Diagnosing lactose intolerance 

It's important to visit your GP if you think you or your child may have lactose intolerance, as the symptoms can be similar to other conditions.

Before seeing your GP, keep a diary of what you eat and drink, and what symptoms you experience. Tell your GP if you notice any patterns, or if there are any foods you seem particularly sensitive to.

Your GP may suggest trying to remove lactose from your diet for two weeks to see if it helps to relieve your symptoms. This will provide further evidence of whether you're lactose intolerant.

Further testing

Other tests aren't usually needed, but your GP may sometimes suggest further tests to:

  • help confirm the diagnosis
  • find out how much lactase (the enzyme used to digest lactose) your body is producing
  • try to determine what might be causing your lactose intolerance

Some of the main tests that may be used are described below.

Hydrogen breath test

A hydrogen breath test is a simple way of determining if you may be lactose intolerant.

You'll be asked to avoid eating or drinking during the night before the test. When you arrive for the test, you'll be asked to blow up a balloon-like bag. This sample of your breath will be tested to find out how much hydrogen is present, measured in parts per million (ppm).

You'll then be given a drink of lactose solution and your breath will be tested every 15 minutes over the next few hours to see if the level of hydrogen changes.

If your breath contains a large amount of hydrogen (more than 20ppm above your baseline) after consuming the lactose solution, it's likely that you're lactose intolerant. This is because lactose intolerance can cause the bacteria in the colon (large intestine) to produce more hydrogen than normal.

Lactose tolerance test

In a lactose tolerance test, you'll be given a drink of lactose solution and a blood sample will be taken from your arm using a needle. The blood will be tested to see how much glucose (blood sugar) it contains.

If you're lactose intolerant, your blood sugar levels will either rise slowly, or not at all. This is because your body is unable to break down the lactose into glucose.

Milk tolerance test

In a milk tolerance test, you'll be given a glass of milk (about 500ml) and your blood sugar levels will be tested. If your blood sugar levels don't rise after drinking the milk, you may be lactose intolerant.

Small bowel biopsy

A small bowel biopsy is rarely used to diagnose lactose intolerance. However, it may be carried out to see if your symptoms are being caused by another condition, such as coeliac disease.

In a small bowel biopsy, a sample of your small intestinal lining is taken using an endoscope (a thin, flexible tube with a light and a tiny cutting tool at the end) that's passed down your throat. This will be carried out under local anaesthetic, so it won't hurt.

The sample of intestinal lining will be tested to see how much lactase it contains. If it only contains a small amount of lactase, it's likely you're lactose intolerant. The sample can also be examined to look for signs of possible underlying conditions such as coeliac disease.

Page last reviewed: 01/04/2016

Next review due: 01/04/2018