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 that you seem particularly sensitive to.

Your GP may suggest that you try removing lactose from your diet for two weeks to see if it helps to relieve your symptoms. This will provide further evidence that you are lactose intolerant.

Further testing

Other tests are not usually needed, but your GP may sometimes suggest further tests to help confirm the diagnosis, to find out how much lactase (the enzyme used to digest lactose) your body is producing or to 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 and useful way of determining if you may be lactose intolerant.

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

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

If your breath contains a large amount of hydrogen (more than 20 ppm above your baseline) after consuming the lactose solution, it is likely that you are 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 will be given a drink of lactose solution, and then a sample of blood 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 are 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 will be given a glass of milk (about 500ml) and your blood sugar levels will be tested. If your blood sugar levels do not rise after drinking the milk, you may be lactose intolerant.

Small bowel biopsy

As a small bowel biopsy is an invasive surgical procedure, it 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 is passed down your throat. This will be carried out under local anaesthetic, so it will not 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 is likely you are lactose intolerant. The sample can also be examined to look for signs of a possible underlying conditions such as coeliac disease.


Page last reviewed: 28/04/2014

Next review due: 28/04/2016