Small Intestinal Bacterial Overgrowth (SIBO) occurs when excessive microorganisms, typically large intestine flora, colonize the small intestine. This imbalance causes uncomfortable symptoms, including chronic diarrhea, bloating, and abdominal pain. The hydrogen and methane breath test is the primary non-invasive diagnostic method. It measures gasses produced by these microbes after consuming a specialized sugar solution, detailing activity within the digestive tract.
The Mechanics of SIBO Breath Testing
The SIBO breath test relies on the principle that only microorganisms produce hydrogen (\(\text{H}_2\)) and methane (\(\text{CH}_4\)) by fermenting carbohydrates. These gasses are absorbed into the bloodstream, travel to the lungs, and are exhaled in the breath for measurement. The test begins with a baseline measurement of \(\text{H}_2\) and \(\text{CH}_4\) before the substrate is consumed. This baseline confirms correct fasting and ensures readings are not elevated due to recent food intake.
The patient drinks a sugar solution (the substrate), which feeds any bacteria present in the small intestine. Breath samples are collected at regular intervals, usually every 15 to 20 minutes, for up to three hours. If an overgrowth is present, bacteria ferment the substrate, producing a measurable rise in gas levels early in the testing period. The pattern of gas production helps localize where the fermentation is occurring.
Interpreting Hydrogen SIBO Results
Hydrogen-dominant SIBO is diagnosed when a significant rise in \(\text{H}_2\) gas is detected early, indicating small intestine fermentation. The standard diagnostic criterion, according to the North American Consensus, is a rise of 20 parts per million (ppm) or more above the baseline reading. This elevation must occur within the first 90 minutes to be considered a positive result.
The timing of this rise is crucial. Since the substrate typically reaches the large intestine around 90 minutes, an early peak suggests bacterial fermentation in the small intestine. A rise occurring significantly later than 90 minutes is likely a normal colonic response. Hydrogen-dominant overgrowth is associated with symptoms of diarrhea.
Analyzing Methane Levels (Intestinal Methanogen Overgrowth)
Methane gas in the breath is interpreted differently from hydrogen, leading to a diagnosis of Intestinal Methanogen Overgrowth (IMO) rather than SIBO. Methane is produced by single-celled organisms called Archaea (methanogens), not bacteria. These methanogens consume the hydrogen produced by bacteria to create methane.
The diagnostic threshold for IMO is a methane level reaching 10 ppm or higher at any point during the breath test. Unlike hydrogen SIBO, the absolute level of methane is the determining factor for a positive diagnosis, whether it occurs in the small or large intestine. High methane levels are associated with slower transit time and are linked to symptoms of constipation.
The Significance of Substrate Choice (Lactulose vs. Glucose)
The choice of sugar solution (substrate) significantly impacts result interpretation. The two most common substrates are lactulose and glucose, each having unique digestive properties. Glucose is a simple sugar rapidly absorbed by the small intestine, typically within the first few feet.
Because glucose is absorbed quickly, a positive result is highly specific for overgrowth in the proximal (upper) small intestine. However, glucose may fail to detect overgrowth located further down, potentially leading to a false-negative result. In contrast, lactulose is a synthetic sugar that is not absorbed and travels the entire length of the small intestine.
Lactulose’s non-absorbable nature means it can detect microbial overgrowth anywhere in the small intestine, making it more sensitive for distal SIBO. The drawback is that lactulose eventually reaches the large intestine, which can lead to a false-positive result if transit time is very fast. Interpreting a lactulose test requires careful consideration of the time taken for the gas to rise to distinguish true small intestinal fermentation from normal colonic activity.