H pylori diagnostic breath tests rely on a simple chemical reaction which is based on the natural behaviour of the bacteria. Naturally occuring gastric urea is made up of 99% carbon isotope 12C and 1% carbon isotope 13C. The breath test uses urea enriched with 13C (that is, 99%).
Patients first drink a sachet of orange juice or citric acid. This rapidly closes the duodenal sphincter to contain the stomach contents. They are then asked to blow through a straw into a glass tube with a screw cap lid. This provides the baseline sample.
Next, they consume a drink containing 13C enriched urea (about 100 ml) and after 30 minutes repeat the blowing exercise into a second tube. This provides the post-dose sample. Both samples are sent away for carbon dioxide isotope analysis by mass spectrometry (laboratories usually return the results within a few days).
The level of 13C in the baseline sample will be normal. If levels rise in the post-dose sample, this suggests the presence of H pylori. The enriched urea provided by the testing kit must have been broken down to produce high levels of 13C in the breath and this implies the presence of urease excreting H pylori in the stomach.
If the post-dose 13C concentration is 3.5 parts per 1000 more than the baseline sample, the test is considered positive for H pylori. The higher the level of 13C, the greater the extent of the infection. Levels of 30-40 parts per 1000 over the baseline result are typical of H pylori infections.
Two back-up tubes are generally also taken at baseline and post-dose stages.
The sensitivity of such diagnostic kits is 96% and the specificity is 100%. 
Figure.
1 Virtually all duodenal ulcers and 80% of gastric ulcers are associated with Helicobacter pylori infections1
Figure.
2 H pylori survives in gastric acid by excreting large amounts of urease. This enzyme breaks down any urea in the stomach to ammonia and carbon dioxide
Figure.
3 The ammonia neutralises any acid found directly in the vicinity of the bacteria, allowing the bacteria to survive
Figure.
4 The carbon dioxide is absorbed into the blood stream and then released from the lungs
References
- Graham DY. Helicobacter pylori infection in the pathogenesis of duodenal ulcer and gastric cancer: a model. Gastroenterology. 1997;113(6):1983–1991. doi: 10.1016/s0016-5085(97)70019-2. [DOI] [PubMed] [Google Scholar]