The primary test for SIBO is a breath test that measures gases produced by bacteria in your small intestine. You drink a sugar solution, then breathe into collection tubes at regular intervals over two to three hours. The pattern of gases in your breath tells your doctor whether bacteria are fermenting food in the wrong part of your gut.
How the Breath Test Works
Bacteria in the small intestine produce gases when they break down sugars. Those gases get absorbed into your bloodstream, travel to your lungs, and show up in your breath. A SIBO breath test exploits this by giving you a measured dose of a specific sugar, either lactulose or glucose, then tracking the gases you exhale over time.
You’ll provide a baseline breath sample before drinking the solution, then give additional samples every 15 to 20 minutes. The test typically lasts about three hours. Most tests measure hydrogen and methane, though newer versions also measure hydrogen sulfide, a third gas linked to a different type of bacterial overgrowth. Samples are analyzed either in real time at a clinic or sent to a lab if you’re using an at-home kit.
What the Numbers Mean
A positive result depends on how high your gas levels rise and how quickly. The North American Consensus and the American College of Gastroenterology use these thresholds:
- Hydrogen: A rise of at least 20 parts per million (ppm) above your baseline within 90 minutes of drinking the sugar solution indicates SIBO.
- Methane: A level of 10 ppm or higher at any point during the test suggests methanogenic overgrowth, sometimes called intestinal methanogen overgrowth (IMO). This type is strongly associated with constipation.
- Hydrogen sulfide: A level of 3 ppm or higher at any point during the test is considered elevated. Higher levels correlate with more severe diarrhea.
Standard two-gas tests only measure hydrogen and methane. If your symptoms lean toward diarrhea but your hydrogen and methane come back normal, hydrogen sulfide overproduction could be the missing piece. The three-gas version of the test is less widely available but worth asking about if your initial results don’t match your symptoms.
Glucose vs. Lactulose: Two Sugar Options
Both sugars are used as the test substrate, but they behave differently in your gut. Glucose is absorbed quickly in the upper part of the small intestine, so it’s better at detecting overgrowth in that area. The tradeoff is that it can miss bacteria living further down. Lactulose isn’t absorbed at all and travels the full length of the small intestine before reaching the colon, giving it a wider detection range.
The catch with lactulose is that it always reaches the colon eventually, and colon bacteria produce hydrogen too. If the sugar passes through your small intestine quickly, the gas spike from your colon can look like a SIBO-positive result when it’s really just normal colonic fermentation. This is the single biggest source of false positives on the lactulose test. Transit time varies a lot from person to person, and in many healthy people and those with IBS, food reaches the colon faster than expected.
How Accurate Is the Breath Test?
Breath testing is the most practical and widely available option for diagnosing SIBO, but it has real limitations. The lactulose breath test in particular has low sensitivity and low specificity, meaning it misses some true cases and flags some people who don’t actually have overgrowth. A significant portion of the signal it picks up reflects how fast food moves through your gut rather than how many bacteria are present.
The glucose breath test is more specific but can miss overgrowth in the lower small intestine. Neither test is perfect, which is why doctors often interpret results alongside your symptoms, medical history, and response to treatment rather than relying on the test alone. A negative breath test doesn’t definitively rule out SIBO, and a positive one doesn’t always confirm it.
What About the Small Intestine Culture?
For years, collecting fluid directly from the small intestine and culturing the bacteria was considered the gold standard. A thin tube is passed through the mouth or nose, down through the stomach, and into the upper small intestine during an endoscopy. The fluid sample is then grown in a lab to count bacterial colonies.
This method has largely fallen out of favor. The Mayo Clinic notes that experts no longer recommend it because the tube can pick up bacteria from the mouth on its way down, contaminating the sample and producing a false diagnosis. The procedure is also invasive, expensive, and only samples one small spot in a roughly 20-foot-long organ. Bacteria may be overgrown in a section the tube never reaches.
Preparing for a Breath Test
Preparation starts weeks before test day and directly affects accuracy. The general timeline looks like this:
Four weeks before: stop all antibiotics and probiotics unless your gastroenterologist specifically says otherwise. These can suppress or alter the bacteria you’re trying to detect.
One week before: stop taking laxatives, including common over-the-counter options like senna and bisacodyl. Laxatives speed up transit time and can distort your results.
The day before: eat only plain, low-residue foods. That means baked or broiled chicken, fish, or turkey, steamed white rice, eggs, broth, and simple seasonings like salt and pepper. For beverages, stick to water, plain coffee, or unsweetened tea with no milk, sugar, or artificial sweeteners. The goal is to starve the bacteria of anything fermentable so your baseline reading is clean. Stop eating entirely 12 hours before the test, and stop drinking all liquids at midnight.
On test day, don’t smoke, exercise vigorously, or chew gum before or during the test. All of these can alter your breath gas readings. You’ll also want to brush your teeth before your first sample to reduce the chance that mouth bacteria skew early readings.
At-Home Test Kits vs. Clinic Testing
At-home SIBO breath test kits follow the same basic protocol as in-clinic tests. You receive a set of labeled collection tubes, a sugar solution, and instructions for timed sampling. You drink the solution, breathe into the tubes at specified intervals, then mail everything to a lab.
A large analysis of over 1,500 home breath tests found that more than 95% were performed and analyzed successfully. The most common issues were high baseline hydrogen from poor preparation (2.6% of tests) and invalid samples at key time points, likely from faulty technique (2.1%). Actual patient errors like mislabeling tubes were extremely rare, under 0.2%. Home testing is a viable alternative when getting to a clinic is difficult, though you do need to follow the prep and timing instructions carefully for reliable results.
Other Tests Your Doctor May Order
Breath testing is the frontline diagnostic tool, but your doctor may pair it with additional tests to assess the downstream effects of SIBO or rule out other conditions. Blood tests can check for vitamin deficiencies, particularly B12, iron, and fat-soluble vitamins like D, which bacteria in the small intestine can deplete or prevent you from absorbing. A stool test can check for fat malabsorption, a sign that SIBO is interfering with normal digestion.
In some cases, imaging like a CT scan or MRI may be ordered to look for structural problems in the intestine that could be causing the overgrowth in the first place, such as strictures, adhesions from prior surgery, or motility disorders. These tests don’t diagnose SIBO directly but help explain why it’s happening and guide treatment decisions.