This is a test that measures the ability of your intestines to digest lactose. It is used to diagnose a deficiency of intestinal lactase (the enzyme used to digest lactose).

There are several ways to perform this test. A typical procedure is described below, but be sure to follow the specific instructions you receive.

After an overnight fast (no food intake), you are given 50 grams of flavored liquid containing lactose to drink. After a defined period of time, tests are performed to determine whether your body can break down the lactose and absorb it.

The preferred, non-invasive method is the measurement of breath hydrogen content. In this method, breath samples are collected as you exhale. The exhaled gases are analyzed for hydrogen, a byproduct of bacteria that breakdown the lactose that is not absorbed.

If this test is not available, blood samples can be taken at various time to measure the amount of glucose, a product of lactose breakdown, in your blood. These blood samples are taken immediately prior to the test and then at various times after drinking the lactose solution (usually 30, 60, and 120 minutes).

Blood Collection:

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

Fast for 8 hours and avoid strenuous exercise for 8 hours before the test.

The breath sample is collected in either a special bag or syringe, and should not be associated with any pain or discomfort.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.