How to Read and Interpret SIBO Test Results

A SIBO breath test report is a graph of the gases your gut bacteria make after you drink a sugar solution, and reading it comes down to how much those gases rise and how fast. Most results can be sorted into positive or negative using a couple of widely agreed cutoffs. Because the test only measures gas in your breath, though, it is an indirect look at what is happening deep in your gut, so even a clear-cut result is a strong hint rather than proof. Which sugar you drank and how carefully you prepared can also change what the same numbers mean.

The Numbers That Make a Result Positive

Your report lists gas readings over time. Each sample is measured in ppm (parts per million), a way of saying how much of a gas is in your breath. The first sample, taken before the sugar drink, is your baseline, or starting level. Hydrogen results are judged by how far later readings climb above that starting level, not by the raw number alone.

To walk through your own report, find the zero-minute reading and note it as your starting level. Subtract it from each later hydrogen reading to see the rise, then check whether a rise of 20 ppm or more lands at or before the 90-minute mark. Scan the methane column separately for any value of 10 ppm or above.

The two gases get different rules because hydrogen starts at a different level in everyone, so the change matters more than the raw number. Methane-making microbes produce it steadily, so a persistently elevated reading means something even if it never climbs.

Samples are usually taken every 15 to 20 minutes over two to three hours, so the graph shows a trend rather than a snapshot. Timing matters as much as size. The sugar usually reaches the large bowel, which is full of bacteria, at some point after 90 minutes, though in people with a fast gut it can arrive sooner, so a rise that appears only late may come from normal large-bowel bacteria rather than overgrowth in the small bowel. Some labs use lower hydrogen cutoffs than 20 ppm, which catches more cases but also flags more people who don’t have SIBO.4PubMed Central. Breath Tests for the Non-invasive Diagnosis of Small Intestinal Bacterial Overgrowth: A Systematic Review With Meta-analysis A lab comment like “borderline” usually means a rise just under the cutoff, which deserves a conversation with your doctor.

What Hydrogen, Methane and Hydrogen Sulfide Each Tell You

Hydrogen-dominant results are the most common type.5PubMed Central. An Oral Botanical Supplement Improves Small Intestinal Bacterial Overgrowth (SIBO) and Facial Redness: Results of an Open-Link Clinical Study Higher hydrogen levels tend to go with more severe diarrhea,6PubMed Central. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms and the main hydrogen-making bacteria differ between people with diarrhea-type and constipation-type IBS.7PubMed Central. Methanogens and Hydrogen Sulfide Producing Bacteria Guide Distinct Gut Microbe Profiles and Irritable Bowel Syndrome Subtypes

Methane is made by microbes that aren’t strictly bacteria, and they may live in the large bowel as well as the small.8PubMed Central. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome – An Update They feed on hydrogen that bacteria produce and turn it into methane, so the two gases are linked. That also helps explain why a methane result can come with a lower hydrogen reading on the same graph. Guidelines now call a methane-positive result IMO (intestinal methanogen overgrowth) rather than SIBO, so your report may use that name; it reflects the same methane finding, just named more accurately.9PubMed Central. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome – An Update Higher methane tends to go with constipation.10PubMed Central. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms

Both columns can cross their cutoffs on the same test. Your doctor may then choose treatment aimed at both groups of microbes, and mixed symptoms such as alternating diarrhea and constipation can fit this pattern. The gas type matters for treatment in general: methane-makers often respond more slowly than hydrogen-makers, so your doctor may plan an IMO result differently. It’s worth asking which gas drove your positive before you start anything.

Some newer tests also measure hydrogen sulfide.11PubMed Central. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms Higher levels tend to go with diarrhea, urgency and belly pain.12PubMed Central. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms No standard cutoff for it appears in the main North American guidelines, so a number on that line is harder to interpret.

These symptom patterns are tendencies across groups of people, not rules. If your report and symptoms don’t line up, that is useful information for your doctor rather than a sign the test failed.

