Lactose intolerance is usually diagnosed by doing a trial run. If you avoid lactose for a few weeks and your symptoms improve, but when you go back on to lactose your symptoms return, then you probably have lactose intolerance.
Another simple test is to compare whether someone can tolerate lactose-free milk but not ordinary milk.
Other tests that are sometimes done include breath tests and blood glucose tests to see if your body is producing enough of the enzyme lactase, which is used to break down lactose.
Occasionally, people with severe symptoms, where the diagnosis is not clear, will have a biopsy of their small intestine. In this test, a sample of bowel wall is taken during an endoscopy (when a camera is passed down into the intestines). This is an invasive test and is not commonly used.
Last reviewed: November 2018