Article Gastric ulcer H. pylori Helicobacter infection Helicobacter pylori Immunochromatography Sensitivity Serum antibody test Specificity Stool antigen test abdominal pain adolescent adult age blood cell count blood sampling controlled study developing country diagnostic accuracy diarrhea disease duration dyspepsia feces analysis female halitosis heartburn human immunoaffinity chromatography immunoassay major clinical study