What does the test involve? In this test, the person drinks a beverage containing fructose, lactose, or lactulose, and then they are asked to breathe out into a bag so that the breath is analyzed at regular intervals. Raised levels of hydrogen in the breath indicate improper digestion. The breath sample will be analyzed for hydrogen content to determine if you are able to properly break down the lactose or sucrose, or if you have bacterial overgrowth. The test for glucose lasts about 2 hours and the test for lactose, fructose, and sucrose lasts about 2-3 hours.