Health and MedicineMedical testGlucose tolerance test: how it is done and normal values

Glucose tolerance test: how it is done and normal values

The glucose tolerance test (or oral glucose tolerance test) is useful to know if a person suffers from prediabetes or diabetes. Find out how to do it.

What is the glucose tolerance test?

Also known medically by the name of  glucose tolerance curve, it is a type or test of blood tests especially used in the diagnosis of prediabetes, type 2 diabetes mellitus (the most common), or gestational diabetes during the pregnancy. Why? Mainly because it helps to understand how the body is able to break down sugar (glucose).

It should not be confused with the sugar test during pregnancy, which is commonly and medically known by the name of the O’Sullivan Test, and which is carried out between weeks 24 and 28 of pregnancy, although depending on the Autonomous Community. In which the pregnant woman is (in Spain) it is possible that two measurements are made, one in the first trimester or the next a little later.

This time we will talk about one of the most common glucose tolerance tests, called oral glucose tolerance test (OGTT). There is also what is known as an intravenous glucose tolerance test (IVTT), but it is less common and is rarely used for diagnosing diabetes.

How the glucose tolerance test is performed: the steps you must follow:

Days before carrying out the tolerance test, it is advisable to follow a normal, routine lifestyle, eating normally. Then, it is essential not to eat or drink anything for 8 hours before the time of the exam. Likewise, you cannot eat or drink anything during the exam.

Before taking the glucose solution, a blood sample is taken, which will allow knowing the fasting blood glucose values, before having ingested anything.

Then you have to drink a glucose solution (it can be glucose or dextrose), which can be somewhat unpleasant at first as it tends to taste very, very sweet. Many doctors tell the patient that its taste is similar to that of a soft drink, but the truth is that it is actually much sweeter. The most common is that the amount of glucose reaches 75 grams.

Later, new blood samples will be taken every 30 to 60 minutes immediately after you have ingested the glucose solution. A third blood sample may be taken.

Since it is a test that can take up to 3 hours, it is essential that the person remains in the laboratory. Going for a walk or exercising is not recommended. Neither obviously eats anything nor drinks anything except the glucose solution administered by the laboratory.

That is, we can summarize the glucose tolerance test as follows:

  1. A first fasting blood sample is obtained.
  2. The glucose solution is swallowed, slowly and gradually.
  3. Wait 30 or 60 minutes and a second blood sample is obtained.
  4. Another 60 minutes (2 hours total) is waited, and a third and final blood sample is obtained.

Glucose Tolerance Test Values

The reference values ​​of the glucose tolerance curve refer to the value that glucose possesses 2 hours after glucose ingestion. And are next:

  • Normal fasting values: 60 to 100 mg/dl.
  • Normal: less than 140 mg/dl.
  • Prediabetes: between 140 to 199 mg/dl.
  • Diabetes: equal to or greater than 200 mg/dl.

Thus, if the patient has fasting glucose values ​​above 126 mg/dl, and a blood glucose equal to or greater than 200 mg/dl 120 minutes (2 hours) after having ingested the glucose solution, it is quite probable who suffers from diabetes.

Oral glucose tolerance test (OGTT) values

It is also possible to take into account other values ​​that are also considered normal depending on the time elapsed after ingestion of the glucose solution. Typically, for a glucose tolerance test with 75 grams of glucose, normal values ​​are as follows:

  • Fasting: 60 to 100 mg/dl.
  • 1 hour later: less than 200 mg/dl.
  • 2 hours later: less than 140 mg/dl.

This test is usually performed in patients with a high fasting blood glucose level (above 125 mg/dl), but which is not high enough to be diagnosed with diabetes.


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