SIBO Breath Testing: How It Works and How to Read Your Results

Q: Do I need to do a breath test before starting treatment?

Not always. Testing is most valuable when it will change the decision, and subtype does change the decision, so in most SIBO presentations it earns its place. Where symptoms are unambiguous and finances are limited, treatment can be started on clinical grounds and testing done later.

Written by Karly Raven, BHSc - Naturopath and SIBO Specialist, Nourished Gut Clinic
Last updated: August 2026

A SIBO breath test is one of the few gut tests where the preparation matters as much as the result.

I say that because a significant proportion of the tests I review in clinic are either uninterpretable or misleading, and almost always for avoidable reasons. Someone ate a fermentable meal the night before. Someone stopped collecting at 90 minutes. Someone was told their test was negative when the curve told a different story.

As a naturopath with over six years in clinical practice at the Nourished Gut Clinic, breath test interpretation is a large part of what I do, both for my own clients and for the practitioners I mentor. I reviewed the diagnostic evidence in detail in the ACNEM Journal in June 2026, in Hydrogen, Methane and Hydrogen Sulfide SIBO: A Literature Review of Pathophysiology, Diagnostic Approaches and Phenotype-Specific Treatment Strategies.

This article explains how the test works, what the thresholds actually are, and how to read a result properly.

What a SIBO breath test measures

You drink a measured sugar solution. If organisms in your small intestine ferment that sugar, they produce gas. Some of that gas crosses into your bloodstream, travels to your lungs and is exhaled.

You collect breath samples at set intervals, usually every 20 minutes for two to three hours. The lab measures the gases in each sample and plots them as a curve.

Humans do not produce hydrogen, methane or hydrogen sulphide in any meaningful quantity. Microbes do. So the gas in your breath is microbial in origin, and the timing of the rise tells us roughly where in the digestive tract it is being produced.

Lactulose or glucose, and why it matters

Two substrates are in common use, and they answer slightly different questions.

Glucose is absorbed high in the small intestine. A rise means overgrowth is proximal. It is more specific but can miss overgrowth further down.
Lactulose is not absorbed by humans at all, so it travels the full length of the small intestine and into the colon. It has better reach but a higher false positive rate, because rapid transit can deliver it to the colon early and produce a rise that looks like SIBO but is not.

Neither is definitively superior. Lactulose is more commonly used in Australian naturopathic practice because of the reach, and because most of the people I see have already had a negative glucose test.

What matters more than substrate choice is that whoever interprets the result understands the limitations of the one you used.

SIBO breath test thresholds explained

The ACG Clinical Guideline on SIBO (Pimentel et al., American Journal of Gastroenterology, 2020) sets out the criteria most laboratories now use.

Hydrogen: a rise of at least 20 parts per million above baseline within 90 minutes
Methane: a level of 10 ppm or more at any point during the test

Note the difference in how those two are defined. Hydrogen requires a rise within a time window. Methane requires a level, at any point, with no window at all. That is deliberate. Methanogens can colonise the large bowel as well as the small intestine, so the timing tells you less.

This is also why a flat methane line at 11 ppm across the whole test is a positive result, even though nothing appears to have happened. I see that misread regularly.

Two gas versus three gas testing

Breath testing measures hydrogen and methane. Hydrogen sulphide is the third gas of clinical interest, but it cannot currently be measured by breath testing in Australia, and the diagnostic criteria proposed for it have not been validated.

That third gas matters because hydrogen sulphide producing organisms consume hydrogen while they work. A person with significant sulphide overgrowth can produce a flat looking hydrogen curve, be told their test is negative, and be sent away with symptoms nobody has explained.

Research in the American Journal of Gastroenterology in 2022 by Villanueva-Millan and colleagues found that hydrogen sulphide producers and methanogens map to distinct microbial profiles and distinct IBS subtypes. In practice, sulphide skews toward diarrhoea and urgency.

Because the gas cannot be measured here, the hydrogen sulphide pattern is recognised and treated clinically on presentation rather than confirmed on a test. A suggestive history plus a flat test is still clinically meaningful. It is not proof of nothing.

One distinction worth making, because the two get confused: three gases is not the same thing as three substrates. Adding a third substrate, as below, still measures the same two gases.

You can read more about how the gases map to subtype in the three types of SIBO.

Fructose as a third substrate

Most SIBO testing uses lactulose or glucose. In my practice I also test with fructose, and it answers a question neither of the others can.

Fructose is absorbed through a specific transporter with limited capacity. Where that capacity is exceeded or impaired, unabsorbed fructose travels on and gets fermented, producing bloating, wind, pain and loose stools that look exactly like SIBO because the mechanism is the same.

Testing with a fructose substrate separates two situations that are usually lumped together.

Primary fructose malabsorption. Transporter capacity is limited in its own right, independent of any overgrowth. This is managed by matching intake to capacity, and it does not disappear when SIBO clears.
Secondary fructose malabsorption. The malabsorption is a consequence of the overgrowth and the mucosal disruption that comes with it. Treat the SIBO properly and absorption usually improves.

