset of people who are asymptomatic with SIBO2
In light of the above, test results should be interpreted with caution by a suitably qualified medical professional, alongside a detailed clinical history. Even then, results are not free from ambiguity and they only complete part of the picture.