Helicobacter pylori antigen testing is FDA approved for use as a noninvasive diagnostic test of H pylori infection and as a test to determine eradication after treatment.
H pylori antigen testing has 3 distinct forms.
The first developed was an enzyme immunoassay (EIA), which uses polyclonal anti– H pylori capture antibody absorbed to microwells. This qualitative test results in a color change that indicates a positive or negative result. There are specific cutoff values of certain optical density, read by spectrophotometry, that are set by manufacturers for positive, negative, and equivocal results.
An EIA utilizing a monoclonal antibody was then developed that eliminated equivocal readings, creating better differentiation between results.  Possible results are as follows:
Positive: H pylori antigen detected
Equivocal: See Interpretation
Negative: H pylori antigen not detected
Lastly, there is an immunochromatographic test with a monoclonal antibody. It has a positive line to indicate a positive reading accompanied by a control line.  Possible results are as follows:
Both positive and control lines present: H pylori antigen detected
Control line only present: H pylori antigen not detected
H pylori antigen testing is FDA approved for use as a noninvasive diagnostic test of H pylori infection and as a test to determine eradication after treatment.
In the pretreatment setting, positive results indicate infection with H pylori, and negative results indicate lack of infection. In the posttreatment setting, negative results indicate eradication if testing is performed at least 4 weeks after therapy is stopped. Positive results indicate treatment failure. Equivocal results with the polyclonal EIA test indicate that the optical density detected falls between positive and negative cutoff values, and the test likely needs to be repeated. 
Collection and Panels
H pylori antigen testing uses one fresh random stool sample.
Samples must be kept cool at 2-8°C and are useable for up to 3 days, after which they must be frozen (at -20°C).
Testing kits vary by manufacturer. In-office, rapid stool antigen tests allow the specimen to be placed directly into a single device used for testing, and results can be read within 10 minutes.
Watery stools (ie, diarrhea) or concentrated stools (ie, in patients with severe constipation) cannot be reliably used as samples because studies have not validated their efficacy.
Testing for infection eradication is more accurate when performed 4-8 weeks posttreatment. 
H pylori antigen testing is not typically performed within a laboratory panel but as a standalone test.
H pylori antigen is a protein constituent of the H pylori bacterium, which is shed in human stool. This bacterium finds its way into the body via oral-to-oral or fecal-to-oral transmission, typically in childhood. Risk factors include an infected sibling and poor living conditions.  The bacteria start in the gastric antrum, where they are most abundant, and move proximally.  Most of the bacteria invade the mucous layer, but approximately 20% attach to the gastric epithelium via outer membrane proteins. 
H pylori has multiple factors that promote its survival, including flagella that help it to move into the mucous layer despite gastric peristalsis. It also produces urease, which helps it survive the acidic environment by neutralizing acid and modifying the mucosal viscosity.  The bacterium can also lie dormant in the coccoid form, which improves its survival.  These factors allow H pylori to persist indefinitely.
H pylori causes chronic gastritis by manipulating the host’s immune system to cause an inflammatory response that predisposes to malignant transformation. It does this in part by invoking a large cytokine release and interacting with host immune cells. A critical example of its manipulation is its ability to recruit neutrophils and avoid phagocytosis by them. However, if it is engulfed, it manipulates the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system to release reactive oxygen species into the extracellular space to cause local damage. 
Although H pylori infection nearly guarantees chronic active gastritis, only 10%-15% of hosts develop peptic ulcer disease (PUD), and even fewer develop cancer. [3, 5] This appears to be related to a complicated interplay between host and bacteria. It is believed that certain strains of bacteria interact with the host’s immune system in such a way as to lead to severe persistent inflammation and potentially malignant transformation.
For example, CagA -positive strains of H pylori are believed to be quite virulent, causing increased cytokine production, leaky cell membranes, and other structural changes. These strains are believed to increase the risk of severe or atrophic gastritis and gastric adenocarcinoma. VacA, which forms vacuoles in cell membranes, leading to cell apoptosis, is a gene present in all strains but has particular virulence with certain sequence combinations. It targets not only gastric epithelial cells but also immune cells, providing carcinogenic potential and a way to evade the immune system, respectively.  Host genetic polymorphisms are thought to affect the degree of the host’s immune response. 
