Lactose tolerance test
What is this test?
This test measures glucose (sugar) levels in blood. This test is done after a period of fasting followed by drinking a fixed amount of lactose (milk sugar). It is used to help determine how well the body is able to absorb lactose. This test is also used when lactose intolerance is suspected.
What are other names for this test?
- Lactose challenge test
- LTT - Lactose tolerance test
Why do I need this test?
Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. You may need this test if you have:
- Lactose intolerance
When and how often should I have this test?
When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.
Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.
The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.
How should I get ready for the test?
Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.
Fast before the test.
How is the test done?
Common sites to collect a capillary blood sample are the fingertip and earlobe. Infants often have a capillary blood sample taken from the heel of the foot. Once the site is selected, the healthcare worker may heat the area with a warm compress to ensure adequate blood flow. The area will be cleaned with antiseptic. A small needle is used to make a cut in the skin surface, and the area may be squeezed gently to produce blood. The blood is collected in small collection device.
A sample of blood will be collected to determine your initial blood glucose level. You will be asked to drink a liquid that contains lactose. Additional blood samples will be collected over a period of two hours at 15 to 30 minute intervals. These samples will be used to monitor changes in your blood glucose level.
How will the test feel?
The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.
During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.
What should I do after the test?
After capillary blood collection is complete, cotton will be placed over the site and held firmly until the bleeding has stopped. A bandage or cotton may be secured over the site.
What are the risks?
Blood: During a blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this test.
What are normal results for this test?
Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following is considered to be a normal result for this test:
- Adults and Children: Rise in blood glucose levels of >20-30 mg/dL (1.1-1.7 mmol/L) above fasting level 
What follow up should I do after this test?
Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.
Where can I get more information?
- National Digestive Diseases Information Clearinghouse (NDDIC) - www.digestive.niddk.nih.gov
- International Foundation for Functional Gastrointestinal Disorders - http://www.iffgd.org
- Academy of Nutrition and Dietetics - http://www.eatright.org
 Shaw AD & Davies GJ: Lactose intolerance problems in diagnosis and treatment.. J Clin Gastroenterol. 1999; 28:208-16.
 Swagerty DL Jr, Walling AD, & Klein RM: Lactose intolerance. Am Fam Physician 2002; 65(9):1845-1850.
 Arola H: Diagnosis of hypolactasia and lactose malabsorption. Scand J Gastroenterol 1994; 29(Suppl 202):26-35.
 The American Gastroenterological Association: What diagnostic tests are used to detect lactose intolerance. American Gastroenterological Association. Bethesa, MD, USAAvailable from URL: http;//www.gastro.org/clinicalRes/brochures/lactose.html. As accessed May 9, 2005.
 Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.
Last Updated: 1/4/2018