Lactose intolerance - Diagnosis 

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


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The 5 comments posted are personal views. Any information they give has not been checked and may not be accurate.

MissTified said on 22 November 2014

The thing is, you advise there ARE tests...
So why, oh why do my Doctors not do them?
I had to establish my intolerance via a homoeopathic clinic, costing an arm and a leg.
Hence MissTified...

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cobob04 said on 25 April 2014

I have been diagnosed with Lactose Intolerance a number of times during my life. After the third test it was concluded I was not getting better and it would be a life time issue. now I suspect my son has this issue although not a bad as me. He can have a small yoghurt, milk in serial and a small amount of chocolate. Any more than this and he is in trouble, stomach cramps followed very shortly by a very runny bowel movement. He was referred to the hospital doctor who after no testing informed his school he was not suffering from lactose intolerance and that there was no actual test for this condition. As may be seen on this site there are tests that may be done to check on this condition. I had the breath test done which gave me a level of intolerance - my result was 98% so no lactose food/drink at all. I would suspect my son has a level of about 60%. But who would do the test to finally get an answer on his condition?

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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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Blood tests

Blood tests are carried out for a variety of reasons and can provide a wide range of information