Various tests have been developed for the
detection of H. pylori, each with their specific advantages and
disadvantages. The available tests are generally divided into invasive tests,
based on gastric specimens for histology, culture, or other methods, and
noninvasive tests, based on peripheral samples, such as blood, breath samples,
stools, urine, or saliva for detection of antibodies, bacterial antigens, or
urease activity. The choice of a specific test for an individual patient
depends on local experience and the clinical setting. In research protocols, a
combination of two methods is often applied. In daily clinical practice, use of
a single test is generally adequate, and most tests are sufficiently accurate
to be used for this purpose. For routine diagnostic purposes, histology, urea
breath testing, and culture are currently most often used, whereas the use of
serology is most appropriate for large epidemiological studies. In
hospital-based care, many patients undergo endoscopy, which is then combined
with an invasive test for H. pylori. Otherwise, breath tests and serology are
commonly used. For children, fecal antigen tests offer the opportunity to
assess H. pylori status
without the need for endoscopy or vena puncture.
Endoscopic Diagnostic Tests (Invasive tests)
In patients who have not been on a PPI (proton pump
inhibitor) within 1–2 wk or an
antibiotic or bismuth within 4 wk of endoscopy, the rapid urease test (RUT)
provides an accurate, inexpensive means of identifying H.
pylori. For patients who have been
taking a PPI, antibiotics, or bismuth, endoscopic testing for H. pylori should include biopsies from the gastric
body and antrum for histology with or without rapid urease testing. Though
culture or polymerase chain reaction (PCR) are the primary means by which
antibiotic sensitivities can be determined, neither is widely available for
clinical use and therefore, cannot be routinely recommended.
There
are presently four biopsy-based diagnostic methods for H. pylori infection.
These include the RUT, histology, culture, and PCR.
Rapid Urease Testing-
The
RUT identifies active H. pylori infection through the organism’s urease
activity. Gastric biopsies are obtained and placed into an agar gel or on a
reaction strip containing urea, a
buffer, and a pH-sensitive indicator. In the presence of H. pylori’s urease,
urea is metabolized to ammonia and bicarbonate leading to a pH increase in the
microenvironment of the organism. A change in color of the pH sensitive
indicator signifies the presence of active infection. Commercially available
kits yield results in 1–24 h. Medications that reduce the density and/or urease
activity of H. pylori, such as bismuth-containing compounds,
antibiotics, or PPIs, can decrease the sensitivity of the RUT by up to 25%.
Though controversial, acute ulcer bleeding at the time of testing may decrease
the sensitivity and negative predictive value of the RUT. As a result of the
patchy distribution of H. pylori infection after antibiotics or PPIs, it
is recommended that biopsies for the RUT be obtained
from
two sites, the body at the gastric anglularis and greater curvature of the
antrum. The simplicity, low cost, and relatively rapid results make the RUT a
practical and cost effective
means
of testing for H. pylori in patients not taking antibiotics, bismuth, or
PPIs who require upper endoscopy. Unfortunately, the usefulness of the RUT in
routine clinical
practice
has been compromised by the widespread use of PPIs as an empiric treatment for
upper GI symptoms. As such, the RUT can rarely be used as a sole means of
identifying H. pylori infection. More commonly, the RUT is combined with
other endoscopic or non endoscopic
modalities to establish the presence or absence of this infection. No studies
have been performed to define the duration of
a PPI’s deleterious
effects on the sensitivity of the RUT. Data with the urea breath test (UBT)
suggest that PPI therapy can cause false-negative test results for 1–2 wk (68,
69). As the UBT and RUT rely upon the identification of H. pylori’s urease
activity, it is reasonable to suggest that PPIs should be withheld for 1–2 wk before
performance of the RUT. In situations where a patient has not taken a PPI for a
period of 1–2 wk before their procedure, the sensitivity of the RUT is likely
sufficient to justify its use as a single test for H. pylori.
Histology-
Histology
has been considered by some to be the gold standard for detection of H.
pylori. Unfortunately, histology is an imperfect gold standard as the
detection of H. pylori relies upon a number of issues including the
site, number, and size of gastric biopsies, method of staining, and the level of
experience of the examining pathologist. A significant advantage of histology
over other diagnostic methods is the ability to evaluate for pathologic changes
associated with H. pylori infection such as inflammation, atrophy,
intestinal metaplasia, and malignancy. Certainly the absence of chronic
gastritis is a potent negative predictor for the presence of H. pylori infection.
As the prevalence and density of H. pylori varies throughout the
stomach, particularly in the face of medications that may reduce the density of
H. pylori, multiple biopsies are needed for accurate diagnosis. It is
therefore recommended that a minimum of three biopsies be obtained, one from
the anglularis, one from the greater curvature of the corpus, and one from the
greater curvature of the antrum, to maximize the diagnostic yield of histology.
A recent study found that the addition of corpus biopsies to antral biopsies increased
the detection of H. pylori infection by 10% when compared with antral
biopsies alone. Similar to the RUT, the sensitivity of histology is
significantly affected by the use of medications such as bismuth, antibiotics,
and PPIs. Although widely available and capable of achieving sensitivity and
specificity of >95%, the cost and need for properly trained.
Culture
Culture
is another highly specific method for identifying active H. pylori infection.
Conceptually, culture is attractive because it not only provides a means by
which to identify infection, but also allows characterization of antimicrobial
sensitivities. Unfortunately, culture is not as sensitive as RUT or histology.
