A hydrogen breath test measures the amount of hydrogen in your child's breath, and helps diagnose several digestive problems your child may have, including lactose (and other kinds of sugar) malabsorption, and bacterial overgrowth in the small bowel.
For this test it is important for your child to fast overnight. At the test, a drink will be provided with the sugar (lactose, for example) to be tested. After drinking the liquid, your child will then blow into a machine that will measure the hydrogen content in her exhaled breath.
Hydrogen levels will be elevated if the sugar is malabsorbed (or in certain other diseases). The test can take several hours to complete because multiple samples of breath are needed.