Helicobacter pylori Stool Antigen and Serology

Helicobacter pylori is a spiral Gram negative bacterium that colonises the gastric mucosa and causes chronic gastritis, peptic ulcer, gastric adenocarcinoma and MALT lymphoma. Tests are divided into non-invasive (stool antigen, urea breath test, serology) and invasive (endoscopic biopsy for rapid urease test, histology, culture and molecular testing).

Stool antigen testing is the main laboratory test, used for diagnosis and for confirming eradication. Serology detects past or present infection and cannot confirm eradication.

About 44% of the world population is infected, with prevalence over 70% in parts of Africa, Asia and South America and under 30% in many high income countries. Most carriers are asymptomatic. The lifetime risk of peptic ulcer is 10 to 15%, and of gastric cancer 1 to 3%. H. pylori is responsible for 75% of gastric cancer. Resistance to clarithromycin and metronidazole exceeds 15 to 20% in many regions and has made empirical therapy less reliable.

Indications. Peptic ulcer disease (active or past), dyspepsia in patients under the age threshold for endoscopy (test and treat), MALT lymphoma, early gastric cancer after resection, first degree relatives of patients with gastric cancer, unexplained iron deficiency anaemia, immune thrombocytopenia, long term low dose aspirin or NSAID use with ulcer risk, before long term proton pump inhibitor therapy in selected patients, and confirmation of eradication after treatment.

Specimen. Stool, a formed sample of 1 to 5 g in a clean container, refrigerated and tested within 3 days or frozen. Serum for antibody testing. Gastric biopsy (antrum and body) for invasive tests. Patients stop proton pump inhibitors 2 weeks, and antibiotics and bismuth 4 weeks, before non-serological tests.

Stool antigen test.

• Monoclonal enzyme immunoassay and rapid immunochromatographic formats detect H. pylori antigen in stool.
• Monoclonal laboratory assays: sensitivity 90 to 95%, specificity 92 to 97%. Polyclonal assays and rapid tests perform less well (sensitivity 80 to 90%).
• Positive: active infection. Negative: no active infection, if the patient was off proton pump inhibitors and antibiotics for the required period.
• Used to confirm eradication at least 4 weeks after finishing treatment.

Serology (IgG).

• Detects antibody, with sensitivity 85 to 90% and specificity 80 to 85%. Positive in past and present infection, and antibody remains for months to years after eradication.
• Useful only in high prevalence populations or patients who cannot stop proton pump inhibitors, in bleeding ulcer (where other tests have false negatives), in gastric atrophy or MALT lymphoma, and has no role in confirming cure.
• Local validation is needed, and a positive result in a low prevalence area with no prior treatment supports a decision to treat.

Urea breath test. Has the highest accuracy (sensitivity and specificity over 95%) and is the alternative to stool antigen for diagnosis and confirmation of eradication.

Invasive tests.

• Rapid urease test on antral biopsy: sensitivity 85 to 95%, specificity 95 to 100%. Reduced by proton pump inhibitors and bleeding.
• Histology with Giemsa or immunohistochemistry: assesses gastritis, atrophy and intestinal metaplasia. Two biopsies from the antrum and one from the body or incisura improve yield.
• Culture and susceptibility testing, and molecular tests (PCR for clarithromycin resistance mutations in 23S rRNA, gyrA for fluoroquinolone): used after treatment failure, and where resistance is high.

Treatment implications. Regimens contain a proton pump inhibitor with combinations of amoxicillin, clarithromycin, metronidazole, tetracycline, bismuth, rifabutin or levofloxacin for 10 to 14 days. Bismuth quadruple therapy is used where clarithromycin resistance is 15% or more.

Critical values and findings needing immediate action.

• Bleeding peptic ulcer: endoscopic haemostasis first. Test at endoscopy or later once stable, with repeat testing when the first is negative.
• Positive test in a patient with alarm features (dysphagia, weight loss, anaemia, persistent vomiting, age over 55 with new dyspepsia): endoscopy, not treatment alone.
• Gastric MALT lymphoma: eradication is first line, and staging and follow up endoscopy are done.
• Persistent positive stool antigen after two treatments: culture and susceptibility, and specialist referral.

Interferences and pitfalls.

• Proton pump inhibitors, antibiotics and bismuth cause false negative stool antigen, breath and urease tests, by reducing bacterial load. H2 blockers have less effect.
• Gastrointestinal bleeding, gastric atrophy, intestinal metaplasia and gastric cancer reduce bacterial density, and false negatives occur on all tests except serology.
• Serology cannot separate current from past infection and is not used to confirm eradication.
• Watery diarrhoea dilutes stool antigen and gives false negatives. Samples are taken from formed stool.
• Stool antigen performance varies between brands, and kits not validated locally can mislead.
• Eradication is confirmed in all patients with ulcer, MALT lymphoma, early gastric cancer or persistent symptoms. Reinfection in adults is under 2% a year in high income countries and higher in other regions.
• Eradication reduces gastric cancer risk most when done before atrophy, and surveillance endoscopy continues in patients with extensive atrophy or metaplasia.
• Penicillin allergy limits regimens and should be checked.

Special groups.

• Children: testing is reserved for ulcer disease or refractory iron deficiency, and not for functional abdominal pain. Stool antigen is the preferred non-invasive test.
• Pregnancy: testing and treatment are deferred.
• Older adults: higher rates of atrophy and false negative non-invasive tests.
• Regions: high prevalence and high antibiotic resistance in parts of Asia, Africa and Latin America, with population screening for gastric cancer prevention in East Asia.

Follow up tests. Confirmation of eradication by stool antigen or urea breath test at least 4 weeks after treatment (and 2 weeks off proton pump inhibitors), full blood count and ferritin, vitamin B12 in atrophic gastritis, endoscopy with biopsy where indicated, serum pepsinogen I and II in gastric cancer risk assessment, and susceptibility testing after treatment failure.