Invasive Tests

Histopathological examination of gastric biopsy specimens. Helicobacter spp. can be detected in histological preparations of gastric biopsy specimens stained with a variety of methods. Hematoxylin and eosin is a suboptimal choice of stains for the specific task of detecting H.pylori. Reliable special stains include the Warthin-Starry and the Steiner silver stains, the Giemsa, Diff-Quick, and Gimenez stains, and a triple stain, which, by combining modified Steiner staining, hematoxylin and eosin, and Alcian blue, allows the simultaneous visualization of the features of gastritis, including intestinal metaplasia, and the bacteria. Several modified versions of this stain have become available. Anti– H.pylori antibodies for the immunohistochemical detection of H.pylori in paraffin-embedded biopsy specimens have high sensitivity and specificity; some laboratories use them for routine clinical diagnosis.

In situ hybridization and polymerase chain reaction. In situ hybridization may be used for the detection of H.pylori in paraffin-embedded sections, but high cost and technical difficulties have relegated this procedure to the research laboratory. Polymerase chain reaction for the detection of H.pylori infection must also be considered a research tool because of its requirement for a sophisticated molecular biology laboratory, the availability of appropriate primers, and its high price.

Smear, brush, and touch preparations. Smears of gastric mucus and exfoliated epithelial cells, usually with Gram staining, may allow the detection of bacteria within minutes of the endoscopic procedure. Rapid urease tests have made cytologic assays obsolete.

Bacterial culture. H.pylori is best cultured in a microaerophilic and humid atmosphere on culture media requiring fresh horse or sheep blood and antibiotics to suppress contaminants. Cultures for H.pylori are technically more complicated than those performed by the usual clinical microbiology laboratory. Because many clinical facilities are not equipped to perform the time-consuming procedures necessary to culture H.pylori, several methods for transportation have been devised.

Rapid urease tests. These assays exploit the high content of urease of H.pylori. A fragment of gastric mucosa is placed into a broth or in agar containing various concentrations of urea. The urease produced by H.pylori hydrolyzes the urea and releases ammonia, which raises the pH of the broth or agar, and an appropriate indicator (e.g., phenol red) changes color as the pH increases. In the first commercially produced rapid urease test, the CLOtest, the original yellow gel capsule into which the specimen is placed becomes red within minutes to hours, depending on the quantity of bacteria present. Several rapid urease tests are now commercially available. Both their specificity and sensitivity, compared with histopatho­logical examination, are extremely high, in most cases approaching 100%.

Дата добавления: 2015-11-14