Also Known As
Hydrogen Breath Test
Lactose Breath Test
Disaccharide Absorption Test
Oral Lactose Tolerance
Formal Name
Hydrogen Breath Test; Lactose Tolerance Test
This article was last reviewed on
This article waslast modified on 27 March 2019.
At a Glance
Why Get Tested?

To help diagnose lactose intolerance in individuals who have difficulties digesting dairy products, or sometimes as part of an investigation of malabsorption conditions

When To Get Tested?

When you have symptoms such as abdominal pain, bloating, gas, and diarrhoea starting within a few hours after consuming milk and other dairy products

Sample Required?

A series of breath samples exhaled into a collector, or a series of blood samples drawn from a vein in your arm

Test Preparation Needed?

Overnight fasting is required; nothing but water is permitted. Avoid strenuous activities and smoking several hours before testing. You may be instructed to brush your teeth and/or rinse your mouth with water prior to and during the breath test.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Lactose tolerance tests measure hydrogen in the breath or changes in the level of glucose in the blood after a person is given a drink containing a standard amount of lactose, thus determining whether the individual is capable of digesting lactose.

Lactose is a sugar with a complex structure. It is a disaccharide, which means it is composed of two simple sugars joined together. Lactose is found in milk and many other dairy products. Before it can be absorbed and used by the body, it must be broken down into its two simpler sugars, glucose and galactose (monosaccharides). This digestion step is performed by lactase, an enzyme produced by cells lining the small intestine.

If an individual does not produce enough lactase (lactase deficient), then undigested lactose passes through the small intestine to the large intestine, where bacteria break it down, producing hydrogen gas and lactic acid. This process can cause the affected person to experience abdominal pain and bloating, flatulence, and diarrhoea within 30 minutes to 2 hours of consuming milk or other dairy products.

There are 2 types of lactose intolerance.

1. Primary- when your body just doesn’t produce much lactase

Almost all babies are born with the ability to digest lactose, but lactase production normally decreases as an individual ages. About 65-70% of the world's population develops some degree of lactose intolerance by the time they reach adulthood. The intolerance can vary by race and ethnicity, with about 90-95% of northern Europeans retaining their ability to digest lactose and 95-100% of Asians and Native Americans becoming lactose intolerant.

2. Secondary-when the lining of your gut is damaged and temporarily doesn’t produce lactase. This can be produced by any condition that causes damage and irritation to the gut such as gastroenteritis. More serious conditions such as Crohn’s disease, coeliac disease and alcoholism can also cause a secondary intolerance.

In the UK, most cases of lactose intolerance are secondary and are as a result of another illness or condition.

Tests of lactose intolerance are often not necessary. If a doctor suspects a patient has lactose intolerance, then a trial of a lactose free diet may be recommended. If the symptoms improve after 5-7 days of avoidance of lactose-containing foods then the patient will probably not need to have any further tests.

Two different types of lactose tolerance tests are available. They both involve the collection of a fasting sample. The person tested is given a liquid to drink that contains a standard amount of lactose. A series of timed samples is then collected and tested.

Hydrogen breath test
This is the most commonly requested test used to detect and diagnose lactose intolerance. This test measures hydrogen gas in breath samples. With lactose intolerance, undigested lactose reaches the large intestine and is broken down by bacteria, producing excess hydrogen gas. The hydrogen gas enters the circulation and is eventually exhaled by the lungs. This is a simple and non-invasive test to perform.

Lactose tolerance blood test
This test is sometimes used to help diagnose lactose intolerance. This test measures the glucose concentration in the blood samples. It detects the conversion of the ingested lactose into glucose and galactose.

Though these tests are listed as such most laboratories do not offer them routinely. Clinicians mostly suggest a lactose free diet to diagnose this condition.

How is the sample collected for testing?

Breath samples are collected by blowing into a bag or other collection device. Blood samples are obtained by inserting a needle into a vein in the arm. Rarely, a stool sample may be collected in a clean container for analysis.

Is any test preparation needed to ensure the quality of the sample?

Fasting is required before and during testing. The person being tested should not exercise or smoke for several hours before testing. In some cases, additional instructions may be provided by the doctor and/or laboratory. For example, you may be asked to brush your teeth and then rinse your mouth with water prior to the hydrogen breath test and then again after drinking the liquid containing lactose. You may also need to avoid eating slowly digestible food stuffs, such as pasta and beans, mainly dietary fibres the day before the test. It is important to follow any instructions you are given by your doctor or laboratory.

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Common Questions
  • How is it used?

