What is the most common clinical test to evaluate lactose intolerance and what does a positive result indicate?

• A. Hydrogen breath test; positive result indicates malabsorption of lactose due to lactase deficiency
• B. Urine glucose test
• C. Stool occult blood test; indicates GI bleeding
• D. Blood test for IgE antibodies to milk

Answer: (A)

Lactose intolerance arises when the small intestine lacks enough lactase to digest lactose, so lactose passes into the colon where bacteria ferment it. The hydrogen breath test is the most common way clinicians assess this. After a lactose-containing drink, if lactase is deficient, lactose isn’t absorbed and is fermented by colonic bacteria, producing hydrogen. That hydrogen enters the bloodstream and is exhaled in the breath, so a noticeable rise in breath hydrogen over the next few hours indicates lactose malabsorption due to lactase deficiency. A positive result supports lactose intolerance. The other tests don’t answer this question: urine glucose testing isn’t used for lactose intolerance, stool occult blood checks for GI bleeding, and blood tests for IgE antibodies to milk assess milk allergy rather than enzyme deficiency.