Testing for H. pylori
infection has become a very important part of the diagnostic process for gastric and duodenal inflammatory disease, since the presence or absence of infection determines the type of treatment to be applied. Testing is also a useful means of monitoring the effectiveness of courses of antimicrobial treatment. A number of different diagnostic test methods, both invasive and non-invasive, are available.
The collection of biopsy specimens from inflamed or ulcerated regions of the stomach and duodenum during invasive endoscopy is considered to be the reference method for diagnosing H. pylori
infection. The biopsy material can be examined using one, or more, of three different test methods.
- Histological examination – staining and examination of the tissue samples allows both evaluation of cell damage and the detection of H. pylori cells in situ. Histological identification is currently regarded as the ‘gold standard’ of diagnostic tests.
- Urease test – a colorimetric test for the presence of urease enzyme activity (indicative of H. pylori infection) in the biopsy sample. This method can be used to give a rapid indication of infection at the time of the biopsy.
- Culture and isolation of H. pylori – the tissue sample may be homogenised, or inoculated directly onto selective agar media. Typically, cultures are incubated for at least 3-5 days at 35oC under microaerophilic conditions. Isolates can be confirmed as H. pylori by Gram staining, urease reaction and other traditional identification tests. PCR-based methods have also been developed, but are not yet widely available. Culture and isolation is necessary if antimicrobial susceptibility testing is required.
Invasive testing is considered to be highly specific – particularly histological examination – but its sensitivity is partly dependent on the accuracy of the biopsy procedure, since sampling tissue from areas where only low numbers of bacteria are present may give false-negative results. Endoscopy is also an expensive and demanding procedure requiring highly trained, skilled staff and is quite uncomfortable for the patient.
Both histological examination and culture of biopsy samples are reliable methods, but they are time consuming and require specialised laboratory facilities with highly trained staff. Urease testing is much more rapid and less costly, but still requires an invasive procedure to obtain the samples.
Several techniques for non-invasive diagnostic tests for H. pylori
have been developed and tests based on three of these are in widespread use.
People infected with H. pylori
generally have specific IgG and IgA antibodies circulating in their blood and these can be detected by serological tests. A number of commercially available tests have been developed, mostly based on IgG detection, which is considered slightly more sensitive and specific. Antibody detection is usually by laboratory-based qualitative enzyme immunoassay (EIA) methods presented in a microplate assay format, although automated immunoassay systems have also been developed. Rapid tests that can be applied at the point-of-care are also available, either based on immunochromatographic technology (lateral flow tests) or on latex agglutination. Quantitative tests are considered to be of limited value.
The sensitivity and specificity of serological testing varies from about 80% to 95% depending on the specific assay, but it is less time consuming and much less costly than invasive methods and means considerably less discomfort for the patient. However, serological testing cannot be used to monitor the progress of antimicrobial therapy, since patients whose infection has been eradicated may continue to carry serum antibodies specific to H. pylori
for several months after successful treatment.
The breath test utilises the ability of H. pylori
to produce large quantities of urease as a diagnostic characteristic. The patient is required to drink a solution of urea labelled with 13C, or 14C isotopes. If an H. pylori
infection is present, the urea will be metabolised by the bacteria, producing ammonia and labelled carbon dioxide, which can be detected in the patient’s exhaled breath by radioactive counting or mass spectrometry. A breath test based on 13C-labelled urea and using infrared spectrophotometry to measure the ratio of 13C to 12C in a breath sample has been developed into a commercially available product for use at the point-of-care.
Breath testing has been found to have a high sensitivity and specificity (94-98%) and can be applied at moderate cost. It is also suitable for monitoring the effectiveness of treatment as it is specific to active infections and can be used to confirm eradication.
Recently, new tests for H. pylori
infection have been developed that rely on the detection of specific antigens in the stools of infected individuals. These are usually EIA-, or amplified EIA-based tests and several are available in a lateral flow format for use at the point-of-care.
Stool antigen testing has been found to be reliable, inexpensive and easy to use. It also has the advantage of indicating only active infection. This means that it can be used to monitor the eradication of infection by antimicrobial treatment and can also detect repeat infections.