The best test for diagnosing SIBO is the lactulose breath test. Lactulose is a sugar that is made up of a galactose molecule bound to a fructose molecule. Humans do not have the ability to break this bond and therefore cannot utilize this sugar for energy. Bacteria, however, are able to break this bond and utilize this sugar for energy.
The breath test measures levels of methane and hydrogen that we exhale after drinking a lactulose solution. These gases are produced by bacteria in our gut as a by-product of their metabolism and are gases that we do not normally emit. We can determine the types and quantity of bacteria in our gut by measuring the levels of hydrogen and methane present in our breath.
Some bacteria produce methane and others produce hydrogen. Having an overgrowth of hydrogen producing bacteria typically cause symptoms of diarrhea. Having an overgrowth of methane producing bacteria typically cause symptoms of constipation. You can have one or the other or a combo of the two. These different types of bacteria require different types of antibiotics or treatment methods.
To learn more about this test check out the NCNM SIBO Testing Center, which includes a short video about how to do the test.