Gastroenterology• Updated 21 October 2026
Hydrogen Breath Test
A breath test that checks for lactose intolerance or bacterial overgrowth in the small bowel.
Results: Same day Specimen: Breath
Quick Summary
Primary Purpose
To investigate bloating, wind and diarrhoea after certain foods.
What It Measures
Hydrogen and methane in breath after a sugar drink.
Preparation
Special diet the day before and fasting; avoid antibiotics beforehand.
Overview
Undigested sugar is fermented by gut bacteria, producing hydrogen.
The test takes 2–3 hours.
What It Measures
Hydrogen and methane
Breath levels over time.
How It's Performed
- 01
Baseline
Blow into a bag.
- 02
Drink
A sugar solution.
- 03
Repeat
Breath samples every 15–30 minutes.
Interpreting Results & Next Steps
Discuss with Doctor
Your doctor will explain what your result means and what happens next.
If values are High:
A rise in hydrogen suggests lactose intolerance or bacterial overgrowth.
Sources and further reading
Last updated 21 October 2026. For more detail, see these trusted health organisations.
- Lactose Tolerance Tests(opens in a new tab) · MedlinePlus, US National Library of Medicine