Helicobacter pylori (H. pylori) bacteria are a common cause of peptic ulcers (sores in the lining of the stomach, small intestine, or esophagus). In this test, a stool (feces) sample is used to determine if H. pylori antigens are present in your child’s gastrointestinal (GI) system. Antigens are substances that trigger the immune system to fight infection.