Hydrogen breath test

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

Dr Chad OkayFounder, Suna Health

3 min read. Updated .

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 about 2 to 3 hours, and is available on the NHS via gastroenterology referral or privately.

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 from baseline at any point during the test confirms intolerance.
  • Fructose: rise of 20+ ppm hydrogen from baseline at any point during the test 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% 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.

Before you book one

  • Breath tests investigate diagnosed patterns; they are not the first step for new symptoms. A change in bowel habit that has not settled, blood in the stool, unintentional weight loss, or unexplained anaemia need a doctor first, at any age - NICE recommends a simple stool test (FIT) for an unexplained change in bowel habit with no minimum duration.
  • Results need clinical interpretation alongside your history: a positive breath test alone is not a diagnosis, and a negative one does not close the question.
  • If you are paying privately, ask who will interpret the result and what happens next before you pay - a number without a plan is not worth the fee.

Common questions

Can I do this test at home?
Some companies sell home versions, and quality varies. Lab-based testing in a clinic, where sample timing and gas measurement are controlled, is the standard approach.
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 a prescribed course of a gut-selective antibiotic such as rifaximin - the choice and length are the prescriber's call, and methane-positive cases are often treated differently. Diet changes ease symptoms but do not clear the overgrowth on their own. 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, after finishing antibiotics. Confirms eradication and gives a baseline if symptoms return.

Sources

Written by Dr Chad Okay. Contributor: Dr Parisah Hussain, MBChB, BSc (Hons), DRCOG, MRCGP. Last updated 10 September 2026. From Digests.

Digests explains how the body works in general, everyday terms. It is not medical advice. It is not a substitute for someone who can examine you and knows your history, and we do not tell you what dose of any medicine to take.

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