How a SIBO breath test works

What a SIBO breath test measures

SIBO, small intestinal bacterial overgrowth, means more bacteria are living in your small intestine than it normally holds. Our page on what SIBO is covers the condition and what causes it. This page is about the test.

You drink a measured sugar solution, then breathe into a series of tubes at set times. Microbes in your gut ferment the sugar and make hydrogen and methane. Some of that gas passes into your blood, travels to your lungs and leaves in your breath, which is where the lab measures it. That is the principle the North American Consensus on breath testing describes.

Methane comes from a different group of microbes, called methanogens, which are archaea rather than bacteria. That is why a high methane reading is now often described separately, as intestinal methanogen overgrowth, or IMO.

So a report tells you when gas rose and by how much. It cannot see where the microbes were. That gap explains most of the confusion around SIBO testing, and I have written separately about how to tell whether it is SIBO or ordinary bloating.

Lactulose or glucose: why the sugar matters

Breath tests for SIBO use one of two sugars. Glucose is absorbed in the upper part of the small intestine. Lactulose is not absorbed at all, so it travels the whole small intestine and on into the large bowel.

Each has its own weakness. Glucose can be absorbed before it reaches overgrowth lower down. Lactulose reaches the large bowel, which is full of bacteria in everyone, so when your gut moves quickly the rise can arrive early and look like SIBO. A 2025 review describes rapid transit producing false positives on the lactulose test, and notes that glucose tests are not immune to it either.

The numbers make the point. Pooled across 25 studies, 62.3% of people with IBS symptoms tested positive on the lactulose breath test, and so did 33.5% of people without symptoms, a gap that was not statistically significant. On glucose the figures were 20.7% and 4.4%. That is why European and Asian-Pacific guidelines both prefer glucose.

Our at-home SIBO breath test uses lactulose. That is why, when we read one, the timing of a rise is looked at beside how quickly your bowels actually move, rather than taken at face value.

How to prepare for a SIBO breath test

Preparation is the part you control. Everything on the report is measured against your first reading, and that reading has to be low to mean anything. What you ate the day before is one of the things that can push it up.

This is what the North American Consensus recommends.

Labs do not all follow it to the letter. One lab’s instructions, for example, ask for no antibiotics for 2 weeks rather than 4 and no probiotics, laxatives or fibre supplements in the 24 hours before, and say some bloating or wind is possible during and just after collection. NutriPATH, the lab behind our kit, lists incomplete avoidance of high-fibre foods, tobacco smoke and napping during collection among the causes of a falsely high result. The instructions that came with your kit are the ones to follow.

One rule sits above all of it. Do not stop or change a prescribed medicine for a breath test without talking to the doctor who prescribed it.

What happens on the day

Your first breath sample is taken before you drink anything. That is your baseline, the reading every later sample is compared with.

Then you drink the sugar solution. The consensus doses are 10 grams of lactulose or 75 grams of glucose, each with a cup of water, and your kit supplies its own.

After that you collect samples at set intervals. One Melbourne clinic describes breath testing as taking 1 to 3 hours with samples every 15 to 20 minutes, depending on the test, and the consensus says two hours is enough to assess SIBO. Stay awake and avoid exercise while you collect.

How to read SIBO breath test results

A report gives a reading for each gas at each sample time, often drawn as a line. Here is how the North American Consensus reads them, what each reading cannot tell you, and where other guidelines disagree.

Not every guideline reads a breath test this way. A 2024 practice update endorsed by the European and American gut motility societies concluded that breath testing is inaccurate for identifying SIBO in people with irritable bowel syndrome and should be abandoned for that purpose. The same paper reports that European guidelines prefer glucose over lactulose, and that Asian-Pacific guidelines say the 90-minute rule cannot be used with lactulose, because lactulose often reaches the large bowel in less than 90 minutes.

So a result is a set of gas readings, not a verdict. Australian Prescriber noted in June 2026 that overgrowth occurs in a subset of people with irritable bowel syndrome, but that routine testing for it is not of clinical value. A positive does not settle the question on its own, and a negative does not rule SIBO out: when 139 people had both a glucose breath test and a culture, the breath test picked up 42% of the culture positives.

After your result

If it is positive, treating SIBO is a medical decision, made with your GP or gastroenterologist.

If it is negative, that does not mean nothing is going on. Healthdirect lists irritable bowel syndrome, constipation, coeliac disease and food intolerances as the regular causes of ongoing bloating. Those are worth looking at with your GP whatever the breath test said.

Either way, Wellbeing George does not diagnose SIBO and does not treat it. Where a clinical nutritionist fits is alongside your medical care: reading the report beside your bowel habits, medicines and history; looking at B12 and the fat-soluble vitamins, which SIBO can affect, using bloods your GP orders; and working out which foods are worth keeping, without cutting your diet down for months. Our page on how we work with bloating and gut symptoms lays out the rest.