H. Pylori Antigen

H. pylori is behind many peptic ulcers and much of the cancer that arises in the main body and lower part of the stomach. This test tells you whether that bacterium is living in your stomach right now, from a stool sample you collect at home.

The word "now" is the point. A blood antibody test can stay positive long after the infection is gone. Stool antigen testing is different: it looks for bacterial protein in stool, so it is used to diagnose active infection and to prove treatment worked.

What the test actually detects

H. pylori (Helicobacter pylori) is a spiral-shaped bacterium that lives in the mucus layer over your stomach lining and can persist there for decades. As the organisms shed material into the gut, bacterial proteins pass into stool.

Many labs use an enzyme immunoassay. In that method, lab-made antibodies bind to H. pylori proteins in the stool sample. The result is qualitative. It is a yes or no answer about whether the bug is present, not a number to optimize like cholesterol.

A positive result usually means live bacteria are colonizing your stomach and you have an active infection. A negative result means the test found no bacterial protein, so an active infection is unlikely, with some exceptions covered below.

Stomach Cancer Risk

Chronic H. pylori infection is the strongest known risk factor for non-cardia gastric cancer. That is cancer in the main body and lower part of the stomach. The infection can cause years of stomach lining irritation that slowly progresses toward precancer and then cancer in some people.

Meta-analyses estimate that H. pylori accounts for the majority of non-cardia gastric cancers, with estimates ranging from roughly 60 to 90 out of 100 depending on the population and how the studies are done. In a large German population cohort, people with the infection were about five times as likely to develop cancer of the main body of the stomach as those without it. If they carried a CagA strain, the risk was about eighteen times higher.

The risk is not fixed. In a Chinese trial followed for 26.5 years, clearing the infection cut long-term stomach cancer risk by about 43%, and by about 63% in people who had no precancerous changes yet.

That timing is why finding the infection early matters so much. The protection from treatment is greatest when you clear the bug before the lining has started to change. Once precancerous changes are more advanced, eradication still lowers risk but does not remove it, and ongoing endoscopic monitoring is still needed. A positive result is not a cancer diagnosis. It is a chance to remove the cause while the process may still be reversible.

Ulcers and MALT Lymphoma

H. pylori is a leading cause of peptic ulcers, the painful sores in the stomach and upper intestine. Clearing the infection lowers the chance of getting another ulcer.

The same infection is also linked to a rare stomach lymphoma called MALT lymphoma. In some people, that lymphoma can shrink or resolve once the bacteria are gone.

The Esophagus Wrinkle

One finding runs against the grain: people with the infection had a lower risk of esophageal adenocarcinoma in the same German cohort, about 65% lower. This is not a reason to keep the bug around. In some people, long-standing infection lowers stomach acid, which may ease the acid reflux that feeds this type of esophageal cancer.

Read the two findings together and the picture is consistent. H. pylori can reduce one reflux-linked cancer risk while sharply raising cancer risk inside the stomach. Eradication is still the goal.

Why This Beats a Blood Antibody Test

The common blood alternative measures IgG antibodies. These are immune proteins your body can keep making after an infection has cleared. So a positive antibody test cannot tell a current infection from an old one, and it cannot be used to prove cure after treatment.

Stool antigen and urea breath tests answer the question most people actually have: is H. pylori active now? In a 515-person health-check study, stool antigen testing was about as accurate as breath testing and biopsy-based methods, while serology had lower specificity. Specificity is about false positives. That is where antibody testing gets people into trouble.

How Accurate It Is

The best stool antigen tests are lab-run assays that use monoclonal antibodies. Monoclonal means the test uses one consistent antibody target. The exact kit still matters. In head-to-head comparisons, stool antigen kits varied widely in accuracy, so performance depends on which assay the lab runs.

These figures come from systematic reviews and head-to-head assay studies. If you are choosing a test, a lab-run monoclonal EIA is a better bet than a rapid strip, especially when you are confirming that treatment worked.

When Results Can Mislead You

The main way this test misleads is a false negative caused by medication. Proton pump inhibitors suppress H. pylori enough to push a real infection below the detection line. Omeprazole, esomeprazole, pantoprazole, lansoprazole, rabeprazole, and vonoprazan are in this group.

In one 218-person study, acid blockers cut stool antigen sensitivity by about one-fifth, and one assay found only about half of infected people while they were taking these drugs. The evidence is not uniform, since another study found antigen levels were not clearly suppressed after two weeks of one acid blocker. Either way, the standard advice is to stop proton pump inhibitors or vonoprazan for 2 weeks before testing when it is safe to do so. H2 blockers like famotidine can usually be used during that gap.

• Recent antibiotics or bismuth: these knock the bacteria down and can cause false negatives. Wait at least 4 weeks after finishing them before testing.
• Active upper-gut bleeding: stool antigen testing is less dependable during a bleeding ulcer. Retest after the bleeding has settled, often with a breath test or endoscopy-guided testing if the situation is urgent.
• Watery or loose stool: a runny sample can dilute the bacterial protein and lower accuracy, so a formed sample is better.
• Testing too soon after treatment: bacterial protein can linger for weeks. Test at least 4 weeks after therapy, and stay off proton pump inhibitors or vonoprazan for 2 weeks before the test.

Why One Test Is Rarely the End

Because this is a yes or no test, the value comes from the two-step loop: find the infection, then prove it is gone. A positive result should lead to eradication therapy. Treatment should be followed by a test of cure, because symptoms can improve even when bacteria survive.

Do the follow-up stool test at least 4 weeks after you finish antibiotics and at least 2 weeks after stopping any proton pump inhibitor or vonoprazan. After a confirmed cure, reinfection is uncommon in lower-prevalence settings like most of the United States, roughly around 1% per year, so routine yearly retesting is usually not needed unless symptoms return or household exposure continues. Some higher-risk groups, such as Alaska Native populations, have much higher recurrence rates.

What a Positive Result Should Make You Do

A positive test points to a clear next step. In general, everyone found to carry H. pylori is treated, even without symptoms, because eradication lowers ulcer and stomach cancer risk. After treatment comes a test of cure. If that first course fails, the pattern of failure guides what comes next. A stool PCR test reads H. pylori DNA rather than antigen. It can check for clarithromycin resistance and steer the next regimen.

Some situations call for a gastroenterologist rather than treating and moving on: trouble swallowing, unexplained weight loss, black stools, anemia, a family history of stomach cancer, or symptoms that persist after the bug is cleared. In those cases, endoscopy to look directly at the stomach lining is worth considering. If stool and breath tests disagree, confirm with another active-infection method before deciding you are infection-free.