While pangastritis is affiliated with gastric ulcers and cancer, antral-predominant gastritis is associated with duodenal ulcers. It is not fully understood how H pylori causes duodenal ulcers, but the organism is linked to 80% of duodenal ulceration. It is believed that H pylori induces these ulcers by suppressing D-cell production of somatostatin, which leads to increased gastrin levels and thus greater acidity. This leads to metaplasia in the duodenum, allowing H pylori to colonize the duodenum and cause inflammation and ulceration. 
Mucosa–associated lymphoid tissue (MALT) lymphoma is also associated with H pylori infection. It has been proposed that T-cell interaction with H pylori antigen leads to cytokine production, which promotes unrestrained B-cell proliferation. 
H pylori cannot be cleared by the body, likely owing to the bacterium’s ability to manipulate the host’s immune system.  Ultimately, the bacterium must be eradicated through treatment. Triple therapy is the first-line treatment and consists of a proton pump inhibitor and two antibiotics given for 10-14 days. [7, 8]
H pylori is believed to infect about half of the total world population and about 35% of the population in the United States. It is most notably known to cause of PUD, gastric adenocarcinoma, and MALT lymphoma.  Thus, H pylori antigen testing has strong implications. It is approved for use in the diagnosis of H pylori infection and to determine eradication of infection after therapy is completed.
H pylori stool antigen testing is newer than its noninvasive constituents, serology and the urea breath test. Stool antigen testing has a similar application to urea breath testing, which was previously the noninvasive test of choice after therapy to establish eradication. Serology cannot be used because it detects antibody titers, which can persist long after successful therapy. 
All 3 tests are approved for use in diagnosing infection. The advantages of stool antigen testing include its ease, rapid results, cost effectiveness, and easy storage. The test requires only one sample and can be performed easily at home. New rapid in-office tests are being manufactured that can be read in less than 10 minutes, facilitating immediate treatment. 
Chronic gastritis is typically asymptomatic unless it leads to complications, such as PUD, carcinoma, or MALT lymphoma.  Early detection of H pylori and verification of treatment success are important to preventing complications. For instance, upon H pylori eradication, MALT lymphoma completely resolves in over 80% of cases.  However, it is unclear if infection elimination can decrease the risk of gastric lymphoma and adenocarcinoma. 
Noninvasive testing makes clinical and economic sense in the setting of symptomatic disease if the patient is young and symptoms are vague. Alarming symptoms, such as weight loss, bleeding, or dysphagia, and older age at onset, are more suggestive of malignancy. In this case, invasive testing with endoscopy and biopsy is indicated. Invasive testing may also be necessary upon treatment failure to attain antibiotic sensitivities via culture. 
Interestingly, H pylori infection may be protective against gastroesophageal reflux disease (GERD) and its complications, likely because it helps decrease the acid content through its production of urease. [8, 10] Thus, the elimination of H pylori infection for the primary prevention of gastric adenocarcinoma may in turn lead to esophageal cancer. Therefore, in the setting of asymptomatic disease, consideration may be given to the individual patient’s risk factors before testing. 
Investigation has revealed that H pylori stool antigen testing has particular implications for certain populations and circumstances. Cutoff values for EIA testing are set by manufacturers; however, studies show that the use of receiver operating characteristic (ROC) curve to set cutoffs yields more accurate results. Testing with ROC may help improve sensitivity and specificity in certain populations. Non-Western patients with higher-fiber diets and children younger than 5 years are two groups in whom testing is less sensitive and ROC may be beneficial. Results appear to be accurate in patients who have undergone partial gastrectomy and patients with end-stage renal disease (ESRD), but the test may not be appropriate in patients with cirrhosis. 
Although ROC may help improve sensitivity and specificity, there are certain circumstances in which false-positive and false-negative results appear to occur consistently. For instance, results are falsely positive in patients with gastrointestinal bleeding in some studies.  False-negative results have been shown to occur if, before testing, antibiotics or bismuth are used within 4 weeks or proton pump inhibitors are used within 2 weeks.  H pylori antigen testing appears to be more accurate at detecting eradication when performed 4-8 weeks posttreatment.