You should visit your GP if you think that 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 confirm that you are lactose intolerant.
To find out how much, if any, lactase your body is producing, and what might be causing your lactose intolerance, your GP may suggest some further tests.
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 (not eaten) 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 afterwards, your blood sugar levels will be tested. If your blood sugar levels do not rise, you may be lactose intolerant.
A stool (faeces) sample may be taken from babies and young children to test for lactose intolerance. This is because large doses of lactose, such as those given in the lactose and hydrogen breath tests, are dangerous for young children.
Instead, stool samples are taken, and the amount of acid in them is measured. If the baby, or child, is lactose intolerant, there will be a high amount of fatty acid, such as acetate, present. This is created by the reaction between bacteria in the colon and the undigested lactose.
The stool sample can also be tested to determine whether there are any parasites present, such as giardia lamblia, or cryptosporidia, which can also cause stomach problems.
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 confirm whether or not 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). The procedure 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.