Lactose tolerance test

Reviewed by John Pillinger, GP

What is a lactose tolerance test?
A lactose tolerance test measures how well the body breaks down milk sugar (lactose) in the intestines.

What happens during a lactose tolerance test?
The patient must not eat or drink for 8 to 12 hours before the test. After this, a blood sample is taken from the fingertip in order to measure the amount of sugar in the blood.

A solution containing milk, sugar and some lemon is then drunk by the patient. If they are able to break down milk sugar properly in the intestines, the level of sugar in the blood will rise within an hour of drinking the solution.

Further blood samples are then taken to see if this is happening.

If the level of blood sugar does not rise this may mean lactose is not split into galactose and glucose. This is often due to a lack of a certain enzyme (lactase), which breaks down the lactose. This problem is called lactose intolerance.

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