Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Lactose Intolerance

Diagnosis

It's possible you have lactose intolerance if your symptoms improve dramatically when you avoid lactose. A trial period of a lactose-free diet is usually all that is needed to make the diagnosis of lactose intolerance. In some cases, your doctor will want to do tests to confirm the diagnosis.

One test to confirm the diagnosis is the lactose breath hydrogen test. The test is painless and noninvasive. You cannot eat food for several hours beforehand.

You begin the test by drinking a liquid that contains lactose. Your breath is then sampled for hydrogen over a few hours. Normally, very little hydrogen is detected in your breath. However, if you have lactose intolerance, the bacteria in your colon will break the undigested lactose into hydrogen gas. The gas is absorbed into your bloodstream and then moves through the bloodstream to your lungs and exhaled. You will be diagnosed with lactose tolerance if higher-than-normal hydrogen levels are detected during this test. Bacterial overgrowth can also cause a positive test result, so it may be considered as an alternative explanation if your test is positive.

Another test that can be used to diagnose lactose intolerance is the lactose tolerance test. You begin this test by drinking a lactose solution. This test measures blood sugar levels at selected intervals over a few hours to determine your ability to digest lactose. If the lactose is normally digested, it is broken down into glucose, and this elevates your blood sugar level. You will be diagnosed with lactose intolerance if your blood sugar levels do not change during this test, because this shows that the lactose was not digested in the normal way.

A significant number of people who have symptoms that suggest lactose intolerance will have normal results on diagnostic tests. Similar symptoms (but normal results on the tests) can be caused by fructose, sorbitol or other sugars that are not easily digested in the small intestine. Similar symptoms can also result from irritable bowel syndrome.