Flat Lines and High Starting Levels

Some reports show hydrogen staying near zero for the whole test.13The American Journal of Gastroenterology. How to Read and Interpret SIBO Test Results One proposed reason is bacteria using up hydrogen to make hydrogen sulfide.14The American Journal of Gastroenterology. How to Read and Interpret SIBO Test Results So a flat line on a test that doesn’t measure hydrogen sulfide isn’t automatically a reassuring negative, and if symptoms continue, a three-gas test may be worth asking about.

A normal negative usually shows a modest, fairly steady graph with readings under the cutoffs. A flat line sits close to zero for both hydrogen and methane throughout. If yours is flat, don’t treat it as a clean negative. Ask whether to repeat the test with stricter preparation rather than starting treatment on a guess.

A starting hydrogen above about 16 to 20 ppm usually makes the result unreadable.15PubMed Central. Unraveling the Complexities of Small Intestinal Bacterial Overgrowth Leftover food fermenting from the day before raises the first reading,16PubMed Central. Unraveling the Complexities of Small Intestinal Bacterial Overgrowth so you breathe out gas before the sugar even arrives. Since every later reading is judged against that starting level, the rise becomes unreliable to interpret. This is why labs often ask what you ate before they accept a result. A high start is a preparation problem, not a diagnosis.

Why the Sugar You Drank Changes the Picture

Your report should say whether you drank lactulose, a sugar the body can’t absorb, or glucose, which is absorbed early in the small bowel. Glucose is taken up quickly in the first part of the small bowel, so bacteria only get a chance to ferment it if they live in that upper stretch. Lactulose keeps moving and meets bacteria everywhere along the way, including the large bowel.

That is why lactulose can pick up overgrowth in the lower small bowel, but is also easier to trigger with normal bacteria. If it travels quickly, large-bowel bacteria ferment it and produce an early rise in someone without SIBO.

The numbers show the gap. In one group of people with IBS, lactulose tests were positive in about 1 in 3, glucose tests in about 1 in 16.17PubMed. Comparison of lactulose and glucose breath test for diagnosis of small intestinal bacterial overgrowth in patients with irritable bowel syndrome Lactulose was positive in 30% of healthy people too, versus under 1% with glucose.18PubMed. Comparison of lactulose and glucose breath test for diagnosis of small intestinal bacterial overgrowth in patients with irritable bowel syndrome A positive lactulose test alone is often wrong: in that group only about 1 in 9 positive lactulose results matched a positive glucose test.19PubMed. Comparison of lactulose and glucose breath test for diagnosis of small intestinal bacterial overgrowth in patients with irritable bowel syndrome A negative lactulose result, though, was right about 97% of the time.20PubMed. Comparison of lactulose and glucose breath test for diagnosis of small intestinal bacterial overgrowth in patients with irritable bowel syndrome So a positive lactulose result is more likely to need confirmation, while a negative glucose result can still miss overgrowth further down.

How Often Breath Tests Get It Wrong

Across many studies, lactulose tests caught about 4 in 10 real cases and glucose about 5 in 10.21PubMed Central. Breath Tests for the Non-invasive Diagnosis of Small Intestinal Bacterial Overgrowth: A Systematic Review With Meta-analysis Lactulose correctly cleared about 7 in 10 people without SIBO, glucose about 8 in 10.22PubMed Central. Breath Tests for the Non-invasive Diagnosis of Small Intestinal Bacterial Overgrowth: A Systematic Review With Meta-analysis Glucose tests worked better in people who had had belly surgery, catching about 8 in 10 cases.23PubMed Central. Breath Tests for the Non-invasive Diagnosis of Small Intestinal Bacterial Overgrowth: A Systematic Review With Meta-analysis

Fast gut speed can also cause error, since sugar reaches large-bowel bacteria early. After gastric bypass surgery, more than 1 in 4 positive lactulose tests looked false once gut speed was checked.24PubMed. Impact of Oral-Cecal Transit Time on the Interpretation of Lactulose Breath Tests After RYGB: a Personalized Approach to the Diagnosis of SIBO People with true positives in that group responded to antibiotics far more often, about 78% versus 33%.25PubMed. Impact of Oral-Cecal Transit Time on the Interpretation of Lactulose Breath Tests After RYGB: a Personalized Approach to the Diagnosis of SIBO

Breath tests agreed with direct small-bowel samples only about 65% of the time in one study.26The American Journal of Gastroenterology. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth One test may not settle the question, especially if symptoms persist.27The American Journal of Gastroenterology. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth

Because tests flag some people without SIBO, a positive raises the probability but doesn’t settle it. How much it raises it depends on how likely SIBO was beforehand, which is why your history matters. A positive counts for more if you have a known risk such as past bowel surgery, and deserves caution if you have no matching symptoms.

