What to tell clients about an Oral Glucose Tolerance Test

Here is a simple explanation of an oral glucose tolerance test (OGTT) and some points to keep in mind for when your clients ask about it.

What does the test do?

A good place to start is to explain the purpose of an OGTT.

To help your client be prepared, describe the procedure and the preparation needed before the test.

Screening tool

The purpose of an OGTT is to work out how the body manages glucose after a meal. The test is used to screen for pre-diabetes, type 2 diabetes and gestational diabetes.

The results show how quickly glucose (sugar) is cleared from the blood.

Drink a syrupy glucose solution

The OGTT involves a client drinking a syrupy glucose solution after a period of fasting.

A blood sample is then drawn to find out if they are metabolising that glucose effectively. The whole procedure takes just over two hours.

Steps of the test

When explaining the steps of the procedure to your client it is important to be clear and set realistic expectations about wait times at the clinic as part of the testing process.

It may be useful to use diagrams about the small sugary drink and the blood samples taken at different times for clients with low literacy levels or those whose first language is not English. Please see the link at the end of this article.

When you know your client is being asked to have an OGTT, it is important to explain to the client the steps that must be taken to prepare for the procedure.

An 8-hour fast

The client can eat the foods that they normally eat leading up to the fasting period. This will help to give an accurate baseline result.

On the evening before the test, the client will be advised to fast overnight for at least eight hours.

Drinking water is still okay in the fasting period.

Schedule for early morning

It is recommended for clients to schedule their OGTT earlier in the morning.

This is for their comfort as they would have needed to skip their breakfast.

Firstly, on arrival at the clinic on the day of the OGTT, the staff will screen the client, asking a few questions and obtaining details such as date of birth.

Equal to 15 teaspoons of sugar

Secondly, a fasting blood sample is taken. This first blood specimen (specimen 1) will be the baseline result.

Thirdly, the staff will prepare a sweet, syrupy drink made up of a 75g dose of glucose (equal to 15 teaspoons of sugar or five slices of bread). Advise the client to finish this drink within five minutes.

Some clients have found it difficult to consume the glucose drink within five minutes. Sometimes people find it difficult to tolerate this sweet drink and can feel sick or even vomit.

The HbA1C can be an alternative

If the client cannot keep the glucose drink down, all is not lost as the baseline result would have been obtained. An HbA1c check may be used as an alternative under certain circumstances. The doctor will advise on this.

After finishing the drink, the staff will ask the client to stay in the waiting room. It is essential to the effectiveness of the test that the client rests and does not perform any physical exercise, smoke, eat or drink (except water) during this phase of the test. Advise your client to take a magazine, book or mobile phone as they will have to wait in between blood sample tests.

Three blood samples taken

After one hour, the second of three blood samples are collected (specimen 2).

The client then waits for a further hour before the third and final blood sample (specimen 3) is collected.

Once the test is completed the client is free to return to their usual activities, including eating and drinking.

Please follow this link to see the steps for the OGGT, in this example, for gestational diabetes screening.


Join our community of over 33,000 people living with diabetes