Hydrogen Breath Test (SIBO & Lactose)
You drink a sugar solution and breathe into a device every 20 minutes for 2-3 hours. Diagnoses small intestinal bacterial overgrowth (SIBO), lactose intolerance, and fructose malabsorption.
Results
Within 1-3 days.
This article is educational and does not replace your doctor's advice. Seek urgent care for any red-flag symptoms.
What is it?
Bacteria in the small intestine produce hydrogen and methane gas when they ferment certain sugars. These gases are absorbed into the blood and exhaled in your breath. By measuring breath gas levels at intervals after a sugar drink, the test identifies bacterial overgrowth or sugar malabsorption.
Why is it done?
To diagnose SIBO (cause of bloating, diarrhea, gas), confirm lactose or fructose intolerance, evaluate post-cholecystectomy diarrhea, work up unexplained bloating.
Who needs it?
- Suspected SIBO (bloating, gas, diarrhea, especially after meals)
- Suspected lactose intolerance for confirmation
- Suspected fructose malabsorption
- IBS workup in selected cases
Before the procedure
- No fermentable foods (high-FODMAP) the day before — your unit will give a list.
- No antibiotics for 4 weeks before; no bowel prep for 2 weeks; no probiotics for 2 weeks.
- Fast 12 hours before.
- No smoking the day of the test.
- No vigorous exercise just before or during.
During the procedure
You arrive at the unit and provide a baseline breath sample. You drink a measured sugar solution (lactulose for SIBO, lactose or fructose for those tests). You then breathe into a device every 20 minutes for 2-3 hours. Side effects during the test (cramping, gas, diarrhea) are part of the diagnosis. Bring a book or watch something on your phone.
After the procedure
You may have continued bloating or loose stool for several hours. Resume normal diet — but if SIBO confirmed, your doctor will start treatment. Results usually within 1-3 days.
Risks
- Temporary GI symptoms (intended — that's how the test works)
- Diarrhea after lactulose
- No serious risks
When you get results
Alternatives if you decline
Empirical antibiotic trial for SIBO. Trial elimination of lactose or fructose for intolerances. Direct small bowel aspirate culture (invasive, requires endoscopy).
Questions to ask your doctor
Save this list or print it before your appointment — doctors appreciate prepared patients.
- What sugar solution will be used?
- How will my treatment depend on the result?
Medically reviewed by: SGA Patient Education Panel · Last reviewed 24/04/2026