Furthermore, culturing techniques for H. pylori are demanding and costly
and as a consequence, only available in a limited number of clinical
laboratories. Nonculture-based means of determining antibiotic resistance are
being developed but have not been adequately standardized and are not widely
available.
Polymerase Chain Reaction
PCR
is a DNA amplification technique that utilizes the rapid production of multiple
copies of a target DNA sequence to identify H. pylori. This testing
method is highly specific and may be more sensitive than other biopsy-based
diagnostic techniques. A recent study found that PCR was able to detect H.
pylori in approximately 20% of gastric biopsies with chronic gastritis but
no identifiable organisms by histology PCR also provides a means of identifying
mutations associated with antimicrobial resistance (78–80). Although presently
restricted to the research arena, this method may some day provide a practical,
reproducible method for antibiotic sensitivity testing, organism typing, and
organism virulence testing.
Non endoscopic Diagnostic Tests (Non-Invasive
tests)
Antibody testing is inexpensive and widely available.
The UBTs and fecal antigen tests provide reliable means of identifying active H.
pylori infection before antibiotic therapy.
The UBT is the most reliable non endoscopic test to document eradication of H.
pylori infection. The monclonal fecal
antigen test provides another non endoscopic means of establishing H.
pylori cure after antibiotic treatment.
Testing to prove H. pylori eradication
appears to be most accurate if performed at least 4 wk after the completion of antibiotic
therapy.
There
are currently three non endoscopic diagnostic testing methods for H. pylori infection.
Antibody testing identifies an immunological reaction to the infection while
the non endoscopic urease tests and fecal antigen test identify the presence of
active H. pylori infection.
Antibody Tests
Antibody
testing relies upon the detection of IgG antibodies specific to H. pylori in
serum, whole blood, or urine. IgG antibodies to H. pylori typically
become present approximately 21 days after infection and can remain present
long after eradication. Antibodies to H. pylori can be quantitatively assessed
using enzyme-linked immunosorbent assay(ELISA) and latex agglutination
techniques or qualitatively assessed using office-based kits. The advantages of
the antibody tests are their low cost, widespread availability, and rapid
results. Unfortunately, several factors limit the usefulness of antibody
testing in clinical practice. According to studies, the antibody test kits
shows about 85% sensitivity and 79% specificity. One of the limitation is that
antibody tests developed using antigens from one region of the world may not
perform well when applied to patients in another part of the world suggesting that
local validation may be necessary. Finally, antibody tests are of little
benefit in documenting eradication as results can remain positive for years following
successful cure of the infection.
Urea Breath Tests
The
UBT, like the RUT, identifies active H. pylori infection by way of the
organism’s urease activity. In the presence of H. pylori, the ingestion
of urea, labeled with either the nonradioactive
isotope
13C or the radioactive isotope 14C, results in production
of labeled CO2, which can be
quantitated in expired breath. Although the amount of radiation in the 14C
UBT is less than daily background radiation exposure, the 13C test
is preferred in children and pregnant females. Overall, the performance
characteristics of both tests are similar with sensitivity and specificity
typically exceeding 95% in most studies. Test reproducibility has been found to
be excellent. The UBT also provides an accurate means of post treatment testing.
Most tests utilize a citrate test meal (50–75 mg), which is administered before
the labeled urea. A urease blood test, which relies upon the detection of
labeled bicarbonate in a blood sample, also reliably identifies active H.
pylori infection before and after treatment. As the non endoscopic urease tests
rely upon the identification of H. pylori’s robust urease activity, test
sensitivity is decreased by medications that reduce organism density or urease
activity, including bismuth containing compounds, antibiotics and PPIs. It is
currently recommended that bismuth and antibiotics be withheld for at least 28
days and a PPI for 7–14 days prior to the UBT. The UBT is more costly than the
antibody tests or fecal antigen test. The expense of the UBT is largely driven by
equipment costs and the cost of labeled urea. UBTs using lower dose 13C,
which have recently been found to yield excellent performance characteristics,
may in part address this issue.
Fecal Antigen Test
The
fecal antigen test (FAT) identifies H. pylori antigen in the stool by
enzyme immunoassay with the use of polyclonal anti-H. pylori antibody.
Recently, a stool test utilizing a monoclonal
anti-H.
pylori antibody has been evaluated. As both tests detect bacterial
antigen(s) suggestive of ongoing infection, they can be used to screen for
infection and as a means of establishing cure following therapy. Similar to the
UBT, the sensitivity of the FAT is affected by the recent use of bismuth
compounds, antibiotics, and PPIs. Recent studies also suggest that the
specificity of the FAT is reduced in the setting of bleeding peptic ulcer
disease and, for this reason, should not be the sole diagnostic test employed
in this setting. Although the FAT is simple to administer and perform, issues
slowing its widespread use include the unpleasantness of handling and storing
stool, limited availability. The development of in-office stool tests is under
way and may improve upon some of the practical limitations of the currently
available tests. At present,
in-office
tests have not been adequately validated in clinical trials. Based upon the
available data, it is reasonable to conclude that the FAT can be used
interchangeably with the UBT to identify H. pylori before antibiotic
therapy. The polyclonal FAT has been less well validated than the UBT in the
post treatment setting. Compared with the polyclonal test, the monoclonal FAT appears
to provide a more reliable means of proving H. pylori eradication.
