See below. The test that I think of for breath testing is for helicobacter pylori, which is a bacterial infection of the stomach that can cause gastritis or stomach ulcers. This test is accurate and currently seems to be preferred over blood testing for h. Pylori. There have been some breath testing for intestinal bacterial overgrowth problems, but I do not know if it is performed much anymore.
Urease breath test. The h pylori bacteria through its metabolic process produces a chemical that can be detected via a breath test.