Lactose Intolerance Tests


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To find out what is causing your lactose intolerance and how deficient of the lactase enzyme you are your GP may ask you to do some further tests. These can include:

  • Hydrogen breath test
  • Lactose tolerance test
  • Milk tolerance test
  • Stool sample
  • Small bowel biopsy

Hydrogen Breath Test

The hydrogen breath test in probably the most simplistic of all of the tests for lactose intolerance. It is used to diagnose two conditions: bacterial overgrowth in the gut or lactose intolerance.

Usually, little hydrogen is detected when you breathe out. If undigested lactose reaching your colon it will ferment, which releases hydrogen gas among others. Such gases are absorbed by the intestine and subsequently released during exhalation. Such events happen in lactose intolerance, hence the hydrogen breathe test can be diagnostic for lactose intolerance.

The test usually takes around 2 to 3 hours in total. It measures the amount of hydrogen when you breathe out, which is measured in parts per million (ppm). Usually, there is hardly any hydrogen detected when you breathe out. However, this is not the case in lactose intolerance, where you see the amount of detectable hydrogen increase. If after an hour from taking the test your hydrogen is raised by more than 20 ppm above your baseline, it is usually thought you have lactose intolerance.

To start with, your baseline hydrogen concentration on exhalation needs to be obtained. This is done by blowing through a small tube into a machine.

After this, you will be asked to drink a lactose solution, and then repeat the previous steps of blowing into the machine which measures the hydrogen concentration. To check for lactose intolerance further breath samples are taken at regular intervals: every 15 minutes for the initial 30 minutes and then every 30 minutes for the next 2 hours. It must be noted that the lactose solution can make some nauseous.

For the hydrogen breath test to be effective, a few instructions should be followed:

  • Do not eat or drink anything (including water) for 12 hours before the test
  • Do not take antibiotics for 4 weeks before the test
  • Do not smoke, sleep or exercise for at least 30 minutes before the test
  • Try to avoid anywhere where people are smoking as cigarette smoke, even second-hand smoke can affect the validity of the test

Your local GP will be able to provide you with more specific instructions for their practice.

Lactose tolerance test

This test looks at how well your body can digest the lactose, the main sugar in milk. It involves taken a blood sample, which is then processed to find out how much glucose is present in your blood.

A blood sample is taken with a needle, before and after consuming a lactose filled solution. The first blood sample allows a baseline for the amount of glucose in your blood to be established. After the lactose solution is consumed, blood samples are taken at regular intervals over a 2 hour period.  If your blood glucose doesn’t increase, or increasing only by a fraction this suggests that the lactose is not being properly broken down by the digestive system - this is indicative of lactose intolerance.

During normal digestion, lactose is broken down into the simple sugars glucose and glactose. The liver then converts galactose into glucose. Glucose is small enough cross the gut wall and be absorbed into the blood stream, increasing blood glucose levels. If lactose isn’t broken down, glucose is not produced and so blood glucose levels do not increase.  

Milk tolerance test

The milk tolerance test is basically the same as the lactose tolerance test. The only difference being the drink you consume; in the milk tolerance test a glass of milk (usually about 500ml) is drunk instead of the lactose solution. Your blood glucose levels are measured before and after the drink is consumed, by taking a blood sample. If your blood glucose level doesn’t increase, or only increases by a small amount lactose intolerance may be suspected.

Stool acidity test

The stool acidity test is usually used when none of the other lactose intolerance tests are suitable.

The hydrogen breath and lactose intolerance tests all require; consumption of large amounts of lactose and cooperation during blood samples and exhalation techniques.

In infants, drinking such large amounts of lactose can often be harmful and cooperation for taking blood samples may be an issue. Drinking large amounts of lactose if a lactose intolerant adult may cause side effects such as nausea and abdominal cramping – these are manageable side effects. However, in young infants consumption of a large quantity of lactose could lead to diarrhoea and ultimately dehydration which can be dangerous in young children. So in young children, to assess whether they are lactose intolerant or not a stool sample is taken instead and a stool acidity test is performed. A stool with a pH of less than 5.5 indicates lactose intolerance.

When lactose is not digested properly, as seen in lactose intolerance the bacteria that break down the undigested lactose produce fatty acids such as lactic acid. So, if an infant is lactose intolerant there will be increased amounts of fatty acid present in stools.

Small bowel biopsy

A small bowel biopsy is rarely used diagnostically for lactose intolerance, as it is an invasive surgical procedure. But, it can be used to differentiate between lactose intolerance and other underlying conditions that may cause a similar set of symptoms such as celiac disease.

This procedure is carried out under local anaesthetic allowing a small sample of the small intestine’s lining to be removed without any pain. This is achieved using a long thin tube called an endoscope.

The sample retrieved is then analysed so see how much of the enzyme lactase it contains. Small amounts of lactase present would indicate lactose intolerance, if not; another condition may be the cause of the symptoms. The biopsy can then be further tested to specific if another condition is the cause.


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