The lactose tolerance test measures blood glucose levels after ingestion of a dose of lactose, a sugar typically found in dairy products. Normally, lactose is broken down by the intestinal enzyme lactase into the sugars glucose and galactose for absorption by the intestines. Lactose tolerance test is used to identify people who are unable to digest lactose because of deficient levels of lactase, a condition known as lactose intolerance.

In addition to or instead of the blood test, hydrogen levels in the breath may be measured after ingesting a dose of lactose. The concentration of hydrogen in exhaled air is directly proportional to the amount of lactose that remains undigested.

Purpose of the Lactose Tolerance Tests

  • To detect lactose intolerance (inability to digest and absorb lactose) in people with typical symptoms of the condition, such as abdominal cramping and bloating, flatulence, and diarrhea

Who Performs Lactose Tolerance Tests

  • A physician, nurse or technician will conduct the test

Special Concerns about Lactose Tolerance Tests

  • A false-positive result may be caused by engaging in strenuous exercise prior to the test or the presence of an intestinal malabsorption disease other than lactose malabsorption.
  • People with diabetes mellitus may experience a rise in blood glucose levels despite lactose intolerance, yielding a false-negative result.

Before the Lactose Tolerance Tests

  • Do not eat or drink anything and avoid strenuous exercise for 8 hours before the test.
  • Inform your doctor if you regularly take any medications, herbs, or supplements. Some of these agents may affect blood glucose levels and must be briefly discontinued before the test.
  • Do not smoke before the test.

What You Experience

  • An initial blood sample is drawn from a vein, usually one in your arm.
  • The examiner then instructs you to drink a solution containing a specified dose of lactose dissolved in water.
  • Three additional blood samples will be taken after 30, 60, and 120 minutes.
  • If the hydrogen breath test is being conducted, you will also be asked to exhale into a special device at these times.
  • The procedure takes about 2 hours.

Risks and Complications of Lactose Tolerance Tests

  • There are some slight risks associated with having blood drawn. These risks include excessive bleeding, fainting or feeling light-headed, hematoma (blood accumulating under the skin) and infection.

After the Lactose Tolerance Tests

  • Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
  • You may leave the testing facility promptly after the test is completed.
  • If you are lactose intolerant, you may experience symptoms such as flatulence, bloating, diarrhea, or gas pains.
  • You may resume your normal diet and any medications withheld before the test.
  • Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.

Results of Lactose Tolerance Tests

  • Your blood glucose levels are analyzed. If lactase is deficient, lactose will not be broken down and the glucose levels will not rise as expected.
  • The presence of accompanying symptoms such as cramping and diarrhea further suggest, but do not confirm, a diagnosis of lactose intolerance.
  • In order to confirm that the presence of lower-than-expected blood glucose levels is due to lactose intolerance, you may need to undergo an oral glucose tolerance test.
  • If the test results indicate the presence of a severe lactase deficiency, there may be a variety of culprits, including inflammatory bowel disease and malabsorption syndromes. Additional testing will usually be done to identify the precise cause.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 16 Jan 2012

Last Modified: 11 Sep 2015