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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, 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 two 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: 

  • Gas

  • Stomach cramps

  • Bloating

  • Diarrhea, especially after you eat or drink milk and other dairy products such as ice cream and cheese

More than half of people of certain ethnic backgrounds have lactose intolerance.

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.

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 health care provider.

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 with absorption.

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 four blood samples, which are drawn through a needle from a vein in your arm. Your health care 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 three intervals after you drink the lactose – at 30 minutes, one hour, and two 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

  • Have diabetes

  • Smoke

  • Take antibiotics within one month of your test

  • Have a malabsoption syndrome 

How do I get ready for this test?

This test requires an eight-hour fast. Don't smoke or exercise for at least eight hours before you have this test. Don't take antibiotics for one month before this test. Be sure your doctor 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.