SIBO (small intestinal bacterial overgrowth) breath testing measures the production of hydrogen and methane gases by bacteria in the gut. These breath test results offer important insights into bacterial overgrowth.  First, individuals ingest glucose or lactulose, which are fermented by bacteria in the small intestine. By assessing the changes in gas production during the breath test, SIBO can be diagnosed. This article is part one of a three-part series discussing common trends and diagnostic criteria for both glucose and lactulose breath testing.

Glucose Breath Test Results

Glucose is absorbed in the small intestine relatively quickly, making it an excellent substrate for detecting bacterial overgrowth in the upper part of the small intestine.

Positive Glucose SIBO Breath Test Results (Hydrogen or Methane Elevation within 120 minutes)

• Hydrogen Rise: An increase in hydrogen gas of 12 ppm or more within the first 120 minutes meets the diagnostic criteria for SIBO (Figure 1). This indicates that bacteria in the small intestine are fermenting glucose before it can be fully absorbed.

  

• Peak Methane Levels: A peak methane level of 10 ppm or more within 120 minutes suggests the presence of methane-producing bacteria in the small intestine, meeting the criteria for SIBO (Figures 2 & 3). Methane-producing bacteria are often associated with constipation.

  

  Figure 3 Peak Methane Level: Methane gas activity elevated throughout test (baseline to 180 minutes).

• Combined Hydrogen + Methane Rise: An increase in the combined total of hydrogen and methane gas of 12 ppm or more within the first 120 minutes meets the diagnostic criteria for SIBO (Figure 4). This indicates bacteria in the small intestine ferment the glucose before it can be fully absorbed.

  

   

Negative Glucose SIBO Breath Test Results

In healthy individuals, glucose absorbs before it reaches the large intestine, and there is no significant rise in hydrogen or methane within the first 120 minutes (Figure 5). Gas production occurring after 120 minutes reflects normal fermentation in the large intestine.

 

Lactulose Breath Test Results 

Lactulose is not absorbed by the body and passes through both the small and large intestines, making it useful for detecting bacterial overgrowth in both regions. 

Lactulose Positive SIBO Breath Test Results (Early Hydrogen or Methane Rise within 120 minutes)

• Hydrogen Rise: A hydrogen increase of 20 ppm or more within the first 120 minutes meets the diagnostic criteria for SIBO, indicating fermentation in the small intestine by bacteria (Figure 6).Figure 6 Hydrogen Rise: An increase in hydrogen gas. Lactulose is not absorbed in the intestines. Therefore, large intestinal bacterial fermentation is observed (typically 140-180 minutes).
• Peak Methane Levels: A peak methane level of 10 ppm or more within 120 minutes suggests the presence of methane-producing bacteria in the small intestine. This meets the criteria for SIBO (Figures 2 & 3). Methane-producing bacteria are often associated with constipation.
• Combined Hydrogen + Methane Rise: An increase in the combined total of hydrogen and methane gas of 15 ppm or more within the first 120 minutes meets the diagnostic criteria for SIBO (Figure 4). This indicates bacteria in the small intestine ferment lactulose. 

Lactulose Negative SIBO Breath Test Results

In a negative test, there will be no significant gas rise in the first 120 minutes. Additionally, gas production typically rises after this period, reflecting normal lactulose  fermentation within the large intestine (Figure 5).

Common Trends in Hydrogen and Methane Gas Production

• Hydrogen-Dominant SIBO: Patients with diarrhea-predominant symptoms (IBS-D) often show elevated hydrogen levels. A sharp rise in hydrogen gas, particularly early in the breath test, suggests bacterial fermentation of glucose or lactulose in the small intestine. (Figures 1 & 6)
• Methane-Dominant SIBO: Patients with constipation-predominant symptoms (IBS-C) usually present with elevated methane levels. High methane gas, especially when peaking at 10 ppm or more within 120 minutes, indicates the presence of methanogenic archaea. The presence of these bacteria can slow intestinal transit. (Figures 2 & 3)
• Mixed Hydrogen and Methane Production: Some patients may show elevated levels of both hydrogen and methane gases, leading to a mix of symptoms such as alternating diarrhea and constipation. This pattern can indicate the presence of both bacterial overgrowth and methanogens in the small intestine. (Figure 4)
• Double Peak Patterns:  For lactulose, a double peak may be observed. If SIBO is present, the first peak occurs within 120 minutes due to small intestinal fermentation. As lactulose reaches the colon, fermentation by colonic bacteria cause the second peak (Figure 7). Double peak results used to be diagnostic for SIBO but are not anymore due to improvements in testing methodology. However, double peaks seen on testing can assist in differentiating between small and large intestine specimen assessments. 

  

Conclusion

Both glucose and lactulose are good substrates for SIBO breath testing. For glucose breath tests, a rise in hydrogen levels or combined gases of 12 ppm or more within 120 minutes meets the criteria for diagnosing SIBO. For lactulose breath tests, hydrogen levels rising by 20 ppm or more or 15 ppm (combined gases) within 120 minutes also indicate SIBO. Peak methane levels of 10 ppm or more indicate methane-dominant SIBO. These tests help clinicians distinguish between hydrogen-dominant and methane-dominant SIBO, leading to more targeted treatment plans.

This is the first blog in a three-part series. Part 2 discusses fructose, lactose, and sucrose sugar malabsorption patterns. In part 3, learn about atypical breath test patterns.