The primary test for SIBO (small intestinal bacterial overgrowth) is a breath test that measures gases produced by bacteria in your gut. It’s noninvasive, takes about two to three hours, and can be done at a clinic or at home with a mail-in kit. A positive result is defined as a rise in hydrogen of at least 20 parts per million (ppm) above your baseline within 90 minutes of drinking a sugar solution.
How the Breath Test Works
The test relies on a simple biological principle: bacteria in your gut ferment carbohydrates and produce gases, primarily hydrogen and methane. Those gases get absorbed through your intestinal wall into your bloodstream, travel to your lungs, and come out in your breath. In a healthy gut, most of this fermentation happens in the large intestine. When bacteria have overgrown in the small intestine, gas production spikes earlier than expected.
You drink a measured dose of a sugar substrate, then breathe into collection tubes at regular intervals, typically every 15 to 20 minutes for two to three hours. The lab analyzes those samples for hydrogen, methane, and in some newer tests, hydrogen sulfide. If gas levels rise sharply within 90 minutes, that early spike suggests bacteria are fermenting the sugar in your small intestine rather than your colon.
Glucose vs. Lactulose Substrates
Breath tests use one of two sugar substrates, and they have different strengths. Glucose is absorbed quickly in the upper small intestine, so it only detects overgrowth in that region. Lactulose isn’t absorbed at all, meaning it travels the full length of your small intestine and can theoretically detect overgrowth further down. The tradeoff is accuracy: a meta-analysis found the glucose breath test has a sensitivity of about 55% and specificity of 83%, while lactulose comes in lower at 42% sensitivity and 71% specificity.
In practical terms, the glucose test is better at confirming SIBO when it’s present, and less likely to give a false positive. Lactulose casts a wider net but produces more ambiguous results. Your provider will typically choose based on your symptoms and clinical picture. The American College of Gastroenterology recognizes either 75 grams of glucose or 10 grams of lactulose as acceptable substrates.
What the Numbers Mean
Your results will show gas levels in parts per million at each time point. Here’s what counts as a positive result:
- Hydrogen: A rise of 20 ppm or more above your baseline within 90 minutes indicates SIBO.
- Methane: A level of 10 ppm or higher at any point during the test suggests methanogen overgrowth. About 15% to 30% of people harbor gut organisms that convert hydrogen into methane, so measuring both gases matters.
- Hydrogen sulfide: Newer three-gas tests flag levels of 3 ppm or higher as abnormal. This is a relatively recent addition and not yet available through all labs.
Methane-producing organisms (technically archaea, not bacteria) are now recognized as a distinct condition sometimes called intestinal methanogen overgrowth, or IMO. If your hydrogen levels stay flat but methane is elevated, a standard hydrogen-only test would miss it entirely. That’s why dual-gas or triple-gas testing has become the preferred approach.
Preparing for the Test
Preparation starts the day before. You’ll follow a restricted diet for 24 hours to clear residual fermentable material from your gut, which prevents leftover food from producing gas that skews results. The allowed foods are plain: baked or broiled chicken, fish, or turkey seasoned with only salt and pepper, white bread, plain steamed white rice, eggs, and clear broth. You can drink water, black coffee, or plain tea.
The avoid list is long. No fruits, vegetables, dairy, nuts, seeds, beans, pasta, whole grains, or high-fiber cereals. Essentially, anything that gut bacteria could readily ferment needs to be off your plate. After midnight the night before, you fast completely: no food, water, gum, or mints until the test is finished.
You’ll also need to stop certain medications beforehand. Antibiotics are typically discontinued at least two to four weeks before testing, and proton pump inhibitors and laxatives may need to be paused as well. Your ordering provider will give you specific timing for any medications you take. On the morning of the test, avoid smoking or vigorous exercise, both of which can affect breath gas levels.
At-Home Kits vs. Clinic Testing
At-home SIBO breath test kits have become widely available, and they use the same collection method as clinic-based tests. You receive a set of breath collection tubes, the sugar substrate, and detailed timing instructions. You drink the solution and breathe into tubes at the specified intervals, then mail everything back to the lab. As long as you follow the preparation and timing instructions carefully, at-home results are considered comparable to in-clinic testing.
The main advantage of clinic testing is supervision. A technician keeps you on schedule and ensures you’re using the collection device correctly. At home, the burden falls on you. If you miss a collection window or accidentally eat something during the fast, your results may be unreliable. Some providers prefer clinic testing for patients who’ve had inconclusive results before or who have complex symptoms that warrant closer monitoring during the test.
Why Results Can Be Misleading
Breath testing has real limitations. The lactulose test, in particular, is essentially measuring how fast material moves through your gut. In people with faster intestinal transit, the sugar reaches the colon sooner, and the resulting gas spike looks identical to a SIBO pattern. Transit time to the colon is remarkably short in many healthy people, and especially in those with IBS. This means the lactulose test can yield false positives at a high rate.
The glucose test avoids the transit problem since glucose gets absorbed before reaching the colon, but it can only detect overgrowth in the upper portion of the small intestine. If bacteria are concentrated further down, glucose may miss them entirely. Neither test performs particularly well in patients with IBS, which is frustrating given that IBS and suspected SIBO overlap frequently. A Mayo Clinic review noted that the poor accuracy of breath testing in patients with gut-brain axis disorders makes it problematic for routine use in that population.
False negatives happen too. If your particular bacterial overgrowth produces mostly hydrogen sulfide, a standard two-gas test measuring only hydrogen and methane will come back normal. Timing errors during collection, incomplete fasting, or recent antibiotic use can also flatten results.
The Gold Standard: Small Bowel Aspirate
Direct sampling of fluid from the small intestine, called a jejunal aspirate, is considered the reference standard for diagnosing SIBO. During an upper endoscopy, a small amount of fluid is collected from the small bowel and cultured in a lab. If the bacterial count exceeds a specific threshold, SIBO is confirmed.
In practice, this test is rarely used. It requires an invasive procedure, is expensive, and can miss overgrowth if bacteria are concentrated in a section of the small intestine that wasn’t sampled. Contamination from mouth or stomach bacteria during the scope can also produce misleading results. For these reasons, breath testing remains the frontline diagnostic tool despite its imperfections, and aspirate culture is reserved for cases where breath test results don’t match the clinical picture.
What to Do With Uncertain Results
Given the limitations of breath testing, a negative result doesn’t definitively rule out SIBO, and a positive result doesn’t always confirm it. Many gastroenterologists interpret breath test results alongside your symptoms, medical history, and risk factors. Conditions like prior abdominal surgery, diabetes, low stomach acid, or structural abnormalities in the small intestine all raise the likelihood of true SIBO.
If your first test is negative but your symptoms strongly suggest SIBO, your provider may repeat the test with the other substrate, order a three-gas test that includes hydrogen sulfide, or pursue a trial course of treatment based on clinical suspicion alone. Testing is one piece of the diagnostic puzzle, not the final answer on its own.