Glossary · Markers and tests
Breath test
In one sentence
A breath test is a non-invasive investigation that measures gases in exhaled air, mainly hydrogen and methane, after a sugar is swallowed, to assess carbohydrate malabsorption or small intestinal bacterial overgrowth.
Also known as: hydrogen breath test · hydrogen and methane breath test · methane breath test · breath testing
What is a breath test?
Human cells do not produce hydrogen or methane. When these gases appear in exhaled air, they come from microbes fermenting carbohydrate in the gut: bacteria produce hydrogen, and some methanogenic archaea use that hydrogen to make methane. Part of the gas diffuses into the blood and is breathed out, so a breath sample offers an indirect window on intestinal fermentation.
In gastroenterology the term also covers the urea breath test for Helicobacter pylori, which works on a different principle. This entry refers to hydrogen and methane breath testing.
How it works
After a baseline sample, the person drinks a standard dose of sugar and breath samples are collected at intervals. The 2017 North American Consensus (Rezaie et al., Am J Gastroenterol) set out these doses and criteria:
| Substrate | Dose | Main use |
|---|---|---|
| Glucose | 75 g | SIBO |
| Lactulose | 10 g | SIBO |
| Fructose | 25 g | Fructose malabsorption |
| Lactose | 25 g | Lactose malabsorption |
- SIBO: hydrogen rise of ≥20 ppm above baseline within 90 minutes.
- Sugar malabsorption: hydrogen rise of ≥20 ppm above baseline during the test.
- Methane: ≥10 ppm at any point is considered methane-positive.
The consensus advises ruling out SIBO before testing for malabsorption of a specific sugar, to avoid false positives, and supports measuring methane, because methanogens consume hydrogen and a hydrogen-only test can underestimate fermentation. Exact thresholds may differ between laboratories.
Preparation and limitations
Preparation matters. Among other measures, the consensus recommends avoiding antibiotics for four weeks beforehand and fasting for 8-12 hours, with a low-fermentable diet the day before. Even so, the test remains indirect: rapid transit, a microbiota that produces little hydrogen or a sampling error can distort the curve. It should be treated as one biomarker among several, never in isolation.
Example
A GP refers a patient with recurrent bloating and constipation. The gastroenterology team requests a lactulose breath test measuring both gases. Hydrogen stays flat, but methane sits at 18 ppm from the baseline sample onwards. The report flags a methane-positive pattern, which is associated with slower transit; how to act on it is the clinician’s decision.
Breath tests at Xenogene
Xenogene offers two breath tests, both measuring hydrogen and methane by gas chromatography with CO₂ used to correct sample quality:
- the SIBO breath test, with lactulose (10 g) or glucose (75 g) and sampling over 2-3 hours;
- the fructose breath test, for fructose intolerance due to malabsorption, with sampling over about 3 hours.
Both require dietary preparation and a prescription from a healthcare professional, who interprets the result. They can be combined with microbiota analysis in the integrated profiles.
Frequently asked questions
How do I prepare for a hydrogen breath test?
Preparation usually involves a low-fermentable diet the day before, several hours of fasting, and no smoking or vigorous exercise during the test. Antibiotics, laxatives and some other medicines can affect the result, but they should not be stopped without advice: the laboratory and the requesting clinician set the protocol.
How long does a breath test take?
It depends on the substrate and protocol. After a baseline sample, further samples are taken at set intervals over roughly 2-3 hours, during which the person rests and does not eat.
Does a positive breath test confirm a diagnosis?
Not on its own. It is an indirect test with false positives and negatives related to the substrate, gut transit and preparation. It informs the assessment, and interpretation belongs with the clinician who requested it, alongside the clinical history.
Related terms
- SIBO
SIBO (small intestinal bacterial overgrowth) is the presence of excessive numbers of bacteria in the small intestine, accompanied by digestive symptoms such as bloating, wind, abdominal pain or diarrhoea.
- Fructose intolerance
The term covers two distinct conditions: fructose malabsorption, a common digestive problem, and hereditary fructose intolerance (HFI), a rare genetic metabolic disease caused by deficiency of the enzyme aldolase B.
- Microbiota
The microbiota is the community of living microorganisms (bacteria, archaea, fungi, protozoa) that inhabits a specific site of the body, such as the gut, mouth, skin or vagina.
- Biomarker
A biomarker is a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, disease processes or responses to a therapeutic intervention. It may be a molecule, a genetic variant or an imaging finding.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
Updated on