Several tests are available to help doctors diagnose lactose intolerance. The latter test is especially useful in diagnosing lactose intolerance in infants. However, most people don't need a doctor's diagnosis to know they suffer from lactose intolerance. They know because of what happens every time they drink a glass or two of milk. The level of lactose intolerance is highly variable from person to person.
Some people have symptoms after they drink or eat even small quantities of dairy products.
Others find they have symptoms after drinking milk, but have no symptoms after eating ice cream or aged cheeses. One reason aged cheese is often tolerated when other dairy products are not is because as cheese ages, the lactose in it is gradually transformed into lactic acid. Lactic acid doesn't cause problems for people who are lactose intolerant. Many of these products are now readily available in supermarkets. Stores offer 100 percent and 75 percent lactose-reduced milk.