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Tests and diagnosis
By Mayo Clinic staffYour doctor may diagnose lactose intolerance based on your symptoms and your response to reducing the amount of dairy foods in your diet. Your doctor can confirm the diagnosis by conducting one or more of the following tests:
- Lactose tolerance test. You'll need to avoid eating before this test, to ensure accurate results. Once at the doctor's office, you'll drink a liquid that contains high levels of lactose. When this lactose reaches your digestive system, the lactase enzyme normally breaks it down into glucose and galactose, which can be absorbed by your bloodstream. In this test, you'll give blood samples over a two-hour period to measure your glucose level. If your glucose level isn't rising, it means you aren't properly digesting and absorbing the lactose-filled drink.
- Hydrogen breath test. This test also requires you to drink a liquid that contains high levels of lactose. Then your doctor measures the amount of hydrogen in your breath at regular intervals. Normally, very little hydrogen is detectable. However, undigested lactose reaches your colon and ferments, causing hydrogen and other gases to be released, absorbed by your intestines, and eventually exhaled. Large amounts of exhaled hydrogen indicate that you aren't fully digesting and absorbing lactose and that you're probably intolerant.
- Stool acidity test. Infants and children suspected of having lactose intolerance take a stool acidity test. The amount of lactose required for the lactose tolerance test or the hydrogen breath test may be dangerous for infants and children. The stool acidity test measures the amount of acid in the stool. Undigested and unabsorbed lactose ferments in the colon, creating lactic acid and other acids that can be detected in a stool sample.
