Introduction
Helicobacter pylori infection is common in China, with an overall estimated prevalence of 55.8% [
1]. It is an important gastric pathogen that can lead to several gastroduodenal disorders including chronic gastritis, gastric atrophy and peptic ulcer disease, and less commonly, to gastric adenocarcinoma and mucosa associated lymphoid tissue (MALT) lymphoma [
2,
3].
Helicobacter pylori is able to convert urea into carbon dioxide and ammonia via its urease enzyme, where the ammonia is used to neutralize the acid for its survival in the stomach [
4]. Based on this feature of
H. pylori, the urea breath test (UBT), a non-invasive
H. pylori infection diagnostic method was developed. This requires a patient to swallow a capsule containing a dose of urea labeled with carbon-13 (
13C) or carbon-14 (
14C) isotope. If the patient is an
H. pylori carrier, the labeled urea will be hydrolyzed by the bacterial urease enzyme within the stomach, resulting in the release of labeled carbon dioxide which is then absorbed into the bloodstream and expelled from the lungs in a few minutes. Hence the amount of labeled carbon dioxide within a patient’s breath sample can be measured to determine current
H. pylori infection status [
5,
6].
Due to its accuracy, simplicity and non-invasive nature, the UBT has been the preferred method of many medical professionals for testing
H. pylori infection in their patients. Both the
13C-UBT and the
14C-UBT are widely used. The former utilizes the stable
13C isotope of carbon while the latter uses the radioactive
14C carbon isotope. Nevertheless, it is important to mention that both are naturally occurring isotopes and the radiation exposure from the
14C-UBT is even lower than that from background radiation [
7]. In fact, the
14C-UBT has been approved by the Food and Drug Administration (FDA) of the United States for its usage in everyone, including children and pregnant women [
8].
While UBT is useful in detecting H. pylori infection, we noticed that several UBT results were contradictory to the outcomes determined via histopathology examination, prompting us to reconsider the diagnostic accuracy of the commercial UBT kits used for screening H. pylori infection in our hospital. In this study, we recruited 484 individuals who underwent endoscopic examination at Shenzhen Kuichong People’s Hospital, among which 217 and 267 were tested for H. pylori infection using the 13C-UBT and 14C-UBT, respectively. By comparing the outcomes to that of histopathology examination of gastric biopsies, which is the gold standard for diagnosing H. pylori infection, we assessed the diagnostic performance of both UBT kits. Additionally, as these commercial kits available for use at our hospital provide only a cutoff value for H. pylori positivity, resulting in high positive rates, we therefore sought to introduce an “indeterminate zone”. Should a UBT value fall within the indeterminate range, we would like to recommend a repeat UBT or the use of another H. pylori diagnostic method to confirm the presence or absence of an infection.
Discussion
The urea breath test is widely accepted as an accurate non-invasive method for diagnosing H. pylori infection. In the present study, we assessed the diagnostic performance of the 13C- and 14C-UBT commercial kits used in our hospital by comparing each UBT outcome against that of histological examination, which was considered the “gold standard” reference method for determination of H. pylori infection. Importantly, based on histology, the 14C-UBT was significantly more accurate than the 13C-UBT in determining H. pylori infection status in patients aged 18–30 years, whereas, in patients older than 50 years of age, the 13C-UBT method was more accurate than the 14C-UBT.
The
13C-UBT measures the ratio of labeled CO
2 to human respiratory CO
2 in the breath. Hence its outcome can be affected by one’s gender, age, urea hydrolysis rate and CO
2 production rate [
15]. Therefore, in the event where there were many false-positive
13C-UBT results among the young patients, it is possible that these individuals have a relatively low basal CO
2 production rate and/or a high urea hydrolysis rate, releasing breath with a proportionally higher quantity of labeled CO
2 and thus, generating a false-positive DOB value. In the situation where
14C-UBT generated substantially more false-positive results than
13C-UBT in older patients, some of these patients might have hypochlorhydria, a condition where there is a low-level production of gastric acid and which is commonly associated with aging, leading to the growth of urease-producing non-
H. pylori bacteria originating either from the oral cavity or the intestine and thus, a UBT-positive outcome [
16‐
18].
Also, it is also possible that the four-hour fasting time (prior to ingesting a capsule containing 50 mg 13C isotope-labeled urea) of this current study is insufficient to empty the stomach in some of these individuals. This situation is most likely to generate a less-acidic gastric environment, which would be rather permissive for the growth of other bacteria with urease activity that could eventually induce a false-positive UBT reaction. Therefore, a longer fasting period, potentially overnight when possible, should be considered the preferred option before testing. Additionally, attention to detail when performing the tests could improve the accuracy. As an example, cleaning teeth and mouth immediately prior to the test might decrease gastric contamination from swallowed oropharyngeal (urease positive) bacteria. Ensuring the patient was sitting quietly prior to the test would lower the amount of endogenous CO2 resulting in a slightly higher breath enrichment of the isotope.
Depending on populations and the doses of
13C-urea or
14C-urea, no one-size-fits-all UBT cutoff value can be used to define whether an individual is
H. pylori-positive or -negative [
19‐
22]. In our study, to overcome the low specificity of each UBT kit, two optimal cutoff points, indicating UBT-positive and -negative, respectively, along with an indeterminate zone to address UBT readings that are inconclusive, were established. The intermediate zone, defined as ranging from 3% to 10.3% DOB for
13C-UBT, and from 87–237 DPM for
14C-UBT, contained at least half of the false-positive test results in this study. By using new optimal cutoff values and including an indeterminate range, the false positive rates can be greatly reduced. More importantly, we suggest that for any patient who had an indeterminate UBT result, a repeated UBT or other diagnostic test such as stool antigen or serum antibody test should be performed to confirm
H. pylori infection, avoiding misdiagnosis and unnecessary antibiotic treatment.
Limitations of study
We concede that there are limitations in this study which was performed in a busy clinical setting rather than in a formal research environment. Despite with the new cutoffs and the establishment of an indeterminate range, the specificity of each UBT kit was only improved to approximately 77%, which is still considerably lower than reported (
10). The lower specificity of each UBT kit used in this study was probably due to the misdiagnosis of
H. pylori infection when histology alone was used as the reference method, as its accuracy depends on the skills of the operator, the size and number of biopsies taken and whether or not the biopsy site contained
H. pylori or missed it by chance.
To address this issue, a larger cohort study with a more even distribution of different age categories, as well as the ability to test every study participant using both 13C- and 14C-UBT, and the concordant use of two or three methods as references should be further performed. For example, rapid urease test, bacterial culture, histology and even PCR, would create a truer “gold standard” and allow for better comparison of the diagnostic accuracy of both tests and the validation of our suggested indeterminate zone. At the same time, observation of how the tests were administered in a busy clinical setting could increase the value of the test even further.
Conclusions
Via comparing the UBT outcomes to that of histopathology examination, we demonstrated that both 13C- and 14C-UBT kits used in this study have high sensitivity but low specificity. Based on ROC analysis and the maximum Youden index method, new optimal cutoff values were identified and used to establish an indeterminate range (3–10.3% DOB for 13C-UBT and 87–237 DPM for 14C-UBT), improving the specificity from 62.2% to 76.7% and 54.7% to 76.9% for the 13C- and 14C-UBT, respectively. We strongly suggest that for any patient who had an indeterminate UBT result, a repeated UBT or other diagnostic test should be performed to confirm H. pylori infection, avoiding misdiagnosis and unnecessary antibiotic treatment. For future studies, a larger cohort study with two or three methods as references should be further performed to validate our suggested indeterminate zone.
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