Diabetes Basics

Oral Glucose Tolerance Test

A glucose tolerance test is performed in order to determine the rate of clearing of glucose from blood. It is called Oral Glucose Tolerance test (OGTT) because glucose is administered orally. The objective of doing so is to test for insulin resistance, diabetes, and sometimes reactive hypoglycaemia. It may be done as a part of other tests (metabolic panel tests, for example).

How to Prepare for OGTT?
Individuals are asked to continue with their regular pattern of carbohydrate (the main source of glucose) consumption in the days following up to the test. However, like all other tests, individuals are supposed to fast for 8 to 14 hours. They are allowed to drink water in the meanwhile.

How to Go About the Test
The test should be ideally conducted in the morning, in between 7 o'clock to 8 o'clock. The reason to do so is because glucose tolerance exhibits a diurnal pattern with a significant drop in the afternoon.

The test subject is first weighed and then administered a glucose solution for consumption. The weight of glucose dose (in grams) is calculated by multiplying the subject's weight with 1.75. The maximum dose of glucose can not exceed 75 grams. It is dissolved in a glass of water and consumed. Despite the fact that it is extremely sweet and may make you puke, it is important within 5 minutes.

Now comes the slightly painful part. In order to know the rate of discharge of glucose blood is drawn out at regular intervals to know glucose (and sometimes insulin) levels. For diabetes, a sample may be collected twice- once immediately after consumption of glucose dose and then two after 1 hour interval.
 
A Word of Caution
The dose of glucose to be administered to a test subject should be given only if the subject weighs over 43 kg (94lb). If this is ignored the test result would be a false one.

OGTT-What do the Results Indicate?
When glucose tolerance test is conducted for gestational diabetes the individual is given a 100-gram oral glucose dose. Normalcy is indicated when the following readings are observed.

* Fasting: less than 95 mg/dL
* 1 hour: less than 180 mg/dL
* 2 hour: less than 155 mg/dL
* 3 hour: less than 140 mg/dL

Normal blood values for a 75-gram OGTT dose (used to check for Type 2 diabetes) should indicate the following:

* Before the test: 60 -100 mg/dL
* 1 hour after the intake of dose: less than 200 mg/dL
* 2 hours after the intake of dose: less than 140 mg/dL.

If, after 2 hours, the value is in between 140-200 mg/dL, prediabetes is indicated. However, if the reading is above 200 mg/dL it is a clear sign of diabetes mellitus.