Lactose Intolerance Test: How It's Done & Results

A lactose intolerance test may involve a hydrogen breath test or an oral lactose tolerance test. A doctor may also recommend other tests, such as a stool acidity test, genetic testing, or an intestinal biopsy.

Preparation usually includes fasting for about 8 hours before the test. Antibiotics may need to be avoided for 4 weeks, while lactose, dietary fiber, and FODMAPs may need to be avoided for 24 hours.

Lactose intolerance occurs when the body produces too little lactase, the enzyme needed to digest lactose. It can cause symptoms such as abdominal bloating, excessive gas, diarrhea, stomach pain, and headache.

How to prepare

Preparation for a lactose intolerance test may include:

• Fasting completely for at least 8 hours for adults and children, and at least 4 hours for infants

• Stopping lactulose, lactose-containing products, laxatives, prokinetic medications, and probiotics 24 hours before the test and avoiding them throughout the testing period

• Reducing physical activity for 2 hours before and during the test to help prevent hyperventilation

• Not smoking for at least 2 hours before the test

• Avoiding antibiotics for at least 4 weeks before the test

• Avoiding lactose, dietary fiber, and FODMAPs for 24 hours before the test, as they may increase hydrogen levels

• Not undergoing bowel preparation for a colonoscopy or intestinal surgery during the 2 weeks before the test

However, preparation for a lactose intolerance test can vary depending on the testing method. It is important to follow the instructions provided by the doctor and the laboratory performing the test.

Is fasting required?

Yes. A lactose intolerance test generally requires fasting. For both the hydrogen breath test and the oral lactose tolerance test, adults and children are typically asked to fast for at least 8 hours before testing.

For infants, fasting recommendations are usually shorter, typically about 4 to 6 hours, depending on the laboratory's protocol.

Main tests

The main tests used to evaluate lactose tolerance include:

1. Hydrogen breath test

The hydrogen breath test is one of the main tests used to diagnose lactose intolerance.

It is the most commonly performed test and is considered the gold standard for diagnosing lactose intolerance.

How it is done: The person slowly breathes into a small tube or device that measures hydrogen levels in the breath. The person then drinks 25 to 50 g of lactose dissolved in water and repeats the breath test every 30 minutes for 3 to 4 hours.

When the small intestine cannot properly digest and absorb lactose, the undigested lactose passes into the colon. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), intestinal bacteria then break it down and produce gases such as hydrogen and methane, which enter the bloodstream, travel to the lungs, and are detected in the breath.

Results: Lactose intolerance may be indicated when the hydrogen level increases by 20 ppm or more compared with the initial measurement, or when the methane level increases by at least 10 ppm.

To diagnose lactose intolerance, the doctor also considers whether symptoms such as bloating, cramping, or diarrhea become worse during the test.

2. Lactose tolerance test

The oral lactose tolerance test is used to evaluate the intestine’s ability to digest and absorb lactose. This test measures blood glucose levels after the person consumes a specific amount of lactose.

How it is done: A nurse collects an initial blood sample. The person then drinks a liquid containing a specific dose of pure lactose, which is generally 50 g for adults.

Additional blood samples are collected at regular intervals to measure changes in blood glucose levels.

Results: If the blood glucose level increases by less than 20 mg/dL, the lactose may not have been adequately absorbed, which indicates lactose malabsorption.

In this situation, the doctor may order a glucose tolerance test to rule out a general problem with glucose absorption.

The oral lactose tolerance test has a sensitivity of approximately 75% to 94% and a specificity of about 96%. However, it may produce inaccurate results in people with small intestinal bacterial overgrowth, or SIBO, or delayed gastric emptying.

This test is also not considered appropriate for people with diabetes because changes in glucose metabolism can interfere with the interpretation of the results.

3. Stool acidity test

A stool acidity test can be performed in people of any age. However, it is most commonly used for infants and young children who may have difficulty completing breath or blood tests.

Undigested lactose in the stool leads to the production of lactic acid, which makes the stool more acidic than normal. This increased acidity can be detected through laboratory testing.

How it is done: The child or adult is given a dose of lactose, their usual milk, or their usual infant formula.

A stool sample is then collected and sent to a laboratory, where its acidity and the presence of other substances are evaluated.

Results: Lactose intolerance may be indicated when the stool is more acidic than normal, with a pH of 5.0 or lower.

4. Intestinal biopsy

A small intestinal biopsy is rarely recommended because it is invasive. It is generally performed only to rule out other serious conditions, such as celiac disease.

This test directly measures lactase activity and has an estimated sensitivity of approximately 95% and a specificity of about 90%.

How it is done: During an upper endoscopy, the doctor removes a small tissue sample from the lining of the small intestine.

The sample is analyzed in a laboratory to measure lactase enzyme activity. This may involve placing the sample on a testing plate and observing whether a color change occurs.

Results: If the sample does not change color, it suggests that there was no reaction and that significant lactase activity is not present, supporting a diagnosis of lactose intolerance.

5. Genetic testing

A genetic test for lactose intolerance uses molecular biology techniques, such as polymerase chain reaction, or PCR, to identify specific genetic variants associated with whether the body continues producing lactase during adulthood.

How it is done: A small blood sample or saliva swab is collected and sent to a laboratory for molecular testing.

Results: In populations of Indo-European ancestry, the TT genotype is associated with lactase persistence. This means that the person is expected to continue producing adequate amounts of lactase and is more likely to tolerate lactose.

The CT genotype is associated with a partial reduction in lactase production, while the CC genotype is associated with a greater reduction in lactase production.

Genetic testing can identify primary hypolactasia, which is the natural and inherited decrease in lactase production that occurs over time. However, it cannot detect secondary lactose intolerance, which occurs when another disease or infection damages the intestinal lining and reduces lactase activity.