What is the lactose breath test?

The lactose breath test (LBT) is the main diagnostic test for lactose intolerance. In a LBT, a fasted patient drinks a lactose solution. Breath hydrogen is measured before drinking the lactose, and then again every 20 minutes for the next 2–5 hours. A rise in breath hydrogen of ≥20 ppm is considered positive for lactose intolerance. Frequently, breath methane is measured along with hydrogen, and a rise of ≥10 ppm in breath methane is considered positive.^[1] Both hydrogen and methane are byproducts of bacterial metabolism; a sizeable increase in one or both after fasted lactose ingestion indicates that the lactose the patient drank is available to their intestinal bacteria for fermentation, and is therefore not being broken down in the small intestine. Some patients may also experience digestive symptoms during the LBT, but this isn’t required for diagnosis.