The SIBO Breath Test: How to Read Your Results, and Why a Negative Test Doesn't Mean You're Fine

By Dr. Sarah Khan, PhD, MBA, Functional Nutritionist NYC | Hashimoto's, Hormone, Autoimmune & Gut Health Specialist

You have been bloated for months. Maybe years. Someone finally mentioned SIBO, and now you are trying to work out how you actually get tested, whether the test is worth doing, and what happens if it comes back negative when you know something is wrong.

This is the guide I wish every person had before they tested. It covers what the breath test actually measures, how to read your own results, the two situations where a negative result means nothing, and how to decide whether testing is the right next step at all.

What the Breath Test Actually Measures

The test does not detect bacteria. It detects the gases they produce.

You drink a sugar solution, usually lactulose or glucose. If organisms are overgrowing in your small intestine, they ferment that sugar and release gas. That gas crosses into your bloodstream, travels to your lungs, and comes out in your breath. You collect breath samples into tubes at set intervals, typically every 15 to 20 minutes over two to three hours.

The pattern of which gas rises, how high, and how early is what tells the story.

Lactulose or glucose

Glucose is absorbed early in the small intestine, so it mainly detects overgrowth in the upper portion. It produces fewer false positives 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 can detect overgrowth further along. It picks up more cases but is more prone to false positives if transit is unusually fast.

Neither is universally better. What matters more is that whoever interprets the result knows which substrate you used.

How to Read Your Own Results

Most people are handed a number and a word. Here is what those numbers generally mean, based on the North American Consensus criteria that most labs follow.

Hydrogen: a rise of 20 ppm or more above your baseline within the first 90 minutes is generally considered positive for bacterial overgrowth. The timing matters as much as the number. A rise that only appears at 120 minutes or later may simply reflect the sugar reaching your large intestine, which is normal.

Methane: a level of 10 ppm or above at any point during the test is generally considered positive. Note the difference: methane does not need to rise, and the timing does not matter. A flat methane reading sitting at 15 the whole way through is still positive. This trips up a lot of people reading their own results, because nothing appears to be happening on the graph.

Hydrogen sulfide: most standard two-gas tests do not measure it at all. Only a three-gas panel will.

If your report shows a rise and a diagnosis, ask which substrate was used, what your baseline was, and at what minute the rise occurred. Those three details change the interpretation substantially.

The Two Situations Where a Negative Result Means Nothing

This is the part that sends people away believing their gut is fine when it is not.

1. You have hydrogen sulfide overgrowth and a two-gas test

Hydrogen sulfide is produced by organisms that consume hydrogen. So when they are overgrowing, hydrogen gets used up rather than released, and a two-gas test can show a flat line or even a drop. The report reads as negative or inconclusive.

The symptom picture is distinctive: rotten-egg or sulfur-smelling gas, diarrhea, reactions to sulfur-containing foods like garlic, onions, eggs and cruciferous vegetables, and often bladder symptoms or histamine-type reactions alongside.

A flat line plus sulfur symptoms is one of the strongest signals in gut testing that the wrong test was run. A three-gas panel is what you need.

2. You have SIFO, which breath testing cannot detect

Small intestinal fungal overgrowth involves yeast rather than bacteria or archaea. Yeast does not produce the gases the test measures, so breath testing is simply blind to it.

Suspect SIFO if you have been treated for SIBO repeatedly with partial or no lasting improvement, if symptoms flare notably with sugar and refined carbohydrates, or if there is a history of repeated antibiotics, long-term acid suppression, or steroid use.

Do You Actually Need to Test?

I want to be honest about this rather than sell you a test.

Testing genuinely helps when your symptoms could plausibly be several things and the result would change what you do, when your dominant symptom is constipation and you need to know whether methane is involved because that changes the whole approach, when you have relapsed after previous treatment and need to know whether the same pattern returned or a different one emerged, or when you want a baseline to retest against later.

