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Hydrogen and Methane Breath Tests

This is usually done to identify small intestinal bacterial overgrowth. Longer tests may also be used to identify malabsorption of sugars such as lactose or fructose which are usually absorbed in the small bowel. Tests are usually undertaken using lactulose, glucose, fructose (for fructose malabsorption) or lactose (lactose malabsorption)


These tests assess the amount of hydrogen or methane expelled in the breath of individuals following the ingestion of a sugar. Humans have no innate pathway by which they can produce hydrogen or methane, therefore any hydrogen or methane produced by the breakdown of sugar is produced by bacteria.


Usually sugars take between 60-90 minutes to travel from the mouth to the end of the small bowel. It is therefore assumed that a rise in hydrogen or methane before 60 minutes is up reflects production of these gases by bacteria residing in the small bowel. Thus a rise of 20 parts per million of hydrogen, or 12 parts per million of methane, in the breath within the first sixty minutes of ingestion of the test sugar is presumed to reflect small intestinal bacterial overgrowth. A rise after this time, particularly after 90 minutes is deemed to originate from the large bowel. There are of course many variables in this test. Some patients (despite a preceding fast) will have colonic bacteria that ferment sugars from the outset of the test. Patients with rapid GI transit (the food rushes through the bowel) may find that the sugars don’t take as long as 60 (or even 30 minutes) to reach the large bowel where colonic bacteria can be expected to ferment the sugar thence giving a false positive test. Patients who have had surgery to their bowel which makes it shorter or speeds their small bowel transit (as may occur in ileal resection) may also have reduced transit times giving false positive readings. In addition several drugs may impact upon the speed it takes for sugars to leave the stomach and travel through the small bowel.


Usually the doctors or scientists will consider this when requesting this test.

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