Lactose tolerance test

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).

How the test is performed

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.

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

For infants and children:
The preparation you can provide for this test depends on your child’s age and experience. For specific information regarding how you can prepare your child, see the following topics:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel

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.

Why the test is performed
This test is used to diagnose lactase deficiency.

Lactose is a disaccharide (a type of sugar) in dairy products. During digestion, lactose is broken down into glucose and galactose, two simple sugars, by the intestinal enzyme lactase. Lactose-intolerant patients have a deficient or absent lactase enzyme. Bacteria metabolize the undigested lactose, producing gases such as hydrogen and other acid products.

Normal Values

In the breath test, a peak rise in hydrogen content of 12 parts per million over the fasting (pre-test) level is considered positive.

In the blood test, a rise in plasma glucose of more than 30 mg/dl within 2 hours of ingesting the lactose is considered normal. A rise of 20-30 mg/dl is inconclusive. A rise of less than 20 mg/dl is abnormal.

Note: mg/dl = milligrams per deciliter

What abnormal results mean

  • Lactase deficiency

An abnormal test should be followed by a glucose tolerance test to exclude an abnormality in absorption of glucose before concluding that there is a lactase deficiency.

What the risks are

The risks associated with having blood drawn are:

  • excessive bleeding  
  • fainting or feeling lightheaded  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Special considerations
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Johns Hopkins patient information

Last revised: December 7, 2012
by Mamikon Bozoyan, M.D.

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