Lactose Tolerance Test

This test determines if a person is lacking an intestinal enzyme called lactase.

In normal digestion, the milk sugar called lactose is changed. Lactose is a disaccharide, which means two sugars. Digestion breaks down lactose into two monosaccharides, which means one sugar. The monosaccharides are glucose and galactose. The enzyme called lactase carries out the breakdown of lactose into glucose and galactose. If lactase is missing or if there is not enough, that person cannot digest lactose and is then lactose intolerant.

Who is a candidate for the test?

This test may be ordered to help diagnose lactose intolerance.

How is the test performed?

The individual swallows 100 grams of lactose. Blood samples are taken 30, 60, and 120 minutes after the lactose is swallowed. A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding.

What is involved in preparation for the test?

Specific instructions should be requested from the healthcare provider.

What do the test results mean?

Normally, individuals are able to convert the lactose that they have swallowed into glucose in the blood. Persons who have little or no lactase will be unable to convert the lactose normally into glucose. In people who are lactose intolerant, a rise in blood glucose of more than 2 milligrams per deciliter (mg/dl) will appear within 2 hours of lactose ingestion.

Author:David T. Moran, MD
Date Written:
Editor:Cafiero, Celeste, MA
Edit Date:05/22/00
Reviewer:Sandy Keefe, RN, MSN
Date Reviewed:08/07/01