Hydrogen Breath Test: What It Diagnoses & How to Prepare

What Is the Hydrogen Breath Test?

The hydrogen–methane breath test is a simple, non-invasive diagnostic tool that measures the concentration of hydrogen and methane gases in your exhaled breath to detect various gastrointestinal disorders. The test is based on the principle that when certain sugars or carbohydrates are not properly digested and absorbed in the small intestine, they reach the colon, where bacteria ferment them, producing hydrogen gas as a byproduct.

This hydrogen is then absorbed into the bloodstream, transported to the lungs, and exhaled in your breath, where it can be measured using specialised equipment. By analysing the pattern and levels of hydrogen and methane gases in your breath samples over a period of time, doctors can identify specific digestive issues and guide treatment decisions.

Conditions Diagnosed by the Hydrogen Breath Test

• Lactose Intolerance: This occurs when the body lacks sufficient lactase, the enzyme needed to digest lactose (milk sugar). Undigested lactose ferments in the colon, causing symptoms like bloating, diarrhoea, and abdominal pain.
• Fructose Malabsorption: Similar to lactose intolerance, fructose malabsorption involves the inability to properly absorb fructose (fruit sugar), leading to fermentation by gut bacteria and digestive symptoms.
• Small Intestinal Bacterial Overgrowth (SIBO): SIBO is characterised by an abnormal increase in the number of bacteria in the small intestine, which can interfere with normal digestion and cause bloating, diarrhoea, and malnutrition.
• Irritable Bowel Syndrome (IBS): The hydrogen–methane breath test may help identify overlapping conditions such as SIBO or carbohydrate malabsorption that can mimic or contribute to IBS-like symptoms.

Additionally, the test can be used to evaluate less common carbohydrate intolerances, such as sucrose or sorbitol malabsorption, and to assess orocecal transit time.

How the Hydrogen Breath Test Works

1. Baseline Measurement: You'll provide an initial breath sample by exhaling into a collection bag to establish your baseline hydrogen levels before consuming the test sugar solution.
2. Sugar Solution Consumption: You’ll drink a solution containing a specific type of carbohydrate substrate (such as lactose, lactulose, fructose, or glucose) that will challenge your digestive system.
3. Bacterial Fermentation: If the sugar is not properly digested in the small intestine, it travels to the colon (or encounters bacteria in the small intestine in cases of SIBO), where anaerobic bacteria ferment the unabsorbed carbohydrates.
4. Hydrogen Production and Absorption: The bacterial fermentation process produces hydrogen gas, which is absorbed through the intestinal walls into the bloodstream.
5. Transport to Lungs: The absorbed hydrogen travels through the bloodstream to the lungs, where it diffuses into the air spaces.
6. Breath Collection at Intervals: You'll provide Breath samples every 15 to 20 minutes over a period of two to three hours, depending on the substrate used as your body digests the sugar solution.
7. Measurement and Analysis: Each breath sample is analysed to measure the hydrogen concentration in parts per million (ppm).

Types of Substrates Used in Testing

• Lactose: Used to diagnose lactose intolerance
• Fructose: Used to diagnose fructose malabsorption
• Lactulose: Used to assess small intestinal transit time and, indirectly, to screen for SIBO
• Glucose: Also used to diagnose SIBO
• Sucrose: Used to detect sucrose intolerance
• Sorbitol: Used to identify sorbitol intolerance

Preparing for Your Hydrogen Breath Test

1. Fast for 12 hours before the test, avoiding all food and beverages except water.
2. Avoid high-fibre foods, dairy products, and fermentable carbohydrates (such as beans, onions, and whole grains) the day before your test, as these can affect bacterial fermentation patterns.
3. Inform your doctor about any medications or supplements you're taking, as some may need to be temporarily discontinued before the test (such as antibiotics, probiotics, or laxatives), according to the NHS Trust.
4. Refrain from smoking or engaging in strenuous physical activity for at least 2 hours before and during the test, as these can influence hydrogen production and breath measurements.
5. On the morning of the test, brush your teeth without using toothpaste or mouthwash, as these products may contain sugars that could interfere with results.

Which Foods and Medications to Avoid Before the Test

• Complex carbohydrates (bread, pasta, grains)
• Fiber-rich foods (fruits, vegetables, legumes)
• Dairy products (milk, cheese, yoghurt)
• Sugary drinks and artificial sweeteners
• Antibiotics (typically discontinued for at least 4 weeks before testing)
• Probiotics (discontinued 4 weeks prior)
• Laxatives and anti-diarrhoeal medications
• Proton pump inhibitors (PPIs) — discuss with your doctor before stopping

Always consult your doctor for personalised guidance on which medications to temporarily stop before the test.

What to Expect During the Test Procedure

On the day of your hydrogen breath test, you'll arrive at the testing facility having fasted for at least 12 hours. The technician will verify that you've followed all pre-test instructions and collect an initial baseline breath sample.

