Helicobacter pylori (H. pylori) is a common type of bacteria that people usually get infected with during childhood. In most cases, the infection does not cause symptoms. However, some people with H. pylori infection eventually develop inflammation of the stomach (gastritis) or ulcers in the stomach or upper small intestine. Gastritis and ulcers cause abdominal pain and, sometimes, bleeding. Doctors often treat gastritis and ulcers caused by H. pylori with a combination of several antibiotics that are given for several days. Treatment gets rid of H. pylori in most patients, but 10% to 20% of patients may have persistent infection despite treatment. Continuing abdominal pain does not reliably identify patients with persistent H. pylori infection because there are other reasons why people can have continued pain. Ideally, a simple noninvasive test could be used to identify patients who do not respond to traditional treatment and who need additional treatment. One proposed test is a stool test (called the stool antigen test). The accuracy of this test has not been well studied.