Noninvasive Tests

Serology. The development of new techniques has minimized the problems of cross-reactivity that plagued first-generation serologic tests. Currently available tests are highly reliable. The large numbers of studies aimed at the discovery of an optimal diagnostic test for H.pylori infection have also provided valuable information on the immune responses to this organism. For example, the selection of H.pylori strains as sources of antigen is critical to the specificity and sensitivity of a test, and it is imperative to evaluate specific tests in the population to which it would be applied before selection of a test for use in specific settings. This population specificity highlights the importance of the many different geographic strains that infect different world populations.

Simplified “in-office” immunoenzymatic tests. Several in-office devices have been developed for the rapid detection of IgG anti– H.pylori antibodies. Most of them consist of disposable kits that provide a yes/no answer within a few minutes of placing a drop of serum in a well that is preabsorbed with antigen and an immunoenzymatic detection system. Although some of these tests are accurate, results have been generally less than the minimum required sensitivity and specificity of 90%. Antibodies (mostly of the IgG class) against H.pylori have been detected in the saliva and the urine of infected patients. The specificity and sensitivity of urine tests have been found to be satisfactory in several studies, particularly in Japan.

Stool antigen assay. An enzymatic immunoassay (HpSA) that detects H.pylori antigens in stools (thus providing information on the presence of current infection) has become available for the diagnosis of H.pylori infection and for monitoring the response to therapy. The test is similar to an enzyme-linked immunosorbent assay, using polyclonal anti– H.pylori antibody absorbed to microwells. A large European multicenter study has yielded encouraging results.

Urea breath tests. The urea breath tests are among the most important and innovative methods to detect H.pylori infection. These tests rest on the ability of H.pylori to produce large quantities of urease. The ingestion of a solution containing urea is rapidly followed, in an infected patient, by the production of ammonia and carbon dioxide. The latter rapidly appears in the subject’s breath. If the ingested urea is labeled either with the radioactive isotope 14C or with the nonradioactive isotope 13C, then the exhaled carbon dioxide will also be labeled and, therefore, measurable by an appropriate detection method. When 14C-labeled urea is used, the general method consists of the ingestion of a solution or a capsule containing quantities between 0.5 and 10 µCi of the isotope-labeled urea. When 13C-labeled urea is used, test subjects are given a solution of 125 g of 99.9% labeled urea followed by a meal aimed at increasing its permanence in the stomach. After a period of time, the subject inflates a balloon, which is immediately sealed and sent to a laboratory for the detection of the isotope-labeled carbon dioxide. Both types of tests are now well standardized and are approved by regulatory agencies in Europe and North America. The urea breath tests are extremely sensitive and specific and, in contrast to serologic tests, detect current active infection (not evidence of past infection). Their widespread use has made them more affordable, and they have become the test of choice for a variety of populations, including children, pregnant women, and patients who cannot undergo an endoscopic procedure.

Дата добавления: 2015-11-14