You will be required to fast for at least 8 hours prior to the test, and up to 12 hours maximum. People who have to take a fasting blood glucose test will often fast overnight and then get the test done first thing in the morning. A blood sample will be drawn and sent for testing.
Eating or drinking in the 8 hours before the test can affect the results. So can stress, illness, alcohol, caffeine, smoking, and certain medications.
If your test results come back suggesting that you are either pre-diabetic or diabetic, another test will be done (on a different day). If you show signs of diabetes and the test results for both tests are similar, then it is likely that the results are correct.
Sometimes a random or casual blood glucose test is also administered. This test checks your blood glucose level at any time during the day - it doesn't matter when you last ate since blood glucose levels shouldn't normally vary too dramatically over the course of a day. If the result of this random test comes back between 140 mg/dl and 200 mg/dl, it suggests pre-diabetes. Higher than 200 mg/dl suggests diabetes. See our conversion chart for blood sugar levels to convert to mmol/L.
Remember that normal fasting blood glucose can be affected by a variety of things. Your doctor will make a diagnosis based on test results (fasting and random blood glucose tests), patient history, and symptoms.