A SIBO test is a non-invasive breath test that detects bacterial overgrowth in the small intestine by measuring gases produced when bacteria ferment a sugar solution you drink. The test takes about three hours, with breath samples collected every 20 to 60 minutes. It’s the most common way to diagnose small intestinal bacterial overgrowth (SIBO) without an invasive procedure.
How the Breath Test Works
Bacteria and archaea living in your gut ferment carbohydrates and produce specific gases: hydrogen, methane, and hydrogen sulfide. In a healthy digestive system, most of this fermentation happens in the large intestine. When bacteria have overgrown in the small intestine, fermentation starts earlier, and the gases show up in your breath sooner than expected.
During the test, you drink a solution containing either glucose or lactulose (more on the difference below). As bacteria in your small intestine encounter that sugar, they produce gases that get absorbed into your bloodstream, travel to your lungs, and come out in your breath. A technician or at-home kit captures those breath samples at regular intervals, and a lab analyzes the gas levels over time. A spike in gas production within the first portion of the test window suggests bacterial overgrowth in the small intestine rather than normal large-intestine fermentation.
Glucose vs. Lactulose Substrates
The two versions of the breath test use different sugars, and each has trade-offs. Glucose is rapidly absorbed in the upper small intestine, so it’s good at detecting overgrowth in that region with fewer false positives. The downside is that glucose gets absorbed before it reaches the lower portions of the small intestine, meaning it can miss overgrowth further down.
Lactulose isn’t absorbed at all, so it travels the full length of the small intestine and can potentially catch overgrowth in areas glucose would miss. The trade-off: because lactulose continues into the large intestine, normal colonic fermentation can look like SIBO on the results, increasing false-positive rates. If you have fast gut transit, this problem gets worse, since the lactulose reaches the colon earlier than expected.
In general, glucose offers higher specificity (fewer false alarms), while lactulose offers wider reach. Your provider will choose one based on your symptoms and clinical picture.
The Three Gases and What They Mean
Older breath tests only measured hydrogen and methane. Newer devices, like the trio-smart test, also measure hydrogen sulfide, which matters because different gas patterns are associated with different symptom profiles and may guide treatment.
- Hydrogen: Elevated hydrogen is the classic SIBO marker and is often associated with diarrhea-predominant symptoms.
- Methane: Produced by archaea (a type of microorganism distinct from bacteria), elevated methane is linked to constipation. This pattern is sometimes called intestinal methanogen overgrowth, or IMO.
- Hydrogen sulfide: Levels at or above 3 ppm at any point during the test are considered elevated and are associated with diarrhea.
There’s also a pattern called a “flatline” result, where both hydrogen and methane stay extremely low throughout the test (hydrogen stays at or below 3 ppm with no rise greater than 1 ppm above baseline). This is uncommon but shows up more often in people with inflammatory bowel disease. One explanation is that hydrogen-consuming bacteria are converting all available hydrogen into hydrogen sulfide, which older two-gas tests can’t detect. A flatline can also occur if you have very slow stomach emptying (gastroparesis) or recently took antibiotics.
How to Prepare for the Test
Preparation starts well before test day. If you’re taking antibiotics, you’ll need to stop them four weeks beforehand. Motility-supporting medications and laxatives should be stopped at least one week prior. These timelines exist because antibiotics can temporarily suppress bacterial populations and skew results, while laxatives and motility agents alter how quickly food moves through the gut, affecting when gases appear.
The day before the test, you’ll follow a restricted diet limited to low-fiber, easily digested foods. That typically means plain baked or boiled chicken or fish seasoned only with salt and pepper, plus white bread or white rice. You’ll avoid oils and fats, sweeteners and seasonings, dairy products, whole grains, and high-fiber foods. The goal is to starve gut bacteria of their preferred fuel so that any gas produced during the test comes from the sugar solution, not leftover food residue.
On the morning of the test, you fast for 12 hours beforehand. That means no food and no water. You’ll also want to avoid smoking and vigorous exercise, both of which can alter breath gas readings.
What Happens During the Test
The test itself is straightforward. You provide a baseline breath sample, then drink the sugar solution. Over the next three hours or more, you blow into collection tubes or a device at intervals of every 20 to 60 minutes. You sit quietly between samples, and most facilities ask you not to eat, drink, sleep, or exercise during the test. Some providers offer at-home kits that follow the same protocol with pre-labeled tubes you mail to a lab.
The experience is boring more than anything. The sugar solution can cause bloating, gas, or cramping, especially if you do have SIBO, but there’s no pain or sedation involved.
Understanding Your Results
Results are plotted on a graph showing gas levels in parts per million (ppm) at each collection point. Your provider looks for a rise above baseline within the timeframe that corresponds to the small intestine (typically the first 90 minutes or so, depending on the substrate used). A rise occurring later may reflect normal large-intestine fermentation rather than SIBO.
For hydrogen sulfide, the threshold is simpler: 3 ppm or higher at any point during the test is considered elevated.
Context matters when interpreting results. A positive breath test in someone with clear risk factors for SIBO (prior abdominal surgery, slow motility, structural abnormalities) carries more diagnostic weight than the same result in someone without those risk factors.
Accuracy and Limitations
Breath testing is the most practical diagnostic tool for SIBO, but it isn’t perfect. The gold standard is actually a jejunal aspirate, where a thin tube is passed into the small intestine during an endoscopy to collect fluid directly. A bacterial count of 1,000 or more colony-forming units per milliliter in that fluid confirms SIBO. This procedure is invasive, expensive, and not widely available, which is why breath testing is used far more often in practice.
The lactulose breath test has drawn particular criticism. A Mayo Clinic appraisal described it as primarily a measure of intestinal transit rather than bacterial overgrowth, with low sensitivity and specificity for diagnosing SIBO. The glucose breath test performs better in terms of fewer false positives, making it more reliable for people who have known risk factors like disordered gut motility. However, neither version is considered highly accurate for patients whose symptoms overlap with irritable bowel syndrome, where the glucose test’s performance also falls short.
These limitations don’t mean the test is useless. They mean results should be interpreted alongside your symptoms, medical history, and risk factors rather than treated as a standalone diagnosis. A negative result doesn’t definitively rule out SIBO, and a positive result, especially on a lactulose test, doesn’t always confirm it.