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13с urea test: preparation, results, positive, negative
Last reviewed: 23.04.2024
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Since the discovery of the Helicobacter pylori bacterium associated with chronic gastritis and gastric and duodenal ulcers, the methods for its diagnosis have been developed, including a urease test that can detect these microorganisms in a particular patient, confirm their involvement in the development of gastroenterological diseases and prescribe effective treatment.
Enzyme urease as a biomarker of Helicobacter pylori infection
H. Pylori bacteria are protected from the acidic environment in the lumen of the stomach in two ways. First, damaging the mucosa with its flagella, microorganisms get to its lower layers, down to the epithelial cells, where the pH is higher (that is, the acidity is lower). Secondly, the bacteria neutralizes the acid, synthesizing in large quantities a catalytically active high-molecular metalloenzyme urease or urea amidohydrolase.
The use of urease in the diagnosis of Helicobacter is possible due to not only the cytoplasmic activity of this enzyme, but also external interaction with host cells.
Under the influence of urease, the urea urea decomposes to hydrogen nitride (ammonia) and carbon dioxide (carbon dioxide). They react with hydrochloric acid of gastric juice and provide around H. Pylori a zone with neutral acidity, and also support the metabolism of bacterial cells.
That is why urease is the most important factor in H. Pylori colonization of the human gastric mucosa, and the detection of ureolytic activity is considered a biomarker of the virulence of the bacterium, which gastroenterologists - by conducting a urease test - are used to diagnose Helicobacter pylori infection, as well as to control the results of bacterial eradication (eradication).
Diagnosis of H. Pylori includes invasive and non-invasive tests - depending on whether an endoscopic examination of the stomach (fibrogastroduodenoscopy) is required or not. An invasive test is a rapid urease test or an express test for urease (RUT-test), which requires tissue samples (biopsies). 13C urease respiratory test (13C-UBT) - the most common among non-invasive tests.
It should be noted that non-invasive diagnosis of H. Pylori can be performed using blood test for antibodies (specificity at 75%, sensitivity - 84%), urine analysis ELISA (with a sensitivity of 96% and 79% specificity), coprograms for antigens bacteria. More information - Helicobacter pylori infection: antibodies to Helicobacter pylori in the blood
[Indications for the procedure of the urease test
By itself colonization of the stomach mucosa H. Pylori is not a disease, it is a factor of increased bacterial load on the body, which under certain conditions can give impetus to the development of a number of gastric and upper gastrointestinal pathologies.
Indications for the diagnosis of Helicobacter pylori infection, in particular the urease test, are associated with gastritis with increased acidity, antral and atrophic gastritis, duodenitis, peptic ulcer of the stomach and duodenum, gastric MALT-lymphoma. After endoscopic resection of early gastric cancer, a histological examination can be carried out in combination with a rapid urease test, a PHD with a urease test.
Gastroenterologists can prescribe a urease test in patients' complaints of a feeling of heaviness and discomfort in the epigastric region, frequent heartburn after eating, a belch sour or rotten, bitterness in the mouth, nausea, intestinal disorders, aching or cramping pain in the stomach.
Preparation
Preparation for 13C urease respiratory test is that the patient should stop: taking antibiotics 4 weeks before testing, and NSAIDs, preparations of the proton pump inhibitor group (reducing gastric acidity) and antacids or adsorbents used for heartburn - at least two weeks. Acceptance of any medications should be stopped in five to six days, and the use of alcohol-containing drinks and smoking - three days before the test.
Also, about a week before testing, it is recommended not to consume legumes, because beans, peas, lentils, soy and beans have urease (which protects plants from diseases and pests).
In the evening on the eve of the test, a late supper dinner is contraindicated; on the day of testing you need to conduct the usual oral hygiene, and for an hour and a half before the analysis should not drink anything or use a cud.
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Technique of the urease test
Technique:
- first take a sample of basic breathing - is exhaled into a soft plastic container (and hermetically sealed);
- is taken internally with a liquid with the addition of 13C-urea;
- After 25-30 minutes a second sample of exhaled air is taken - into another container.
The samples thus obtained are analyzed on a mass spectrometer with separation of isotopes in the second sample and determination of their concentration. The difference between the values in the second and the first sample is expressed as delta (δ) compared to the baseline. The normal values, that is, the negative values of the labeled 13C atom in uninfected patients, vary between 0.15 and 0.46%, and positive values in the presence of infection are 1.2-9.5% with a urea hydrolysis rate of more than 12-14 μg / min.
And if it is simpler: the detection of carbon dioxide labeled with 13С isotope in the exhaled air indicates that urea has undergone hydrolysis with the urease enzyme H. Pylori, which, in fact, confirms the fact of its presence in the stomach.
Respiratory test for H. Pylori
Test 13C-UBT or 13C urease respiratory test for Helicobacter pylori is one of the most important non-invasive methods for detecting this infection: at 100% sensitivity and specificity 98%, the risk of false positive and false negative results compared with histology and blood test for antibodies is lower by 2 , 3%.
The analysis is based on the hydrolysis of the urease H. Pylori orally administered liquid urea, labeled with an atom (stable non-radioactive isotope) of 13C carbon (50-75 mg of 13C-urea, diluted with 100 ml of liquid).
Urea, labeled with an isotope indicator, undergoes hydrolysis in the stomach, releasing ammonia and containing labeled carbon dioxide atoms, which diffuses into the blood and is released from the lungs when breathing. Labeled carbon dioxide fixes the apparatus for analysis - a mass spectrometer, whose action is based on non-dispersive isotopelective spectroscopy, or an infrared spectroscopy analyzer.
Rapid urease test
A rapid urease test (Rapid Urease Test or RUT) is performed during an endoscopic examination of the stomach and duodenum - using modern endofibroscopes - and simultaneous sampling of biopsy specimens. The biomaterial should be taken from the antral part of the stomach. So fibroadastroduodenoscopy or FGD with a urease test is an invasive diagnostic method.
The resulting biopsy is completely immersed in a finished standardized medium containing agar gel, urea, acid-base indicator phenolsulfophthalein and bacteriostatic (added to prevent the growth of pollutant microorganisms and false positive results).
If H. Pylori bacteria are present in the stomach tissue sample, the urease produced by them will hydrolyse the urea and raise the pH, which is clearly seen by changing the yellow color of the indicator to orange (at pH 6.8) and purple (at pH> 8) 75% of the positive tests change color in 120-180 minutes, and the faster the indicator changes color, the more bacteria are there. But tests that appear negative, stand for 24 hours.
A quick urease test is positive, what is it? A positive RUT test-redness of the indicator-indicates that there are at least 105 H. Pylori bacteria in the biopsy specimen placed in the agar medium, although usually their concentration is higher.
The sensitivity of various modifications of tests varies in the range of 90-98%, and the specificity is 97-99%.
As gastroenterologists note, if the patient has a sharply positive urease test (three crosses are put in the conclusion), this means: pH> 8, and the indicator has changed color in less than 60 minutes from the moment of immersion of the biopsy, which indicates a large amount of H.pylori and a high level of urease expression. An approximate number of bacteria can be counted by histological examination of the biopsy specimen under a microscope, and if the imaging site exceeds 40-50, the level of infection is considered high.
13C urease respiratory test rarely gives false positive results, and in patients with suspected stomach ulcer or duodenal ulcer, one positive test is considered a confirmation of the diagnosis, whereas a negative result should be confirmed by the results of the EGF with a urease test.