Usually... you have to fast the night before the test too. Your doctor should explain everything to you though. If you've ever had the test for gestational diabetes (the extended version) then it's very similar.
I had one of these years ago and it's purpose is to test your body's ability to control blood glucose and insulin levels. For it to be truely effective you must starve for 12 hours and they should give you a glucose drink, then measure over a full 5 hours to test what happens to blood sugar and insulin levels over the next 5 hours. the reason 5 hours is because your blood sugar and insulin levels should be normal after 2.5 to 3 hours, but if you are hypoglycaemic, your bllod sugar levels will drop below normal in the hours that follow. Hypoglycaemia is step 1 on a 5 step path to type 2 diabetes. It can be controlled by diet (essentially most people who have it are overweight) but if you aren't (like me) you may suffer from Insulin Resistance, which is where your body cannot absorb insuline properly leading to high insulin levels, which leads to high blood sugar levels and high cholesterol. Mine was being caused by Lupus. Now the treatment I am on has caused me to come off my insulin process boosting drug, metformin.
I hope this is not too much info, as I don't know why you are having this test, but all the best.
Just spoke to my doctor. The receptionist read it wrong, I dont need a glucose test but a gamma TT blood test hee hee panic over.
but on a worse note a lot of my bloods are abnormal :S and all my inflammatory markers are up :S