The Hydrogen Breath Test (HBT) is a non-invasive diagnostic tool used to investigate common gastrointestinal symptoms like bloating, gas, and abdominal discomfort. The test measures specific gases, primarily hydrogen and methane, found in the exhaled breath. These gases are byproducts of fermentation by gut bacteria, not human cells. By tracking gas levels over time after consuming a specific sugar solution, medical professionals gain insight into digestive function.
The Purpose of the Hydrogen Breath Test
The primary reason a doctor orders an HBT is to confirm or rule out two main digestive conditions: Small Intestinal Bacterial Overgrowth (SIBO) and carbohydrate malabsorption. SIBO is a disorder where an abnormally high number of bacteria colonize the small intestine, which should normally have a low bacterial count. Carbohydrate malabsorption occurs when the body lacks the enzymes needed to properly digest certain sugars, such as lactose or fructose.
In both conditions, unabsorbed carbohydrates travel to parts of the gut where they are fermented by microbes. If SIBO is present, bacteria in the small intestine rapidly ferment the test sugar, producing hydrogen and methane gas that is exhaled. If the body cannot break down a sugar (malabsorption), the undigested sugar passes into the colon, where resident bacteria ferment it, causing a measurable spike in breath gas. The timing and location of the gas spike indicate which condition is present.
Preparing for an Accurate Test
The accuracy of the Hydrogen Breath Test relies heavily on strict patient adherence to preparatory guidelines. To ensure that only the test substrate is being fermented, patients must follow a specific, low-fiber, low-fermentable diet for 12 to 24 hours before the test. This preparatory diet typically restricts foods like whole grains, fruits, vegetables, nuts, seeds, and dairy, recommending instead simple foods such as baked poultry or fish with salt and pepper, and plain white rice.
The influence of gut bacteria on the test requires that certain medications and supplements be stopped well in advance. Patients are usually instructed to discontinue antibiotics and probiotics for up to four weeks prior to the test to prevent skewing the bacterial population. Laxatives, fiber supplements, and prokinetics, which affect gut motility, must also be stopped, often for at least one week. Furthermore, a strict fast is required for at least 12 hours leading up to the test, and activities like smoking and vigorous exercise must be avoided on the day of the procedure. Failure to follow these instructions can result in a high baseline gas reading, which may render the test inconclusive or lead to a false positive result.
Understanding the Testing Procedure
The Hydrogen Breath Test procedure begins with the collection of a baseline breath sample after the required overnight fast. The patient blows into a specialized collection device, such as a syringe or a breath bag, which captures the gas from the very end of the exhalation. This initial sample establishes the individual’s fasting level of hydrogen and methane, measured in parts per million (ppm).
Following the baseline collection, the patient ingests a measured dose of a specific sugar dissolved in water, known as the substrate. The choice of substrate, such as lactulose, glucose, lactose, or fructose, depends on the condition being investigated. After drinking the solution, the patient provides subsequent breath samples at regular, timed intervals, usually every 15 to 30 minutes, for a total duration of two to three hours. Throughout the testing period, the patient is advised to remain sedentary, avoiding eating, drinking, or smoking, to ensure consistent and accurate gas measurements.
Interpreting Normal and Abnormal Results
A normal, or negative, result on a Hydrogen Breath Test is defined by a low baseline gas level that remains below specific thresholds throughout the testing period. A healthy baseline reading for hydrogen is typically less than 16 ppm. If the initial reading is too high, it may suggest inadequate patient preparation or, in some cases, an underlying bacterial issue, necessitating a retest.
For a result to be considered normal, the increase in hydrogen or methane over the baseline must not reach the predetermined diagnostic cut-offs. The interpretation is highly dependent on the substrate used and the timing of the gas spike. A hydrogen rise of less than 20 ppm over the baseline throughout the test is generally considered a negative result for carbohydrate malabsorption.
When testing for SIBO, the timing of the gas increase is particularly important because fermentation occurring early in the small intestine is the diagnostic indicator. A common criterion for a positive SIBO result is a rise in hydrogen concentration of at least 20 ppm above the baseline within the first 90 minutes of the test. Conversely, a rise of 20 ppm or more that occurs after the 90-minute mark is more indicative of normal fermentation by bacteria in the colon, suggesting a normal result for the small intestine.
The test must also account for methane, which is produced by a different type of microbe called archaea that consumes hydrogen. A methane level of 10 ppm or more at any point during the test is often considered a positive result, indicating an overgrowth of these methanogens. Because diagnostic standards can vary between clinical guidelines and the specific test substrate used, a healthcare professional must analyze the full gas curve in the context of the patient’s symptoms and medical history for a proper diagnosis.