Testing adds less when the plan would be identical either way, when the underlying driver is already obvious and unaddressed, or when a negative result would not actually reassure you.

Here is the thing worth sitting with: the test tells you what is overgrowing. It does not tell you why. And the why is what determines whether you stay well after treatment.

The Question the Test Does Not Answer

Overgrowth is always downstream of something. The most common drivers I see in practice:

Impaired migrating motor complex. This is the wave that sweeps your small intestine clean between meals. When it is not firing properly, bacteria are never cleared. This is the single most common reason SIBO recurs.

Low stomach acid, often after years of acid-suppressing medication, which removes a key barrier to bacteria establishing where they should not.

Thyroid dysfunction. Hypothyroidism and Hashimoto's both slow gut transit, which is why thyroid and gut problems so often travel together.

Post-infectious changes. A bout of food poisoning can trigger an autoimmune process affecting the nerves that control motility, sometimes years before symptoms are recognised.

Chronic stress physiology, which suppresses the migrating motor complex directly, lowers stomach acid, and shifts blood flow away from digestion.

This is why relapse rates are high when treatment stops at clearing the overgrowth. Kill the organisms without correcting the conditions that let them establish, and the same conditions produce the same result within months.

It is also why so much of what gets diagnosed as IBS has an identifiable overgrowth pattern underneath that nobody investigated.

What a Good Testing Process Looks Like

If you decide to test, the sequence that produces a usable answer:

First, prepare properly. Preparation requirements vary by lab but usually involve a restricted diet the day before and a fasting period. Getting this wrong is a common cause of unreliable results, so follow your lab's instructions precisely rather than general advice you find online.

Second, test all three gases where possible. A two-gas panel leaves hydrogen sulfide invisible, and that is the subtype most likely to be missed.

Third, interpret the result alongside your symptom pattern rather than in isolation. Constipation with methane means something different from diarrhea with hydrogen.

Fourth, and most importantly, investigate the driver at the same time. Thyroid function, stomach acid, motility history, prior food poisoning, prior surgery. Treating the overgrowth without this is why people end up testing again a year later.

Frequently Asked Questions

How do I get a SIBO breath test?

Breath testing is available through gastroenterologists and through at-home kits ordered by qualified practitioners. At-home kits are collected by you and mailed to the lab, and for most people they are as reliable as in-clinic testing provided the preparation instructions are followed exactly.

Does insurance cover SIBO breath testing?

Coverage varies considerably. Testing ordered through a gastroenterologist is more likely to be covered than a practitioner-ordered at-home kit. Check directly with your insurer, and ask the lab about self-pay pricing, which is often more reasonable than people expect.

What if my SIBO test is negative but I still have symptoms?

A negative result rules out what the test measured, not everything. If you had a two-gas test, hydrogen sulfide was not assessed. Breath testing cannot detect fungal overgrowth at all. And symptoms can also stem from motility problems, bile acid issues, food intolerances, or the gut-brain axis. A negative test is a reason to keep investigating, not to stop.

How accurate is SIBO breath testing?

It is useful but imperfect. False negatives happen with hydrogen sulfide and with fungal overgrowth. False positives can occur with unusually fast transit. This is why results should be read alongside your symptom pattern rather than treated as a verdict on their own.

Should I retest after treatment?

Often yes, particularly with methane-dominant overgrowth, which is the most likely to persist or relapse. Retesting confirms whether the pattern cleared and gives you something objective to compare against if symptoms return later.

How long does the test take?

The collection itself takes two to three hours, during which you stay fasted and collect breath samples at set intervals. Results usually come back within one to two weeks.

Clinical References

• Rezaie A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology, 2017.
• Pimentel M, et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology, 2020.
• Rome Foundation, Rome IV Criteria. theromefoundation.org

This article is educational and is not medical advice. Testing and treatment decisions should be made with a qualified practitioner who knows your history.