A breath test is used to detect the presence of hydrogen or methane-dominant bacteria in the small intestine. The test is conducted by having the patient drink a solution of glucose or lactulose and then breathe into test tubes over a period of 3 hours. The difference in substrate, glucose or lactulose, is that they detect bacterial growth in different areas of the small intestine. Glucose is a sugar that is fermented in the upper (proximal) part of the small intestine; while lactulose is fermented in the lower (distal) part of the small intestine. In my practice, I prefer to do combination glucose and lactulose testing to ensure that the test is highly accurate and avoids any false negatives due to the limitations of using only one test.