Glucose Tolerance Test (GTT) Lab Values

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Determine the significance and clinical use of Glucose Tolerance Testing in clinical practice


Lab Test Name:

Glucose Tolerance Test



In normal conditions, blood sugar levels rise in conjunction with eating a meal. Blood sugar levels then return to a baseline level as the glucose is used up or stored. 

Glucose Tolerance Test is administered to evaluate the body’s response or tolerance to a measured administration of glucose (sugar)


The GTT distinguishes between normal blood sugar response to a meal, and the presence of an abnormal response that could be linked to pre-diabetes, diabetes or gestational diabetes.



Glucose Tolerance Tests are administered to:


  • Screen for gestational diabetes
  • Diabetes diagnostic testing due to symptoms, or family history
  • Medical conditions related to development of diabetes
    • Obesity
    • Metabolic syndrome
    • Polycystic ovarian syndrome


Normal Therapeutic Values:

Normal – 


Fasting- 60-100 mg/dL

1 Hour- <200 mg/dL

2 hours-

  • <140 mg/dL – normal
  • 141-200 mg/dL- impaired glucose tolerance
  • >200 mg/dL – diagnostic of diabetes




Fasting-<90 mg/dL

1 Hour- <130-140 mg/dL

2 hours-<120 mg/dL – normal



Fasting->95 mg/dL

1 Hour- >140 mg/dL

2 hours->120 mg/dL



  • Gray tube
    • Sodium fluoride/potassium oxalate tube


What would cause increased levels?

Increased blood sugar levels following the GTT:

  • Diabetes Mellitus Type I & II
  • Pre-diabetes
  • Gestational diabetes
  • Insulin resistance


What would cause decreased levels?

Decreased levels in response to GTT:

  • Proper endocrine response
  • Vigorous exercise
  • Medications
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Hey everyone, Abby, here from In this lesson, we’re covering the glucose tolerance test. We’ll talk about why it’s indicated, its normal values, when we might see it increase or decrease, and what that means for our patients. Let’s get started! 


A glucose tolerance test, abbreviated GTT, is just like it sounds. It evaluates the body’s response or tolerance to a measured amount of glucose. So, how the test takes place, is that the patient is fasting, their blood sugar is measured, they’re given a measured dose of glucose, and then their body is evaluated an hour and two hours after administration to see what their body or how their body responded. And it distinguishes whether they have a normal or an abnormal response. It can help us detect if there’s insulin resistance or diabetes. 


Some clinical indications, or why we would have this test taken, is to screen for gestational diabetes. Pretty dang common. I have a friend that just had a baby, and I remember her going through this testing. Uh, it’s also tested if there are, uh, symptoms or family history of diabetes and any medical conditions related to the development of diabetes; so, obesity, any metabolic syndrome and even polycystic ovarian syndrome. 


Normal therapeutic values for a patient that is non-pregnant, is going to show a fasting glucose between 60 and a hundred. We’ve seen those values before. An hour after they’re given that metered or measured dose of glucose, they should have an increase in their blood sugar, right? But, a normal response should be less than 200. After two hours, a normal response, their blood sugar should return to less than 140. If it’s any higher than that, um, up to about 200, it’s considered impaired glucose tolerance or insulin resistance, and it’s diagnostic of diabetes If it’s greater than 200, after that two hour timeframe in someone that’s nonpregnant. Someone that is pregnant, their normal fasting blood glucose should be below 90. After an hour, It should be less than 130, but up to 140, and after two hours, should return to less than 120, that would be normal. An abnormal or a diagnostic for gestational diabetes, would be a fasting, uh, value greater than 95, greater than 140 after an hour, and greater than 120 after two hours. That means that there’s just not enough of, uh, either a response from the insulin, so there could be some resistance, um, or a diabetic picture. Collection takes place in a gray tube, you may have seen one of these before. Lab values, when they’re increased, are going to tell us that we have presence of diabetes, whether it be type one or type two, it could also be diagnostic for pre-diabetic, or gestational diabetes, or tell us about insulin resistance. Decreased levels would be that we have a proper endocrine response. This is great. The cells are actually letting that blood glucose in and utilizing it like here. Whereas, if you have vigorous exercise, that also is going to drive glucose deep into the cell via the insulin, so that you’ve got more energy. And certain medications can also cause a decrease in the glucose tolerance test, uh, whether those be medications to actually treat, uh, diabetes or that type of picture, or certain medications, or just going to reduce the blood sugar. 


Linchpins for this lesson are that the glucose tolerance test evaluates the body’s response or tolerance to a measured dose of glucose. And it’s a diagnostic tool for diabetes to put it short. After two hours, someone that’s nonpregnant should have less than 140 milligrams per deciliter and someone that’s pregnant should have less than 120 milligrams per deciliter. When the value is increased, this is going to mean diabetes type two, uh, insulin resistance, or gestational diabetes. It’s a diagnostic tool, whereas, a decreased value, we’ve got a good, healthy response, and it can be related to lifestyle or medication. 


You all did great on this lesson and that wraps it up on glucose tolerance testing. Now, go out for your best self today and as always, happy nursing.


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