Who Should Be Tested? See Helicobacter.
Refractory iron-deficiency anaemia is not considered a good reason for testing anymore.
Gold standard is Gastric biopsies (antrum and corpus) for histopathology during endoscopy. Initial diagnosis of H. pylori infection can be based on either positive histopathology + positive rapid urease test, or a positive culture.
Stool antigen test is pretty reliable. Could simply indicate asymptomatic carriage though. Once you know about it, of course, you need to consider eradication given cancer risk.
You should wait at least 2 weeks after stopping proton pump inhibitor (PPI) therapy and 4 weeks after stopping antibiotics before doing any of these tests.
Detection of antibodies against H. pylori (whether in blood, urine, saliva) are not reliable for use in the clinical setting.
[ESPGHAN and NASPGHAN. Evidence-based guidelines J Pediatr Gastroenterol Nutr. 2016]