SIBO test – Lactulose breath test: procedure, preparation, results

In short: The lactulose breath test shows whether too many bacteria live in the small intestine (SIBO). You do it comfortably at home: after drinking a test solution, you give several breath samples over about three hours. The lab measures hydrogen and methane.

How does the test work?

Lactulose is a sugar the body can't absorb itself. It reaches the gut undigested, where bacteria ferment it. This produces hydrogen and methane, which pass via the blood into the lungs and are breathed out. If these gases rise early – before the lactulose has reached the large intestine – this points to too many bacteria in the small intestine.

Preparation

Your lab will give you detailed instructions. Typical steps are:

• About four weeks before: no antibiotics. Stop probiotics and laxatives beforehand, as agreed with your practitioner.
• The day before: a light, low-fermentation diet, for example rice, eggs, fish or meat – no vegetables, fruit, wholegrains or dairy.
• Before the test: about 12 hours of fasting, water only. No smoking, no exercise.

On the day of the test

1. You give a first breath sample (baseline).
2. You drink the lactulose solution.
3. You then give another sample about every 30 minutes, over roughly three hours in total.
4. You send the tubes to the lab.

During the test, stay at rest, don't eat and drink only a little water.

What do the results show?

• Hydrogen rises early: a sign of hydrogen-producing SIBO.
• Methane is raised: a sign of methane-producing microorganisms, often linked to constipation.
• Both stay low: SIBO is less likely; then we look at other causes.

The values alone are not a diagnosis. I always discuss them together with your symptoms and history – and that's what your treatment plan is built on.

Hydrogen or methane: what's the difference?

Hydrogen is produced by bacteria when they ferment carbohydrates. When mainly hydrogen is raised, bloating, fullness and rather loose stools or diarrhoea are often in the foreground.

Methane is produced differently: not by bacteria, but by so-called archaea. These microorganisms use the bacteria's hydrogen as “food” and turn it into methane. That's why hydrogen can even look normal when methane producers have overgrown. Methane slows down gut movement – so constipation and a hard, sluggish belly are typical. Because methane producers can live not only in the small intestine but also in the large intestine, specialists now often call this IMO (intestinal methanogen overgrowth).

A third gas, hydrogen sulfide, is usually not measured by standard breath tests. It can be linked to diarrhoea and foul-smelling wind. A normal test result therefore doesn't rule out overgrowth in every case.

The distinction matters because hydrogen and methane producers are treated differently. That's why I put together your therapy according to your gas profile.

When should I see a doctor first?

Blood in the stool, unintended weight loss, fever or severe pain should be checked by a doctor first.

Find out more: Bloating assessment · SIBO: when your bloating isn't “normal”

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