    Lactose tolerance testing is requested to help diagnose the cause of lactose intolerance. It may be requested by itself or as part of a larger panel of tests when a secondary cause, such as a malabsorption condition, is suspected. The hydrogen breath test is the most commonly requested test. The blood test for lactose tolerance is not requested as frequently but is used for the same reason.

  • When is it requested?

    Testing is requested when a person has signs and symptoms that suggest lactose intolerance that develop 30 minutes to 2 hours after ingesting milk or other dairy products. Some of these include:

    • Abdominal pain and bloating
    • Diarrhoea (rare in adults)
    • Nausea
    • Flatulence

    The goals of laboratory testing are to diagnose lactose intolerance and distinguish it from other conditions with similar symptoms. Sometimes, if lactose intolerance is suspected based on typical signs and symptoms, the Doctor will suggest a trial of a lactose free diet rather than conducting lactose tolerance tests. If the symptoms improve then this is consistent with lactose intolerance (although it is not diagnostic), and further testing will often not be necessary.

  • What does the test result mean?

    Hydrogen breath test
    A baseline breath sample is taken before giving a lactose-loaded drink followed by serial breath samples taken at frequent intervals over the next few hours. At the same time, the patient will be assessed for symptoms. If the hydrogen gas in a person's breath significantly increases from the baseline, then it is likely that the person is lactose intolerant.

    If the breath samples are negative or low for hydrogen, then it is less likely that the person is lactose intolerant. The signs and symptoms may be due to another cause. However, some people may have lactose intolerance even with a negative result. In these cases, the bacteria in the intestine may not produce hydrogen. If the diagnosis is suspected despite a negative test result, then the doctor may recommend a trial of a lactose free diet to see if symptoms improve.

    Lactose tolerance blood test
    Timed samples of blood are taken and measured for glucose. If the glucose levels do not increase, yet the person still has symptoms consistent with lactose intolerance, then the condition is likely present. Increasing blood glucose levels over the course of the test indicates that signs and symptoms are unlikely to be due to lactose intolerance.

    Care must be taken when interpreting results of the test. People who have diabetes may have an increase in blood glucose even when they do not produce enough lactase.

  • Is there anything else I should know?

    Antibiotics taken within the last month or two prior to testing may decrease the number of normal bacteria in the large intestine and give a false negative hydrogen breath test. False negative results can be seen in patients with lung disease and also in patients who are non-hydrogen producers.

    If food moves more quickly than usual through a person's intestinal tract, that person may experience symptoms associated with lactose intolerance because the lactose has a shorter amount of time to be exposed to and broken down by lactase.

    Bacterial overgrowth in the intestines (more bacteria present than normal) can cause symptoms similar to lactose intolerance, as can a variety of other gastrointestinal disorders.

    Bacterial overgrowth can give false positive results in breath test. Sometimes, these patients show an early rise in breath hydrogen concentration, that is, earlier than 60 minutes after ingestion of lactose.

    Although it is not commonly done, it is possible to test for mutations in the gene that regulates lactase production (the LCT gene). Small bowel biopsy allows for an assessment of lactase enzyme activity but is rarely performed because of the availability of non-invasive tests. However, this may be helpful in some secondary conditions like coeliac disease.

  • Should everyone have lactose tolerance testing?

    Many people have some degree of lactose intolerance, especially as they age, but general testing is not considered necessary.

  • Is the breath test used for other things?

    Yes, other sugar "loads" can be given to examine other sugar (disaccharide) intolerances; however, lactose is the most commonly tested.

    Other breath tests are available to help detect conditions, but they measure different substances and are unrelated to tolerance testing. Two examples are a breath test for Helicobacter pylori infection and alcohol (ethanol) testing.

  • Can I do anything to increase my production of lactase?

    No, lactase concentrations do not change with lifestyle changes. However, many people with lactose intolerance are able to digest small amounts of milk and are often able to tolerate yogurt and hard cheeses.

  • Is there treatment available for lactose intolerance?

    There is no specific treatment to cure lactose intolerance, but it can be managed. Lactose-reduced dairy products are available for those with the condition. Supplements that contain the lactase enzyme can be taken when dairy products are consumed to help break down the lactose and prevent signs and symptoms. Non-dairy sources of calcium and other nutrients found in dairy products are also available for those who cannot tolerate lactose.

    Calcium and vitamin D intake should be maintained in these patients by means of calcium rich foods or supplementation. In patients with secondary lactose malabsorption, successful treatment for primary disorder can restore lactase enzyme activity.

  • Does lactose intolerance only apply to breast milk and cow's milk?

    No, lactose is present in the milk of all mammals, including, for example, goat's milk.

  • Does soy milk have lactose in it?

    No. Most people with lactose intolerance can drink soy milk as well as other plant-based products, such as rice or oat milk.

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