How a SIBO breath test works

How is SIBO diagnosed?

SIBO is diagnosed by a doctor, usually your GP or a gastroenterologist, and the usual test is a hydrogen and methane breath test. You fast, give a first breath sample, drink a sugar solution, then breathe into tubes at set times for about two hours, at a clinic or at home with a kit. There’s no Medicare rebate for breath testing.

The other method is a culture of fluid taken from the small intestine during an endoscopy. It counts the bacteria directly, but it’s invasive, so a breath test is usually where testing starts. The American College of Gastroenterology’s 2020 guideline suggests a glucose or lactulose breath test for people with irritable bowel syndrome symptoms, and the 2022 European guideline on breath testing says either sugar can be used.

How does a SIBO breath test work?

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’s 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’s 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 can’t see where the microbes were. That gap explains most of the confusion around SIBO testing, and I’ve written separately about how to tell whether it’s SIBO or ordinary bloating.

Getting a SIBO breath test in Australia

There are two ways to do it. Your GP or gastroenterologist can send you to a breath testing clinic or hospital lab, where you collect the samples on site. Or you collect them at home with a kit and post the tubes back, which is how our at-home test works. Either way, there’s no Medicare rebate.

The steps are the same in both. At home, your kit’s instructions do the job a clinic’s staff would, so follow them closely.

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 isn’t absorbed at all, so it travels the whole small intestine and on into the large bowel.

Each has its own weakness. The large bowel is full of bacteria in everyone, so when your gut moves quickly a lactulose 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 aren’t 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 wasn’t statistically significant. On glucose the figures were 20.7% and 4.4%. That’s why the 2022 European guideline, while saying either sugar can be used, prefers glucose for its lower false positive rate, and why Asian-Pacific guidelines say the 90-minute rule can’t be used with lactulose.

Our at-home SIBO breath test uses lactulose. That’s 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 accurate is a SIBO breath test?

A breath test is the simplest way to look for SIBO, and it’s the test the American College of Gastroenterology suggests for people with irritable bowel syndrome symptoms. What it measures is gas, so it shows that fermentation happened and roughly when, rather than counting bacteria.

Not every guideline reads it the same 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. Australian Prescriber noted in June 2026 that overgrowth occurs in a subset of people with irritable bowel syndrome, but that routine testing for it isn’t of clinical value. And when 139 people had both a glucose breath test and a culture, the breath test picked up 42% of the culture positives.

So a result is a set of gas readings, not a verdict. A positive doesn’t settle the question on its own, and a negative doesn’t rule SIBO out. That’s why we read a report beside your bowel habits, medicines and history, rather than on its numbers alone.

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 don’t 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. Don’t stop or change a prescribed medicine for a breath test without talking to the doctor who prescribed it.

How do you do a SIBO breath test at home?

1. Follow your kit’s preparation: no antibiotics for four weeks if the test can wait, a low-fermentation day before, then 8 to 12 hours without food.
2. Take your first breath sample before you drink anything. That’s your baseline, the reading every later sample is compared with.
3. Drink the sugar solution in your kit (10 grams of lactulose).
4. Breathe into a tube at each set time, for about two hours.
5. Stay awake, keep your activity to a minimum, and don’t smoke.
6. Note the time of any symptoms, then post the tubes to the lab.

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.

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’s how the North American Consensus reads them, and what each reading can’t tell you.

What happens after a positive or negative result?

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

If it’s negative, that doesn’t mean nothing is going on. Irritable bowel syndrome, constipation, coeliac disease and food intolerances are the regular causes of ongoing bloating. Those are worth looking at with your GP whatever the breath test said, and our guide to testing for food intolerance covers how that one is worked out.

Either way, Wellbeing George doesn’t diagnose SIBO and doesn’t 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.