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Oral Glucose Tolerance Blood Test

Last updated May 9, 2018

Approved by: Maulik P. Purohit MD, MPH

The Oral Glucose Tolerance Blood Test helps determine glucose levels in blood, after ingestion of a known amount of glucose (either 100 g or 1.75 g per kg of body weight). It is used to diagnose diabetes, cancers, and metabolic disorders.

What are other Names for this Test? (Equivalent Terms)

  • OGTT Blood Test

What is Oral Glucose Tolerance Blood Test? (Background Information)

  • Glucose is a simple sugar that serves as the body’s main energy source. Some organs, such as the brain, use glucose exclusively for energy
  • Glucose is obtained through the diet directly and indirectly. Foods may contain glucose; alternatively, more complex sugars, such as maltose, lactose, and starch, are broken down and their components converted into glucose
  • After a meal, blood glucose levels may rise markedly. Conversely, they may fall after a strenuous workout has forced muscles to use up the body’s glucose stores
  • Blood glucose levels must be maintained within a relatively narrow range. Excess glucose in blood may damage tissues, while deficiencies may cause fatigue, lightheadedness, and brain malfunction
  • The body compensates for the various factors that impact blood glucose levels through the use of hormones. Two such sugar-regulating hormones are insulin and glucagon. They play opposing roles:
    • Insulin is made by beta cells of the pancreas. It is released in blood after a rise in blood glucose (such as after a heavy meal), to help lower it to normal levels by stimulating cells to take in glucose and other nutrients
    • Glucagon is made by the alpha cells of the pancreas and has the opposite effect. It is released when glucose levels fall below normal (such as after a workout) and causes cells to produce glucose and release it from their stores
  • Problems with either of these mechanisms may cause abnormalities in blood glucose levels that can even be life-threatening
  • The most common cause of elevated glucose is diabetes mellitus, or simply “diabetes”. There are 3 types of diabetes mellitus, all of which stem from problems with insulin function:
    • Type 1, or insulin-dependent, diabetes results from the autoimmune destruction of beta cells of the pancreas
    • Type 2, or insulin-independent, diabetes results from the desensitization of cells to the effects of insulin
    • Gestational diabetes may occur during pregnancy and result in excessively large babies that exhibit low glucose levels
  • The Oral Glucose Tolerance Blood Test helps determine glucose levels in blood, after ingestion of a known amount of glucose (either 100 g or 1.75 g per kg of body weight). It is used to diagnose diabetes, cancers, and metabolic disorders

What are the Clinical Indications for performing the Oral Glucose Tolerance Blood Test?

Following are the clinical indications for performing the Oral Glucose Tolerance Blood Test:

  • Monitoring glucose levels of individuals with diabetes
  • Family history of diabetes
  • Extensive surgery or injury
  • Frequent urination
  • Increased thirst
  • Fatigue
  • Blurry vision
  • Rapid weight loss
  • Tingling and numbing in the extremities
  • Giving birth to a baby over 9 pounds
  • Obesity
  • History of cardiovascular disease

How is the Specimen Collected for Oral Glucose Tolerance Blood Test?

Following is the specimen collection process for Oral Glucose Tolerance Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test.

What is the Significance of the Oral Glucose Tolerance Blood Test Result?

An initial value for the Oral Glucose Tolerance Blood Test is taken when fasting. The levels are then measured after an interval of 0.5, 1, 2, and 3 hours respectively.

A high value for the test is indicated by an elevated level during any of the 5 measurements, as follows:

  • Initial fasting measurement - greater than 110 mg/dL
  • After 0.5 hour - greater than 160 mg/dL
  • After 1 hour - greater than 170 mg/dL
  • After 2 hours - greater than 125 mg/dL
  • After 3 hours - greater than initial fasting measurement

A high value for the test may indicate:  

  • Diabetes mellitus
  • Latent diabetes
  • Cushing’s syndrome
  • Hyperlipoproteinemia
  • Infections
  • Acute myocardial infarction (heart attack)
  • Pancreatic cancer
  • Eclampsia
  • Cancer metastases
  • Acidotic conditions
  • Duodenal ulcer

A low value (less than 70 mg/dL for the initial fasting measurement) for the test may indicate:

  • Hyperinsulinism
  • Adrenal gland insufficiency
  • Malabsorption
  • Protein malnutrition

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • The Oral Glucose Tolerance Blood Test should not be performed if the resting blood sugar is greater than 200 mg/dL
  • Resting blood glucose levels increase with age. After age 60, glucose levels are usually 10 to 30 mg/dL higher than normal
  • If an individual cannot eat or tolerate glucose, the IV glucose tolerance blood test may be performed instead of the Oral Glucose Tolerance Blood Test
  • Diabetes insipidus is a rare disorder whereby the kidneys cannot conserve water

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

The following DoveMed website link is a useful resource for additional information:


Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Helpful Peer-Reviewed Medical Articles:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: April 1, 2016
Last updated: May 9, 2018