Hydrogen breath test

Hydrogen is a gas that is present in the atmosphere in minute amounts.

It is also produced in the caecum (first part of the large bowel) when carbohydrates are fermented by the colonic bacteria. This is then absorbed into the bloodstream, taken to the lungs and is excreted on the breath.


As hydrogen is not usually present in quantities of more than 20ppm the test is useful in determining:

1. Malabsorption of certain sugars, e.g. lactose (alactasia).

2. Bacterial overgrowth in the small intestine (due to inflammation and strictures as seen in Crohn's disease.


1. Nothing to eat or drink after midnight prior to the test.

2. No smoking from 6pm the night before. (Gases produced by the cigarette smoke may interfere with the measurement).

About the Test

It is a simple, painless test, which takes around 3½ hours. Breath samples are taken using a syringe and mouthpiece. This is placed in the mouth and on breathing out, the plunger is drawn back and the sample is retained in the syringe.

A sample is taken on arrival in the department as a baseline, (i.e. fasting).

This is followed by drinking a sugar solution (depending on what the doctor is looking for).

Samples of breath are taken at half-hourly intervals and placed into a monitor and the amount of hydrogen in the sample is measured.

At the end of the test, results are shown on a graph and analysed by computer.

What happens next?

The doctor will review the results and decide whether a special diet is indicated in order to exclude certain foods which may be causing your symptoms. It may be that foods are not considered to be accounting for the symptoms and further investigations might need to be carried out