Peak H₂ + CO₂

If you live with bloating, gas, abdominal pain, or unpredictable bowel habits, the cause may not show up in a routine stool test or blood panel. Bacteria growing where they shouldn't, in the small intestine, ferment the food you eat and produce gases that you exhale. Measuring those gases is one of the few ways to catch that process directly.

This test captures two of those gases at the same time. Hydrogen comes from gut bacteria fermenting carbohydrates. Carbon dioxide is exhaled by your lungs and serves as a quality check that the breath sample came from deep in your airways. Together, they tell a clinician whether your symptoms are likely driven by bacterial fermentation in the wrong place.

What This Test Actually Captures

Your small intestine is normally home to relatively few bacteria. When too many bacteria settle there, a condition called SIBO (small intestinal bacterial overgrowth), they ferment carbohydrates from your food and release H₂ (hydrogen gas) into your bloodstream. That hydrogen travels to your lungs and gets exhaled. By drinking a sugar solution like glucose or lactulose and then breathing into a collection tube every 15 to 20 minutes, you create a timeline of how much hydrogen your gut bacteria are producing.

The CO₂ (carbon dioxide) part of the measurement does something different. It is not a sign of disease. Carbon dioxide in your breath comes from your normal metabolism, and a sample with too little CO₂ usually means you exhaled mostly air from your mouth or upper airway rather than from deeper in your lungs. The lab uses CO₂ to flag bad samples and, in some protocols, to mathematically correct hydrogen readings that look artificially low. Studies in children found that CO₂ correction changed diagnostic conclusions in only a small minority of cases, with very high agreement between corrected and uncorrected hydrogen readings.

Small Intestinal Bacterial Overgrowth

SIBO is the condition this test is most often used to detect. The North American Consensus considers a hydrogen rise of 20 parts per million or more above your baseline within 90 minutes of drinking a sugar substrate to be a positive result. Compared with the invasive reference standard of small bowel aspirate and culture, the glucose breath test catches roughly 32 to 48 out of every 100 people who actually have the condition, and correctly clears about 86 to 90 out of 100 who don't. Lowering the cutoff to a 12 ppm rise catches more cases but mistakenly flags more healthy people as positive.

Hydrogen is not the only gas that matters. Some people harbor methane-producing microbes in addition to hydrogen-producers, and methane is associated with constipation. Research linking gut microbe profiles to symptom subtypes found that methanogens like Methanobrevibacter smithii are associated with constipation-predominant IBS (irritable bowel syndrome with constipation), while hydrogen sulfide producers map to diarrhea-predominant IBS. If you are testing for SIBO, ordering hydrogen and methane together gives a more complete picture than hydrogen alone.

Carbohydrate Malabsorption

The same breath test format, with a different sugar substrate, can reveal whether you cannot properly digest specific carbohydrates. Drinking a lactose solution and then watching hydrogen rise tells you whether the lactose is being fermented by gut bacteria instead of being absorbed by your small intestine. Reviews report that the lactose breath test has good sensitivity and specificity for diagnosing lactose maldigestion. Fructose breath testing follows a similar logic for fructose intolerance.

What this means for you: if you have years of bloating after dairy or certain fruits, this test can confirm whether the symptoms are caused by a fermentation problem rather than a non-specific food sensitivity. That distinction changes which dietary strategy is likely to actually work.

Why CO₂ Is Measured Alongside Hydrogen

Breath tests can be technically tricky. If you don't exhale deeply enough, your sample is mostly mouth and upper airway air, which dilutes the gases your gut produced. Carbon dioxide solves this. Because your lungs continuously release CO₂, a sample with appropriately high CO₂ is a sample that came from deep in your lungs. A sample with low CO₂ is suspect.

Some labs apply a CO₂ correction factor that scales the hydrogen reading up or down based on how diluted the sample appears. This is a quality control step, not a separate disease marker. Knowing your peak H₂ + CO₂ reading lets your clinician confirm that your hydrogen number is trustworthy before acting on it.

Reference Ranges

These thresholds come from the North American Consensus on hydrogen and methane breath testing in gastrointestinal disorders. Cutoffs vary by sugar substrate, by lab, and by whether your provider is using a stricter or more sensitive interpretation. Use them as orientation, not as universal truth.

Compare your results within the same lab over time for the most meaningful trend. Different labs use different sugar substrates, sampling intervals, and gas analyzers, and a value at one lab may not be directly comparable to a value at another.

When Results Can Be Misleading

Breath tests are sensitive to what you do in the hours and days before the appointment. Reproducibility is limited even in well-run protocols, which is why a single positive or negative reading should rarely be the sole basis for treatment.

• Mouth bacteria: a clinical study found that rinsing with chlorhexidine mouthwash before the test significantly reduced hydrogen and methane readings in a majority of participants. Bacteria living in your mouth can ferment the test sugar before it ever reaches your gut, producing gases that look like a positive result.
• Recent diet: eating fermentable carbohydrates the day before testing can elevate baseline hydrogen and obscure the rise that would otherwise signal bacterial overgrowth. Most protocols ask for a low-fermentation diet for 24 hours before testing.
• Recent antibiotics, laxatives, or motility drugs: these can suppress or alter bacterial activity and produce false negatives. Most protocols recommend waiting at least four weeks after antibiotic use before testing.
• Shallow breathing into the collection tube: if your CO₂ reading is low, the sample came from your upper airway rather than deep in your lungs, and the hydrogen value will be artificially diluted. This is the entire reason CO₂ is measured alongside hydrogen.

Why One Reading Is Not Enough

Hydrogen breath testing has documented limits in reproducibility, and the same person tested on different days can produce different curves. A negative result during a quiet symptom period may flip to positive during a flare, and a borderline result deserves a repeat test rather than immediate treatment. Establishing a baseline, retesting if you have a confirmed positive after a course of treatment, and tracking your symptoms in parallel give you a far clearer picture than any single number.

A reasonable cadence: get an initial test when symptoms are present, retest 2 to 4 weeks after completing any antibiotic or dietary intervention to confirm the change, and retest if symptoms return. If your first test is borderline, repeat it before committing to weeks of antibiotic therapy.

What to Do With an Abnormal Result

A positive breath test is a strong signal but not a complete diagnosis on its own. The next step is to look at the pattern alongside your symptoms and any companion tests.

• If hydrogen is positive but methane is not, and you have diarrhea-dominant symptoms: the picture is consistent with hydrogen-driven SIBO, often treated with the gut-targeted antibiotic rifaximin under medical supervision.
• If methane is also positive and you have constipation: the methane-producing microbes typically require a different antibiotic strategy. Discuss this with a gastroenterologist before starting treatment.
• If results are borderline or symptoms don't fit the gas pattern: consider a repeat test on a different sugar substrate, a stool test for inflammation markers like calprotectin, or evaluation by a gastroenterologist for other causes such as celiac disease, inflammatory bowel disease, or pancreatic insufficiency.
• If your symptoms are severe or include blood in stool, weight loss, or anemia: breath testing is not a substitute for endoscopic evaluation. Get a workup with a gastroenterologist.