You'll then drink the sugar solution specific to the condition being tested (e.g., lactose for lactose intolerance or fructose for fructose malabsorption). Over the next 2-4 hours, you'll provide breath samples every 15-30 minutes by exhaling into a collection bag or device.

Breath Sample Collection Process

1. Exhale normally into the mouthpiece of the collection bag until it is fully inflated.
2. Close the bag tightly to prevent any air from escaping.
3. The technician will remove the breath sample from the bag using a syringe.
4. The sample is then injected into the hydrogen breath analyser for measurement.
5. This process is repeated at regular intervals (usually every 15-30 minutes) for the duration of the test.

Interpreting Hydrogen Breath Test Results

Your doctor will interpret the results of your hydrogen breath test based on the pattern and levels of hydrogen in your breath samples over time. Generally, a significant rise in hydrogen concentration (≥20 ppm above baseline within 90–120 minutes) indicates a positive result, suggesting that the sugar substrate was not properly digested and absorbed.

Understanding Positive vs Negative Results

• Positive Results: A positive hydrogen breath test result is typically defined as a rise in breath hydrogen concentration of 20 parts per million (ppm) or more above the baseline level. This indicates that the sugar substrate was not properly digested and absorbed, leading to bacterial fermentation and hydrogen production. A positive result indicates carbohydrate malabsorption or bacterial overgrowth depending on the substrate. For example, an early rise (within 90 min) after glucose or lactulose suggests SIBO, whereas a later rise indicates carbohydrate malabsorption.
• Negative Results: A negative result is characterised by no significant increase in breath hydrogen levels throughout the test period. This suggests that the sugar substrate was effectively digested and absorbed in the small intestine, with minimal bacterial fermentation occurring in the colon. A negative result usually rules out the conditions tested, but false-negatives can occur in individuals who predominantly produce methane or hydrogen sulfide instead of hydrogen. However, it's important to note that false-negative results can occur in some cases, particularly if the individual has recently taken antibiotics or has an altered gut microbiome.

Limitations and Accuracy of the Hydrogen Breath Test

False-negative results can occur if an individual has recently taken antibiotics, which can temporarily suppress the gut bacteria responsible for hydrogen production. Additionally, some people may not have enough hydrogen-producing bacteria in their gut, leading to false-negative results. False-positive results can also occur if an individual has not followed the proper pre-test preparation instructions or has consumed foods or medications that affect hydrogen levels.

Despite these limitations, the hydrogen–methane breath test is generally considered a reliable and validated method for diagnosing lactose intolerance, fructose malabsorption, and SIBO when performed correctly and interpreted in the context of an individual's symptoms and medical history.

Risks and Side Effects

• Mild bloating, abdominal discomfort, or diarrhoea during the test due to the sugar solution consumed
• Dizziness or lightheadedness from fasting or prolonged breath-holding during sample collection
• Rarely, mild gastrointestinal upset or intolerance to the test sugar (such as lactulose) may occur, but allergic reactions are extremely uncommon

If you experience severe or persistent symptoms, consult your doctor immediately.

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FAQs

What does a positive hydrogen breath test mean?

A positive hydrogen breath test indicates that the sugar substrate consumed during the test (e.g., lactose, fructose, or lactulose) was not properly digested and absorbed in the small intestine. This can suggest the presence of a specific digestive disorder, such as lactose intolerance, fructose malabsorption, or SIBO, depending on the substrate used and the pattern of hydrogen elevation.

How should I prepare for a hydrogen breath test?

1. Fast for 12 hours before the test
2. Avoid complex carbohydrates, high-fiber foods, and specific sugars the day before
3. Discontinue antibiotics, probiotics, and certain medications as directed by your doctor
4. Do not smoke or engage in vigorous exercise before or during the test
5. Avoid using toothpaste or mouthwash on the morning of the test

How long does the hydrogen breath test take?

The hydrogen breath test procedure typically takes 2-4 hours to complete. During this time, you'll need to remain at the testing facility and provide breath samples every 15-30 minutes after consuming the sugar solution.

Are there alternatives to the hydrogen breath test?

While the hydrogen–methane breath test is a reliable and non-invasive method for diagnosing certain gastrointestinal disorders, there are alternative tests available, such as lactose tolerance tests, small bowel aspirate and culture, and glucose tolerance tests. Your doctor will determine the most appropriate test based on your specific symptoms and medical history.

Can medications affect my hydrogen breath test results?

Yes, certain medications can affect the accuracy of hydrogen breath test results. Antibiotics, probiotics, laxatives, and proton pump inhibitors can alter the gut microbiome and influence hydrogen production. It's essential to inform your doctor about all the medications you are taking and follow their instructions regarding which ones to discontinue before the test.

References

1. https://my.clevelandclinic.org/health/diagnostics/12360-hydrogen-breath-test
2. https://www.nbt.nhs.uk/our-services/a-z-services/gastrointestinal-gi-physiology/gastrointestinal-gi-physiology-patient-information/hydrogen-breath-test
3. https://www.sth.nhs.uk/clientfiles/File/HBT%20REview%20date%20March%202025.pdf
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC4175689/#limitations