Lactose breath test

What is a lactose breath test?

A lactose breath test, also called a hydrogen breath test, checks whether your body can properly digest lactose, the sugar found in milk and other dairy products. You drink a lactose solution, then breathe into a machine at intervals so your care team can measure the hydrogen in your breath.

Why might I need one?

Your doctor may suggest this test if you get symptoms such as abdominal pain, bloating, wind or diarrhoea after eating or drinking dairy products. The test helps confirm or rule out lactose intolerance, which is when your body can't properly break down lactose.

What are the benefits of having it?

A lactose breath test is a simple, safe way to find out whether lactose intolerance is causing your symptoms, without needing any needles or invasive procedures. Once your doctor has a clear diagnosis, they can help you manage your symptoms, often just by adjusting your diet.

Are there different types of hydrogen breath test?

A lactose breath test is one specific type of hydrogen breath test to see specifically how your body handles milk sugar. Other versions use different sugars to check for other conditions, such as fructose or sucrose for other sugar intolerances, or glucose to check for small bowel bacterial overgrowth (SIBO).

How do I prepare for the test?

You'll need to fast before the test – the care team will let you know exactly how long for. You can drink water, but avoid milk and fruit juice the day before. You also shouldn't smoke, chew gum, sleep or exercise for a period immediately before or during the test itself.

In the day or two before your test:

• avoid high-fibre foods such as fruit, vegetables, wholegrains, beans and bran for 24 hours beforehand
• base your meals around plain foods such as white bread, white rice or pasta, potatoes without their skin, and chicken or fish
• stop taking fibre supplements and laxatives for at least 3 days before the test
• avoid antibiotics for 4 weeks before the test, as these can affect your results.

Tell your care team before your test if you:

• have diabetes, so they can help you manage your blood sugar during the fast
• have had a colonoscopy or other bowel-cleansing procedure in the last 4 weeks
• are pregnant, or have a lung or heart condition
• are allergic to any medications
• have had recent diarrhoea, as this may affect your results.

On the day of the test, you can take your usual medicines, other than vitamins, laxatives and antibiotics, with a small sip of water. Brush your teeth as normal, but avoid using denture adhesive if you wear dentures.

What happens during the test?

The whole appointment usually takes around 2 to 3 hours in total.

1. You'll give an initial breath sample by breathing into a hand-held machine, usually by holding your breath briefly and then blowing steadily into a mouthpiece
2. You'll then drink a cup of water with lactose dissolved in it; some people feel a little nauseous after this
3. You'll give further breath samples at regular intervals, roughly every 15 to 30 minutes, for the next 2 to 3 hours
4. During this time, you shouldn't eat, chew gum, smoke, sleep, or exercise, as this can affect your results
5. You can read or bring a book or magazine to help pass the time between samples
6. You may also be asked to note down any symptoms you notice, such as bloating or abdominal discomfort.

What happens after the test?

You can go back to your normal activities, including driving, straight after the test, and you can eat and drink normally again.

Your results will be sent to your GP or the doctor who referred you, and they'll contact you to discuss them and arrange any treatment you might need. You may occasionally be asked to come back for a repeat test, for example if your first breath sample wasn't low enough to give a clear result.

What are the risks or side effects?

A lactose breath test is very safe. Because it works by giving you lactose to digest, it can sometimes bring on the same symptoms that led to the test in the first place, such as bloating, cramping or diarrhoea, though these should settle afterwards.

The test isn't suitable for everyone. Tell your doctor before your appointment if:

• you have unexplained low blood sugar after eating (postprandial hypoglycaemia), as the test shouldn't be carried out in this case
• you have diabetes, since fasting before the test can increase your risk of a low blood sugar episode (a 'hypo').