The Hydrogen Breath Test (HBT) is a non-invasive procedure used to investigate common digestive complaints such as bloating, abdominal pain, and chronic diarrhea. This diagnostic tool works by measuring the levels of certain gases, specifically hydrogen and often methane, in a patient’s exhaled breath. The HBT helps healthcare providers identify if a patient has difficulty digesting specific sugars or if there is an abnormal overgrowth of bacteria in the small intestine. By analyzing the pattern of gas production after a patient consumes a test solution, the test provides objective data to confirm or rule out specific causes of gastrointestinal distress.
The Scientific Basis of Hydrogen Measurement
The fundamental principle behind the breath test is that human cells do not naturally produce hydrogen gas. Instead, hydrogen is created almost exclusively by the metabolic activity of bacteria residing in the gastrointestinal tract. In a healthy digestive system, most carbohydrates are fully broken down and absorbed in the small intestine, leaving little for the bacteria in the large intestine to ferment. When carbohydrates, such as sugars or starches, are not properly absorbed, they travel undigested to the lower digestive tract.
Upon reaching the colon, or the small intestine in cases of bacterial overgrowth, the resident bacteria begin to ferment these unabsorbed sugars. This fermentation process releases various gases, including hydrogen. The hydrogen gas produced by the bacteria is then absorbed through the intestinal lining into the bloodstream. From the bloodstream, the gas travels throughout the body until it reaches the lungs, where it is exchanged and expelled when a person exhales. By collecting and analyzing the concentration of hydrogen in the breath, the test indirectly measures the amount of unabsorbed carbohydrate that reached the bacteria in the gut.
Common Conditions Diagnosed
The Hydrogen Breath Test is used to diagnose several distinct gastrointestinal conditions, primarily by using different sugar substrates. Small Intestinal Bacterial Overgrowth (SIBO) is a common application where bacteria from the large intestine colonize the small intestine, causing premature fermentation of food. For SIBO testing, a non-absorbable sugar like lactulose or a rapidly absorbed sugar like glucose is administered to see if bacteria are present in the upper digestive tract where they should not be.
The HBT is also used to diagnose carbohydrate malabsorption, which occurs when the body lacks the necessary enzymes to break down certain sugars. Lactose intolerance, the inability to digest the sugar in milk, is diagnosed using a lactose solution as the substrate. If the patient lacks the lactase enzyme, the lactose reaches the colon and is fermented, leading to a measurable spike in breath hydrogen.
Fructose malabsorption is identified by having the patient consume a fructose solution. A positive result indicates that the small intestine is unable to efficiently absorb the fructose, allowing it to pass into the colon for bacterial action. The selection of the specific test substrate—lactose, fructose, or glucose/lactulose—dictates which condition the physician is investigating.
Preparing for and Undergoing the Procedure
A successful and accurate Hydrogen Breath Test requires strict patient preparation to ensure a low baseline level of hydrogen in the gut before testing begins. Patients are typically required to stop taking antibiotics and probiotics for a period of one to four weeks before the test, as these medications can drastically alter the gut bacteria population. Additionally, certain medications, such as laxatives and prokinetics that affect gut motility, must often be discontinued a week before the procedure.
For 24 hours leading up to the test, patients must adhere to a highly restricted, low-fermentable diet, avoiding high-fiber foods like whole grains, fruits, vegetables, and beans. The most rigorous requirement is an overnight fast, typically 12 hours, where nothing is consumed except for plain water. Smoking, chewing gum, and vigorous exercise must also be avoided for at least an hour before and throughout the test, as these activities can skew the gas readings.
The procedure itself begins with the patient providing a baseline breath sample by blowing into a specialized collection device, such as a balloon or a breathalyzer-like machine. This initial sample establishes the patient’s resting hydrogen level, which should be very low after the preparatory fast. Immediately after the baseline is taken, the patient drinks the specific sugar solution—lactose, fructose, or lactulose—dissolved in water.
Following the consumption of the substrate, the patient remains in the clinic and provides subsequent breath samples at timed intervals, usually every 15 to 30 minutes, for a total duration of two to three hours. During this period, the patient is monitored for any symptoms, such as bloating or gas, which are often recorded alongside the gas measurements. This series of timed samples allows the medical team to track the digestive process and pinpoint where fermentation is occurring.
Understanding the Test Results
Interpreting the HBT involves analyzing the parts per million (ppm) of hydrogen and sometimes methane in the collected breath samples relative to the initial baseline reading. A result is generally considered positive for carbohydrate malabsorption, such as lactose or fructose intolerance, if the breath hydrogen concentration rises by 20 ppm or more above the fasting baseline. This elevated reading signifies that a significant amount of the consumed sugar reached the colon and was fermented by bacteria.
For the diagnosis of Small Intestinal Bacterial Overgrowth (SIBO), the threshold is typically a rise in hydrogen of 12 ppm or more over the baseline, particularly when using a glucose substrate. The timing of the gas peak is also an important factor in differentiation. An early rise in hydrogen, usually within the first 90 minutes, suggests fermentation is occurring quickly in the small intestine, indicative of SIBO. Conversely, a hydrogen peak that occurs later suggests the fermentation is happening in the colon, which is the expected location.
Many facilities now measure methane gas concurrently with hydrogen, as a substantial portion of the population produces methane instead of, or in addition to, hydrogen. A rise in methane of 10 ppm or more is considered a positive result. Concurrent measurement helps reduce the chance of a false negative result that could occur if only hydrogen were measured.