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Diagnosis - 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 2 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 are not 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:

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.

The blood will be tested to see how much blood sugar (glucose) 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 do not rise after drinking the milk, you may be lactose intolerant.

Small bowel biopsy

A small bowel biopsy is rarely used to diagnose lactose intolerance.

But it may be carried out to see if your symptoms are being caused by another condition, such as coeliac disease.

Page last reviewed: 25 February 2019
Next review due: 25 February 2022