The Hydrogen Breath Test (HBT) is a non-invasive diagnostic tool used to investigate common gastrointestinal symptoms like bloating, abdominal pain, and diarrhea. The procedure involves a patient consuming a specific sugar solution, after which the concentration of hydrogen and methane gas in their breath is measured at timed intervals. The test works because only specific microorganisms in the digestive tract produce these gases. Measuring these levels provides insight into small intestine function and microbial activity.
The Primary Conditions Indicated by Elevated Hydrogen
A positive hydrogen breath test result points toward small intestinal bacterial overgrowth (SIBO) or carbohydrate malabsorption. Both conditions involve undigested sugars reaching parts of the gut where they should not be, leading to fermentation. The location of this fermentation differentiates the two diagnoses.
SIBO is characterized by an abnormally high number of bacteria colonizing the small intestine, which should naturally contain a relatively low microbial population. These misplaced microbes ferment carbohydrates much earlier in the digestive process, generating a rapid and significant rise in breath hydrogen.
Carbohydrate malabsorption is a failure to properly digest a specific sugar, such as lactose or fructose, in the small intestine. Since human digestive enzymes are insufficient or inactive, the sugar travels undigested into the large intestine. Colonic bacteria then ferment this sugar, producing a positive hydrogen result later in the test timeline.
The Physiological Reason for Hydrogen Production
The fundamental reason a positive hydrogen test is diagnostic is that human cells do not produce hydrogen gas. Hydrogen is exclusively a metabolic byproduct of anaerobic fermentation carried out by certain types of bacteria and archaea. The test relies entirely on this microbial metabolism to generate the measurable gas.
When a carbohydrate is not absorbed by the small intestine lining, it passes to the bacteria, which consume it as a food source. This anaerobic fermentation releases gases, primarily hydrogen. The gas is readily absorbed through the intestinal wall and into the bloodstream.
The blood carrying the dissolved hydrogen gas circulates throughout the body and eventually reaches the lungs. There, it diffuses into the air sacs and is expelled with the exhaled breath. By collecting breath samples over a period of hours, the timing and quantity of hydrogen gas can be precisely measured in parts per million (ppm).
Interpreting Results Based on the Test Substrate
The interpretation of a positive result depends entirely on the specific sugar, or substrate, that the patient ingested for the test. Different sugars are used to target different areas of the gut and diagnose distinct conditions. The most common substrates are glucose, lactulose, lactose, and fructose.
Carbohydrate Malabsorption Testing
Tests using naturally occurring sugars like lactose, fructose, or sucrose primarily diagnose malabsorption. A positive result, generally defined as a rise of 20 ppm of hydrogen above the baseline, indicates that the body’s enzymes failed to break down that sugar. This allows it to reach the colon for bacterial fermentation, and the delayed rise suggests a digestion problem rather than bacterial overgrowth.
SIBO Testing
For the diagnosis of SIBO, glucose or lactulose are the preferred substrates. Glucose is an easily absorbed sugar. A positive result is typically defined as a rise in hydrogen of 12 ppm or more above the lowest point within the first 90 minutes. Since glucose is absorbed high in the small intestine, an early, significant spike suggests bacteria are present in the upper small bowel, fermenting the sugar before absorption.
Lactulose is a synthetic sugar that human enzymes cannot absorb, meaning it travels the entire length of the small intestine. A positive SIBO result on a lactulose test is often defined as a rise of 20 ppm above the baseline within the first 90 minutes. This early rise indicates that bacteria in the small intestine fermented the lactulose before it reached the large intestine.
Next Steps After Receiving a Positive Result
A positive hydrogen breath test is strong evidence that requires consultation with a healthcare provider. The results must be interpreted in the context of the patient’s symptoms and medical history to formulate a definitive diagnosis and treatment plan.
Treatment for SIBO
For a positive SIBO result, treatment often involves targeted antibiotics, such as rifaximin, or the use of prokinetic agents to improve gut motility. In some cases, a temporary elemental diet may be recommended to starve the overgrown bacteria. The overall goal is to reduce the bacterial population in the small intestine and restore the natural balance.
Treatment for Malabsorption
If the test indicates carbohydrate malabsorption, such as lactose or fructose intolerance, the primary course of action is dietary modification. This typically involves reducing or eliminating the specific offending carbohydrate from the diet. Ongoing monitoring or retesting may be necessary to confirm the effectiveness of the treatment.