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Oral glucose tolerance test (OGTT)

This test assesses the body's ability to handle glucose (a type of sugar) consumed in the diet.

What is the purpose of this test? The oral glucose tolerance test is mainly used to screen for and diagnose prediabetes and diabetes.
How to prepare? Fast. Do not eat or drink anything (except water) for at least 8 hours before your blood is drawn.
Diagnostic values* Diagnostic blood glucose values 2 hours after ingestion of 75 g of glucose in adults (not pregnant):
  • Diabetes: ≥ 11.1 mmol/L
  • Prediabetes: 7.8-11.0 mmol/L
Associated tests Related tests include:
How is it done?

OGTT procedure:

  • A blood sample is taken at the start of the test, usually from a vein in your arm.
  • You'll then be given a few minutes to drink a high-glucose solution (typically 75 g of glucose).
  • More blood samples will be taken, usually from a vein in the arm.
    • For the test requiring 2 blood samples, the second sample will be taken 2 hours after you drink the glucose solution.
    • For the test requiring 3 blood samples, the second and third samples will be taken 1 and 2 hours after you drink the solution.

Precautions to take during the test:

  • Do not eat or drink anything (except water).
  • Stay seated; do not walk around.
  • Notify the medical staff if you experience dizziness, nausea, or vomiting.
How long before I get the results? Results are usually ready in a day. It may take longer to receive your results.
* These values are presented for information purposes only. As different laboratories may have different target values, the lab report usually specifies the values used for the test. It should also be noted that test results must be interpreted based on factors such as age, sex, and health condition. Values outside of this range may be considered normal for some individuals.

What does an abnormal test result mean?

High blood glucose values:

  • May indicate diabetes or a risk of diabetes
  • May indicate other conditions such as acromegaly, hyperthyroidism, or problems with the pancreas

Low blood glucose values:

  • May indicate hypoglycemia or a risk of hypoglycemia
  • May indicate other conditions such as liver disease, kidney disease, or hypothyroidism

Medications that may affect test results

The use of certain medications can influence test results. Here are a few examples:

  • Corticosteroids (e.g., prednisone)
  • Tricyclic antidepressants
  • Diuretics
  • Adrenaline
  • Estrogens
  • Lithium
  • Phenytoin

What you should know before going for this test

When you go in for a blood test or any other medical exam, it's always a good idea to bring a complete list of any prescriptions, over-the-counter medications, or natural health products you're taking. Unless otherwise advised, it's best to take your medications as usual on blood test days. If in doubt, don't hesitate to talk to your healthcare professional.

© Copyright Vigilance Santé

The patient information leaflets are provided by Vigilance Santé Inc. This content is for information purposes only and does not in any manner whatsoever replace the opinion or advice of your health care professional. Always consult a health care professional before making a decision about your medication or treatment.

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