That distinction changes the whole dietary plan. Clients with secondary fructose malabsorption are frequently told to avoid fructose indefinitely, when what they actually need is the overgrowth treated and a staged reintroduction afterwards. Being handed a lifelong restriction for a temporary problem is one of the more avoidable things that happens in this field.

It also explains a pattern that otherwise makes no sense. Someone reacts badly to apples, pears, honey and mango but handles onion and garlic without trouble. That is not a classic FODMAP picture, and until you test fructose specifically it looks like random inconsistency.

One clinical point worth understanding: what matters is not simply whether a food contains fructose. It is the amount of fructose in excess of glucose, plus the total free fructose load in the meal. Glucose assists fructose absorption, which is why a food with balanced amounts is often fine while one with excess fructose is not.

After treatment I retest with a fructose substrate before broadening the diet, so that reintroduction is based on measured tolerance rather than hope. You can see how this plays out in practice in how we helped a client overcome SIBO and fructose intolerance.

Preparation is not optional

This is where most tests go wrong, and it is entirely fixable.

24 hours before: a restricted diet, removing fermentable carbohydrates. Your lab will provide the specific list and they do vary.
12 hours before: fasted, water only.
Antibiotics: at least four weeks clear, ideally longer.
Antimicrobial herbs and probiotics: discuss timing with your practitioner. These affect results.
Laxatives and prokinetics: generally paused, but never stop a prescribed medication without checking first.
Morning of the test: no smoking, no exercise, no sleeping during collection. All three change gas readings.

A poorly prepared test is worse than no test, because it produces a number that looks like data and gets treated as one.

What a negative result does not rule out

A negative breath test does not mean nothing is wrong.

It may mean the overgrowth is distal and the substrate did not reach it, or that it is hydrogen sulphide dominant and therefore not captured by the gases we can measure, or that the preparation was imperfect, or that the driver is something other than SIBO entirely, such as fungal overgrowth, bile acid malabsorption, pancreatic insufficiency or a disorder of gut brain interaction.

At the Nourished Gut Clinic I treat the breath test as one input, not a verdict. The curve shape, your bowel pattern, your history, your response to previous treatment and your other testing all sit alongside it. If you want to see how that assessment runs from start to finish, our online SIBO naturopath page sets out the whole process.

If you have a confirmed result and want to know what happens next, SIBO treatment sets out the full picture.

Frequently Asked Questions

How accurate is a SIBO breath test?

It is a useful test rather than a perfect one. Both false positives and false negatives happen, particularly with lactulose where rapid transit can mimic a positive. Accuracy improves considerably with correct preparation, a full collection period and interpretation of the whole curve rather than a single threshold. I would rather have a well prepared test read properly than a more expensive test read badly.

Do I need to do a breath test before starting treatment?

Not always, and this is a genuine clinical judgement call. Testing is most valuable when it will change the decision, and subtype does change the decision, so in most SIBO presentations it earns its place. Where symptoms are unambiguous and finances are limited, I have started treatment on clinical grounds and tested later. What I will not do is run a test simply because it is available.

Why was my SIBO test negative when I still have symptoms?

This is common and it is worth taking seriously rather than accepting as an answer. Breath testing does not capture hydrogen sulphide, so that phenotype is missed. Distal overgrowth can be missed with glucose. Preparation errors flatten curves. And sometimes the driver genuinely is not SIBO. A negative result should open an investigation, not close one.

Can a breath test tell me if I have fructose malabsorption?

Yes, using a fructose substrate rather than lactulose or glucose. It is worth doing because it separates primary fructose malabsorption, where transporter capacity is limited in its own right, from malabsorption that is secondary to SIBO and improves once the overgrowth is treated. That distinction determines whether you are managing something long term or reversing something temporary, and a lot of people are given the long term answer when the short term one applies.

How long does a SIBO breath test take?

Collection runs two to three hours, with a sample every 20 minutes. You do it at home, seated and awake, without eating, smoking or exercising. The 24 hour prep diet and the 12 hour fast beforehand matter more than the collection itself, so read the instructions properly the day before rather than the morning of.

Ready to get a clear answer on your gut?

If you have a breath test result sitting in a folder that nobody has properly explained to you, that is a very common place to be, and it is a fixable one.

If you are not ready for that step yet, the free 3-Day Bloat Fix is a practical starting point you can begin this week.

If you would rather see how we work first, our online SIBO naturopath page covers testing, treatment and what telehealth looks like from anywhere in Australia.

If you want your results interpreted properly and a plan built from them, the Nourished Gut Program is our six month process. It is application only, five new clients a month. Book a free Digestive Strategy Session and we will tell you honestly whether it fits your case.

Karly Raven, BHSc is a naturopath, microbiome restoration specialist, and SIBO expert. She is the founder of the Nourished Gut Clinic and creator of the Feed Forward methodology. Karly works with people with IBS, SIBO, IBD and chronic gut conditions, and mentors health practitioners through karlyraven.com.

This blog is for educational purposes only and does not constitute medical advice. Please consult a qualified health practitioner before making changes to your treatment plan.