IBS and IBD

Hydrogen breath test

Also called: lactose breath test, SIBO breath test, fructose breath test

The hydrogen breath test measures hydrogen gas in exhaled breath after drinking a sugar solution. Humans don't produce hydrogen, but gut bacteria do, when they ferment sugar that wasn't absorbed in the small intestine. The test diagnoses lactose intolerance, fructose malabsorption, and small intestinal bacterial overgrowth (SIBO). It's non-invasive, takes 2 to 3 hours, and costs £150-£300 privately or is available on the NHS via gastroenterology referral.

How it works

You arrive fasted, breathe into a bag for a baseline reading, then drink a measured dose of a specific sugar (lactose, fructose, lactulose or glucose depending on what's being tested). Then you breathe into bags every 15 to 30 minutes for 2 to 3 hours. The lab measures hydrogen and methane in each sample. A rise above the diagnostic threshold indicates malabsorption (sugar reaching the colon) or overgrowth (bacteria in the small intestine producing gas earlier than normal).

What each test diagnoses

  • Lactose breath test: lactose intolerance.
  • Fructose breath test: fructose malabsorption.
  • Lactulose breath test: SIBO (small intestinal bacterial overgrowth).
  • Glucose breath test: SIBO (alternative, more specific in some labs).

Reading the result

  • Lactose: rise of 20+ ppm hydrogen at 90+ minutes confirms intolerance.
  • Fructose: rise of 20+ ppm hydrogen at 90+ minutes confirms malabsorption.
  • SIBO (lactulose): rise of 20+ ppm hydrogen within 90 minutes (early peak).
  • SIBO (glucose): rise of 20+ ppm hydrogen from baseline within 90 minutes (NA Consensus, Rezaie 2017).
  • Methane: a rise of 10+ ppm any time during the test indicates methanogen overgrowth (often seen with constipation-predominant IBS).

Preparing for the test

  1. Avoid antibiotics for at least 4 weeks before.
  2. Avoid high-FODMAP foods for 24 hours before.
  3. Fast for 12 hours before. Water is fine.
  4. No smoking on the morning of the test.
  5. No exercise during the test.
  6. PPIs do not need to be stopped before SIBO breath testing (NA Consensus 2017, Rezaie).

Limitations

  • False negatives: about 10 percent of people are non-hydrogen producers (their gut produces methane only). Modern tests measure both gases.
  • Lactulose SIBO test has higher false positive rate than glucose.
  • Doesn't work well in fast-transit guts (sugar reaches the colon before bacteria in the small intestine can ferment it).
  • Doesn't tell you which bacteria are overgrowing, just that fermentation is happening early.

Common questions

Can I do this test at home?
Some companies sell home versions. Quality varies. Lab-based tests give more reliable results because the timing of sample collection and gas measurement matters. Worth doing in a clinic the first time.
Is the lactose test the same as a milk allergy test?
No. The breath test diagnoses lactose intolerance (inability to digest the sugar). Milk allergy is an immune reaction to milk protein and is tested with skin prick or IgE blood tests.
How is SIBO treated after diagnosis?
Usually 14 days of rifaximin (an antibiotic that mostly stays in the gut) plus dietary changes. Methane-positive SIBO may need rifaximin plus neomycin or metronidazole. Recurrence is common, so the underlying cause (slow motility, low stomach acid) needs addressing too.
Should I retest after treatment?
Often yes, 4 weeks after finishing antibiotics. Confirms eradication and gives a baseline if symptoms return.

Sources