A stomach bacterium called Helicobacter pylori (H. pylori) can cause ulcers in the stomach and upper part of the small intestine. This test involves drinking a small amount of a compound called urea that has been tagged with a radioactive carbon atom (C-13). The urea is broken down by the H. pylori bacteria in the stomach, and the tagged carbon is exhaled in the breath. The amount of tagged carbon in the breath is measured to see if H. pylori is present.