Lactose tolerance tests

  • Definition
    • Lactose tolerance tests measure the ability of your intestines to break down a type of sugar found in milk and other dairy products called lactose. If your body cannot break down this sugar you are said to have lactose intolerance.

  • Alternative Names
    • Hydrogen breath test for lactose tolerance

  • How the Test is Performed
    • Two common methods include:

      • Lactose tolerance blood test
      • Hydrogen breath test

      The hydrogen breath test is the preferred method. It measures the amount of hydrogen in the air you breathe out.

      • You will be asked to breathe into a balloon-type container.
      • You will then drink a flavored liquid containing lactose.
      • Samples of your breath are taken at set times and the hydrogen level is checked.
      • Normally, very little hydrogen is in your breath. But if your body has trouble breaking down and absorbing lactose, breath hydrogen levels increase.

      The lactose tolerance blood test looks for glucose in your blood. Your body creates glucose when lactose breaks down.

      • For this test, several blood samples will be taken before and after you drink a liquid containing lactose.
      • A blood sample will be taken from a vein in your arm (venipuncture).
  • How to Prepare for the Test
    • You should not eat or do heavy exercise for 8 hours before the test.

  • How the Test will Feel
    • There should not be any pain or discomfort when giving a breath sample.

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

  • Why the Test is Performed
    • Your health care provider may order these tests if you have signs of lactose intolerance.

  • Normal Results
    • The breath test is considered normal if the increase in hydrogen is less than 12 parts per million over your fasting (pre-test) level.

      The blood test is considered normal if your glucose level rises more than 30 mg/dL (1.6 mmol/L) within 2 hours of drinking the lactose solution. A rise of 20 to 30 mg/dL (1.1 to 1.6 mmol/L) is inconclusive.

      Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

      The examples above show the common measurements for results for these tests. Some labs use different measurements or may test different samples.

  • What Abnormal Results Mean
    • Abnormal results may be a sign of lactose intolerance.

      A breath test result that shows a rise in hydrogen content of 12 parts per million (ppm) over your pre-test level is considered positive, and means you may have trouble breaking down lactose.

      The blood test is considered abnormal if your glucose level rises less than 20 mg/dL (1.1 mmol/L) within 2 hours of drinking the lactose solution.

      An abnormal test should be followed by a glucose tolerance test to rule out a problem with the body's ability to absorb glucose.

  • Risks
    • There may be slight risks from having blood drawn.

  • References
    • Hogenauer C, Hammer HF. Maldigestion and malabsorption. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger & Fordtran's Gastrointestinal and Liver Disease. 10th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 104.

      Semrad CE. Approach to the patient with diarrhea and malabsorption. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 140.