CLINICAL QUESTION: How does a noninvasive stool antigen immunoassay for detection of Helicobacter pylori infection compare with standard invasive diagnostic methods?
BACKGROUND: The reference standard for the diagnosis of H pylori infection is endoscopic biopsy and histopathologic confirmation of the organism, which is often impractical and cost-prohibitive for patients presenting with dyspepsia. Noninvasive tests include serum antibody titers, the carbon-13 urea breath test, and stool testing with either polymerase chain reaction or antigen enzyme immunoassay. Stool testing may represent an alternative noninvasive diagnostic approach that is both inexpensive and acceptable to patients.
POPULATION STUDIED: The investigators enrolled 104 consecutive patients undergoing upper endoscopy from a university gastroenterology practice. Sex distribution slightly favored men (57%).
STUDY DESIGN AND VALIDITY: All of the patients underwent a rapid urease test, histology, and culture. The reference standard for H pylori infection was a positive result for at least 2 of the 3 tests. The first stool following endoscopy was tested for the presence of H pylori antigen using the Premier Platinum HpSA Immunoassay (Meridian Diagnostics; Cincinnati, Ohio).
The study design was straightforward and readily reproducible. The investigators avoided workup bias by performing all 4 diagnostic tests on all study participants. The authors do not explicitly state whether the tests were conducted in a blinded manner, thus it is unclear if expectation bias was avoided. They detail, however, which investigators performed various study tasks, and from that information we can presume that the tests were performed in a blinded fashion. Because of possible referral bias, a higher prevalence of endoscopic disease would be expected in the study population when compared with that seen in a typical family practice setting.
OUTCOMES MEASURED: The investigators determined the endoscopic diagnoses for all patients. They also calculated the sensitivity, specificity, positive predictive value, and negative predictive value for the stool test.
RESULTS: The endoscopic diagnoses were: nonulcer dyspepsia, 48%; active ulcer disease, 32%; gastric or duodenal erosions, 16%; and gastric polyps, 4%. The sensitivity and specificity for the stool test were 96% (95% confidence interval [CI], 90.6%-100%) and 93% (95% CI, 85.1%-99.5%), respectively. The positive and negative predictive values were 92% and 96%, respectively. The investigators did not calculate likelihood ratios (LRs); however, the data supplied allows the reader to do so. The positive LR was 12.5, and the negative LR was 0.04. Given a prevalence of H pylori infection of 40% (typical for the primary care setting), a positive test result would increase the likelihood of infection to 89%, and a negative result would decrease the likelihood to 3%.
This study describes an accurate inexpensive noninvasive stool antigen immunoassay test for detection of H pylori infection. The cost per assay is approximately $27.1 The test compares favorably with the current reference standard. It represents an improvement over other noninvasive diagnostic methods such as the carbon-13 urea breath test (less costly and more convenient) and serologic antibody titers (at least as accurate and more rapid than laboratory assays). Although confirmation of this study’s findings in a primary care setting would be helpful, the advantages of this test make it an immediately favorable alternative to other available methods.