Hydrogen breath testing is used to evaluate several different gastrointestinal problems including intolerance of various sugars (such as lactose intolerance) and overgrowth of bacteria in the small intestine. Bacteria in the intestinal tract can produce hydrogen when they are exposed to unabsorbed sugars. This hydrogen gets into the bloodstream, is taken to the lungs, and then removed from the body in the breath. To do this test, the child is asked to blow into a bag (to get a baseline reading). Then they are given a measured amount of a specific sugar to drink. At intervals after this, the child blows into a bag. The hydrogen in their breath will be measured. The test takes about 4 hours.