lactose breath test

Last reviewed 01/2018

The breath hydrogen test may be used to diagnose the malabsorption of specific carbohydrates. Sugars malabsorbed in the small bowel are metabolised by colonic bacteria with the production of hydrogen. The hydrogen diffuses rapidly across the colonic mucosa into the blood and can be measured in the breath.

The test is most commonly used to diagnose lactose intolerance but may be used to detect rarer conditions such as hyposucrasia, and glucose-galactose malabsorption.

Patients should avoid high roughage foods the evening before the test in an attempt to achieve a low basal breath hydrogen. Following an overnight fast, brushing the teeth will help eliminate oral fermentation of the substrate during the test.

A basal sample is taken. Fifty grams of substrate in 200 ml of water are then swallowed. End-expiratory breath samples are recorded at 15 or 30 minute intervals for two hours. A positive test is indicated by a rise of 20 ppm of hydrogen above baseline.

False positive results may be caused by bacterial colonisation of the normally sterile small intestine; false negatives, by absence of the colonic organisms responsible for hydrogen production, due to antibiotic treatment or acute gastrointestinal upset.