Lactose Tolerance Test Purpose, Procedure and Result

The lactose tolerance test is a test for diagnosing an intolerance of ingested lactose. Lactose intolerance is a genetic condition in which individuals are unable to digest and absorb the lactose sugar in cow’s milk.

What is lactose intolerance?

Lactose intolerance is the inability to digest the sugar in milk, lactose. Milk is an important source of nutrition for children and even some adults.

With normal lactose tolerance, all of the lactose is broken down (digested) in the intestine by an enzyme into two smaller sugars (galactose and glucose) which then are absorbed into the body.

As a result, normally no lactose reaches the colon.

On the other hand, in individuals with lactose intolerance who lack the intestinal enzyme that breaks down the lactose, the ingested lactose is neither digested nor absorbed in the small intestine and reaches the colon where it is used by the bacteria in the colon.

The bacteria produce chemicals that cause diarrhea, and also produce gas. Abdominal pain also may occur.

How is the lactose tolerance test done?

The lactose tolerance test requires an individual to drink a liquid that contains lactose.

In the small intestine among lactose tolerant individuals, the ingested lactose is split into galactose and glucose which then are absorbed from the intestine and go into the blood.

Several blood samples are taken over a two hour period following the ingestion of the lactose to measure the blood glucose level.

If lactose tolerance is normal, the glucose level in the blood rises due to the absorption of glucose from the intestine. If there is lactose intolerance, the glucose level does not rise.

When is it ordered?

Testing is ordered 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
  • Diarrhea
  • Nausea
  • Flatulence

What does the test result mean?

Hydrogen breath test

A baseline breath sample is taken before giving a lactose-loaded drink. 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.

This can be confirmed after ingestion of lactulose, a synthetic, non-digestible disaccharide sugar that is similarly broken down to hydrogen gas by intestinal bacteria.

Since lactulose is not absorbed by the intestine, the ongoing lack of hydrogen gas production suggests a false negative indicating the person may still be lactose intolerant.

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 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.

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.

Although it is not commonly done, it is possible to test for mutations in the gene that regulates lactase production (the LCT gene).

Potentially, this could identify likely lactase deficiency and take the place of hydrogen breath testing.

What other tests can detect lactose intolerance?

Many clinicians simply make this diagnosis based on improvement of symptoms when lactose is withheld from the patient’s diet and affravation when it is re-introduced.

Other tests used to detect lactose intolerance include the lactose hydrogen breath test and stool acidity test, the latter is primarily used in infants and very young children.

There also is a genetic test which can determine whether an individual has or does not have the intestinal enzyme that is required for the digestion and absorption of lactose.

Source & More Info: Medicine Net and



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