Lactose intolerance is frequently and mistakenly self-diagnosed. In babies, it is also often confused with cow milk allergy. Diagnosis should not be based on clinical symptoms alone and should be made by a physician who has performed certain laboratory tests.

Diagnosis outline

Lactose intolerance is usually diagnosed in the following way:

• A patient is asked to maintain a food diary recording everything they eat or drink
• The physician may instruct a patient to remove lactose from their diet completely for two weeks and reassess the presence or absence of symptoms afterwards. If symptoms are absent, the lactose intolerance is likely.
• A patient may be asked to undergo a blood sugar test. After an overnight fast, a patient drinks lactose solution and the sugar level of the blood is checked. If the patient is intolerant to lactose, blood sugar levels will either rise slowly, or not at all because the sugar from the lactose solution is not absorbed in the blood at a normal rate. A variant of the blood sugar test is the milk tolerance test, in which case a patient is given around 500ml of milk to drink after their overnight fast instead of lactose solution. Again, if blood shows no change in sugar level then the patient is considered to be lactose intolerant.
• The hydrogen breath test is a simple and very useful test that often confirms lactose intolerance. The person is asked to fast overnight, then given a lactose solution to drink after which they breathe into an instrument that measures the concentration of hydrogen (parts per million, ppm) in their breath. People who exhale a large amount of hydrogen (>20 ppm above the initial value) in an hour are diagnosed as lactose intolerant.
• The stool acidity test is another useful test for the condition in infants and young children. If the lactose remains undigested, it is converted by bacteria to lactic acid and other fatty acids that can be detected in the stool. Furthermore, glucose may also be detected in stools.
• One test that may be performed in diagnosing lactose intolerance is a stool bowel biopsy. However, this test is invasive and may require a hospital stay. It is rarely used to diagnose lactose intolerance but more to rule out other conditions such as celiac disease.
• A small amount of tissue is taken from the inside of the intestinal wall under local anaesthetic using an endoscope for guidance. An endoscope is a thin flexible tube with a light source and a camera at its tip that is threaded into the gut via the mouth. The sample of tissue is then stained and examined under a microscope and tested to see how much lactase it contains. Low levels of lactase indicate lactose intolerance.