Lactose Tolerance Test

  • Medical Author:
    Jay W. Marks, MD

    Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

woman with abdominal pain

What is the lactose tolerance test?

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.

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.

Medically reviewed by Joseph Palermo, DO; American Osteopathic Board Certified Internal Medicine


"Lactose intolerance"

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Reviewed on 3/24/2016
Medically reviewed by Joseph Palermo, DO; American Osteopathic Board Certified Internal Medicine


"Lactose intolerance"

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