There are 2 types of test. The one I always have is a 2 hour glucose Tolerance Test (due to having babies weighing more than 4.5kg, and having a strong family history of diabetes).
The Oral Glucose Tolerance Test (OGTT) is a fasting test, you need to fast for 8-12 hours prior to the test.
You will have a fasting blood test, which is a pre-glucose drink test, then you will drink the drink within 5 mins,
you then have another blood test 1 hour from when you finished the drink (one hour post-glucose ingestion) ,
and another blood test an hour after that (2 hours post-glucose ingestion). You can't eat, drink, smoke in between the bloods (water should be ok)
The second is the Oral Glucose Challenge Test (OGCT) which is a non-fasting test, usually drink the glucose drink within 5-10 mins, and have a test 1 hour after that.
Depending what is written on your pathology form from the Dr will depend on which test you are required to have.