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Lactose intolerance

Updated 21 September 2018

Diagnosing lactose intolerance

Diagnosing lactose intolerance on symptoms alone isn’t accurate. Here's how to get a proper medical diagnosis.

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Many people consider themselves lactose intolerant without having been properly tested. Lactose intolerance is a well-known, common digestive problem, and so a lot of people think it could be the cause of their gastrointestinal symptoms.  

However, diagnosing lactose intolerance on symptoms alone isn’t accurate. In fact, about 20% of people who believe they’re lactose intolerant are not.  Testing for lactose intolerance is valuable, as it can exclude the digestive problem and guide you to the true disease that’s causing your symptoms.

The following methods are used to diagnose lactose intolerance:

Hydrogen breath test

The hydrogen breath test is the most reliable, easiest, non-invasive, cost-effective test for diagnosing lactose intolerance. The test measures the ability to absorb lactose by testing breath hydrogen levels following the ingestion of 25-50g of pure lactose after an overnight fast. This is equivalent to about 500ml to 1 litre of milk.

If you are lactose intoleranct, the lactose isn’t digested and absorbed in the small intestine and reaches the colon intact, where bacteria ferment it. Hydrogen, as well as carbon dioxide and methane, are produced. Small amounts of these gasses are absorbed from the colon into the blood and travel to the lungs where they’re excreted in the breath. 

After the ingestion of lactose, samples of breath are collected every 10-15 minutes for 3-5 hours.  These samples are analysed for hydrogen. Some physicians may also test for methane. If hydrogen levels from the breath are greater than 20ppm, it means that the small intestine didn’t digest and absorb all of the lactose – in other words, you don’t absorb lactose properly. If you also experience gastrointestinal problems, you’re classified as being lactose intolerant.

This test isn’t completely accurate. For example, the amount of hydrogen excreted in the breath is proportional to the extent of lactose that’s malabsorbed (i.e. the more hydrogen in the breath, the greater the malabsorption), but the amount of hydrogen in the breath isn’t proportional to the severity of symptoms.

False-negatives or false-positives also do occur with this test. Factors that may lead to incorrect results include:

  • Hydrogen non-excretion. This occurs in up to 20% of individuals with lactose malabsoption and will lead to a false-negative result.
  • Lack of hydrogen-producing bacteria. This occurs in about 10-15% of the population and will lead to a false-negative result.
  • Intestinal motility disorders. These disorders slow down or speed up the transit time in the gut. When the transit time of lactose is sped up through the small intestine, lactose may not be fully digested and absorbed, leading to a false-positive result.
  • Small bacteria overgrowth. Bacteria from the colon move into the small intestine, where these bacteria will ferment the lactose before it can be digested or absorbed. This will result in a false-positive test.
  • Recent treatment with antimicrobials (e.g. antibiotics). This can suppress colonic bacteria and, thereby, the production of hydrogen. This can also lead to possible false-negative results.
  • Eating high-fibre foods before the test.

Lactose tolerance test (blood glucose test)

The lactose tolerance test assesses your ability to absorb lactose by measuring the blood glucose levels following a lactose load. This was one of the first tests that was used to test for lactose intolerance, but is rarely used clinically anymore as the test isn’t sensitive enough.

For this test, lactose is ingested after an overnight fast, and blood samples are collected and analysed for glucose. If the lactose is split into glucose and galactose, and absorbed into the blood, your blood glucose levels will rise more than 2mmol/l and indicate lactose tolerance. A rise in blood glucose of less than 1mmol/l would indicate lactose malabsorption.

Milk challenge

A milk challenge is a very simple way of diagnosing lactose intolerance. You’ll have to fast overnight and then drink a glass of milk in the morning. It’s important that the milk is fat-free to ensure that it isn’t the fat in the milk that’s causing your symptoms. Nothing else must be ingested for 3-5 hours. 

If you’re lactose intolerant, the milk should produce symptoms within 30 minutes to 2 hours of ingestion. If you experience no symptoms, or the symptoms are milder than usual, then it’s unlikely that lactose intolerance is the cause of your symptoms.  It’s important to note that lactose intolerance occurs on a scale: most individuals who have the digestive problem can still tolerate a certain amount of lactose. For this reason, a milk challenge isn’t the most accurate test for diagnosing lactose intolerance.

Lactose-free or elimination diet

This is the most common way used in the self-diagnosis of lactose intolerance. It simply involves going on a diet that eliminates milk and milk products.

If an elimination diet is used for diagnosing lactose intolerance, it should be done with the help of a dietitian who can ensure that your diet remains balanced and healthy, and that all lactose is removed from your diet (note that milk products are often hidden in prepared foods).

Lactose intolerance can be diagnosed if symptoms are resolved by a trial of strict lactose-free eating (and when symptoms reappear with the subsequent reintroduction of dairy foods).

The elimination diet is based on subjective reporting of gastrointestinal symptoms and is therefore not considered reliable.

Genetic testing

Primary lactose intolerance is genetically determined and a couple of mutations have been observed on the DNA. Specifically, the cytosine (C = decrease in lactase production) is exchanged for a thymidine (T = persistent production of lactase) on the lactase gene at position -13910. 

It’s therefore possible to analyse DNA from cells in the blood to determine if your genes are programmed to decrease or continue lactase production. The pattern of the DNA fragments shows whether you’re lactose intolerant (C/C) or lactose tolerant (C/T or T/T).Genetic testing involves a quick, easy and highly specific test that can distinguish between primary (genetic) and secondary (due to another illness) lactose intolerance. It can also be used to diagnose congenital lactose intolerance.

Small intestinal biopsy

A biopsy of the lining of the small intestine is the most direct test for lactase deficiency. The biopsy is obtained through an endoscopy, which involves inserting a long, flexible tube with a small camera down your throat and oesophagus and into your stomach and upper intestine.

The lactase levels of the lining of your small intestines are measured to determine lactase concentrations. The biopsies of lactose-intolerant individuals show normal histologic characteristics, but low or absent levels of lactase concentrations. The procedures used to measure the lactase levels are expensive and often not available, limiting the use of biopsy as a diagnostic tool. 

Diagnosing lactose intolerance in young children

In babies with gastrointestinal symptoms, lactose intolerance has to be formally diagnosed, as there could be other reasons why the infant is experiencing gastrointestinal symptoms.

A hydrogen breath test or intestinal biopsy are generally suggested, although many physicians suggest a trial of lactose-free formula to see if symptoms improve. Other tests may also have to be done to determine why the infant is lactose intolerant.

Stool acidity tests are also sometimes used for infants and young children. This test measures the amount of acid in the stool. With lactose malabsorption, the unabsorbed lactose is fermented by bacteria in the colon, producing (among other compounds) acids that turn the stool acidic. A lactose-intolerant baby or child will develop an acidic stool following a test dose of lactose. This test is used infrequently and the hydrogen breath test is considered superior.

The importance of retesting for lactose tolerance

A number of studies have compared the effects of lactose consumption to placebo in self-reported, lactose-intolerant individuals. Individuals who subjectively report lactose intolerance can generally tolerate small amounts of lactose (the lactose contained in about 250ml of milk).

It’s important to get testing done to ensure that lactose isn’t removed from your diet unnecessarily.

Reviewed by Kim Hofmann, registered dietitian, BSc Medical (Honours) Nutrition and Dietetics, BSc (Honours) Psychology. August 2018.