Lactose Tolerance (Blood)
Does this test have any other names?
Lactose tolerance serum test
What is this test?
This is a test to see whether your body is able to break down lactose. Lactose is
a type of sugar found in milk, cheese, ice cream, and other dairy products. Lactase,
an enzyme in your stomach, breaks down lactose into two simple sugars: glucose and
galactose. Within 2 hours of eating, your blood glucose level should rise. This blood
test measures the increase.
Why do I need this test?
You may have this test if you have symptoms of lactose intolerance. Symptoms include:
Lactose intolerance is more common in people of certain ethnic backgrounds. These
include African, Hispanic/Latino, American Indian, and Asian.
You also might have this test if you have an intestinal problem like inflammatory
bowel disease or a malabsorption syndrome like short gut syndrome. Infants who are
not gaining enough weight may also have this test.
What other tests might I have along with this test?
You may also have a lactose hydrogen breath test, another way to measure your body's
ability to break down lactose. This test measures the amount of hydrogen in your breath.
If you are unable to digest lactose, your breath will have a high amount of hydrogen.
If this test is for your child, he or she may have stool tested for acidity. A child
may have glucose in his or her stool because of undigested lactose.
You may also have a glucose tolerance test. Glucose tolerance is used to diagnose
diabetes. When you have diabetes, either your pancreas does not make enough of the
hormone insulin to break down sugar, or the cells in your body that use insulin become
more resistant to it.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
If your glucose levels increase to greater than 20 milligrams per deciliter (mg/dL)
after drinking the lactose, you likely don't have lactose intolerance or a problem
If your glucose levels don't rise after drinking the lactose, you may be lactose intolerant
or have a problem with absorption.
How is this test done?
This test requires 4 blood samples, which are drawn through a needle from a vein in
your arm. Your healthcare provider will take the first sample before you drink a dose
of lactose, usually 100 grams of lactose diluted in 200 milliliters of water. The
other blood samples are taken at 3 intervals after you drink the lactose – at 30 minutes,
1 hour, and 2 hours.
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Your results might be affected if you:
Exercise strenuously before testing
Eat or drink before testing
Take antibiotics within 1 month of your test
Have a malabsorption syndrome
How do I get ready for this test?
This test requires an 8-hour fast. Don't smoke or exercise for at least 8 hours before
you have this test. Don't take antibiotics for 1 month before this test. Be sure your
healthcare provider knows about all medicines, herbs, vitamins, and supplements you
are taking. This includes medicines that don't need a prescription and any illicit
drugs you may use.