Lactose intolerance - Diagnosis 

Diagnosing lactose intolerance 

Visit your GP if you think you have lactose intolerance. Before visiting 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

Your GP may suggest further tests to find out how much (if any) lactase your body is producing and what might be causing your lactose intolerance.

Hydrogen breath test

A hydrogen breath test is a simple and useful test.

You will be given a drink of lactose solution after you have fasted overnight. The concentration of hydrogen is measured in the air that you breathe out. It is measured in parts per million (ppm).

If after about an hour your breath contains a large amount of hydrogen (more than 20 ppm above your baseline), it is likely that you are lactose intolerant. The baseline is the amount of hydrogen that is present in your breath before drinking the lactose solution.

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 (a bowel condition that is caused by intolerance to a protein called gluten).

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). This will be carried out under local anaesthetic (a painkilling medication), 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, lactose intolerance is likely. The biopsy will also reveal whether there is another underlying cause for your lactose intolerance.

Page last reviewed: 17/05/2012

Next review due: 17/05/2014


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The 3 comments posted are personal views. Any information they give has not been checked and may not be accurate.

London Lady said on 10 April 2012

DororthyMargaret and SRAH, I am fasting this morning for a lactose intolerance test (hydrogen breath test) which is being conducted UCLH. I had a similar fructose test there last month - all part of a big investigation into IBS and bowel issues.

I was looking up to see if there are any side effects when I spotted your comments. If you are in / near London ask your GP for a referral to UCLH gastroenterology department ( or if out of the area, find a similar gastroenterology department at your local hospital and ask for a referral there. It often helps to tell your doctor where you would like to be referred to, rather than expect them to know. They frequently don't!
Good luck!

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dorothymargaret said on 05 April 2012

I have also been told by a GP that there is no test for lactose intolerance. It is all very puzzling

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SRAH said on 18 February 2012

I believe I might be lactose intolerant. And after reading this information (as I wanted to believe I can trust NHS info better than anything else on the web) I kept the said notes and went to see my GP and asked if it was possible to have a test done.

My GP said that from the symptoms I told her it did seem to her like I have lactose intolerance, but that there is no test that can be done to diagnose it.

I said I saw on the NHS website that there were a few test including Hydrogen Breath Test (and that I had fasted hopping I could I have it done then), to which she replied that it was very strange as it wasn't true, and assured me that there is no test that can be done to this effect.

She did suggest the exclusion diet, as mentioned above, but instead of 2 weeks she said it has to be done for 6 weeks.

As an IBS and lactose intolerant patient, I find this discrepancy in information within the NHS very confusing and thus not very helpful. I now completely understand people's need to turn to alternative help such as the Yorktest, which is largely disapproved by GPs.

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Screening and testing

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