A negative isn’t a full all-clear either. With roughly half of real cases missed, it can occur in someone who truly has overgrowth. With strong symptoms or a known risk, it may be treated as inconclusive, and another test can be reasonable. Gas levels and symptoms track only loosely, so treatment decisions combine the result with symptoms and risk factors.

How Preparation Can Skew Your Result

Each one affects the test in its own way. Antibiotics can temporarily lower bacteria numbers, which may give a falsely negative result if you test too soon. Probiotics add bacteria that can ferment the sugar and alter gas readings. Laxatives and motility drugs change how fast the sugar moves, which can shift when the rise appears. Check with your prescriber before pausing acid reducers or any other drug. Some medicines can’t safely be stopped, and your doctor may interpret the result around them.

The day before, avoid fiber, dairy, complex carbs, fermentable foods and alcohol.32PubMed Central. Unraveling the Complexities of Small Intestinal Bacterial Overgrowth Then fast for 12 hours before the test.33PubMed Central. Unraveling the Complexities of Small Intestinal Bacterial Overgrowth Don’t smoke or exercise hard on test day, since both can push readings around.34PubMed Central. Unraveling the Complexities of Small Intestinal Bacterial Overgrowth

In practice, the aim is a bland, low-fermentation day. White rice, plain chicken or fish, and eggs are commonly suggested, but labs differ in what they allow, so follow your kit’s written instructions. If you skipped any of these steps, tell your doctor before reading much into the result.

Home kits generally hold up well. More than 95% give a usable result,35Gut. Common issues with home testing – hydrogen and methane breath testing for SIBO invalid samples happen in about 2 in 100,36Gut. Common issues with home testing – hydrogen and methane breath testing for SIBO and high starting hydrogen ruins about 3 in 100, usually from preparation.37Gut. Common issues with home testing – hydrogen and methane breath testing for SIBO

What Comes After Your Result

A positive result that fits your symptoms and risk factors often leads to a course of treatment, commonly an antibiotic chosen with the gas type in mind. Your doctor may also suggest addressing the cause, since treatment alone may not prevent a return.

If the result is unclear, the next step might be repeating the test with better preparation or using a different sugar drink. It could also be a direct sample or a check for other causes of your symptoms.

Doctors can take fluid straight from the small bowel during an endoscopy, often seen as the most direct test.38The American Journal of Gastroenterology. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth A count above 1,000 bacteria per millilitre is now usually read as SIBO, though some older reports use 100,000.39The American Journal of Gastroenterology. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth It needs sedation, costs more and carries the usual endoscopy risks.40The American Journal of Gastroenterology. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth

Things that slow gut movement, acid-reducing pills, liver disease and weak immunity can all encourage overgrowth.41PubMed Central. CRITICAL APPRAISAL OF THE SIBO HYPOTHESIS AND BREATH TESTING: A CLINICAL PRACTICE UPDATE ENDORSED BY THE EUROPEAN SOCIETY OF NEUROGASTROENTEROLOGY AND MOTILITY (ESNM) AND THE AMERICAN NEUROGASTROENTEROLOGY AND MOTILITY SOCIETY (ANMS) Ask whether a risk factor applies to you. About a third of people with SIBO are eventually also found to have fungal overgrowth, which breath tests don’t detect.42PubMed Central. Small Intestinal Bacterial and Fungal Overgrowth: Health Implications and Management Perspectives Persistent symptoms after a positive test and treatment can have several explanations, and fungal overgrowth is one possibility to ask about.

A repeat test after treatment can confirm that gas levels have returned to normal. A normal retest with continuing symptoms suggests looking for another cause.

For your appointment, useful questions include:

  • Which gas made my result positive, and what does that mean for treatment?
  • Was my preparation good enough to trust this?
  • Should a different test confirm it?
  • When should I retest after treatment?