There are two different tests your doctor can use to measure your blood glucose to determine if you have pre-diabetes or diabetes: the fasting plasma glucose test (FPG) or the oral glucose tolerance test (OGTT). In general, it is recommended that any blood test that is outside the normal range be repeated on a different day to confirm the diagnosis. The American Diabetes Association recommends the FPG because it is easier, faster, and less expensive to perform.
The FPG test is a blood test that determines the amount of glucose (sugar) in the blood after an overnight fast (not eating for at least 8 hours). A fasting blood glucose level between 100 and 125 mg/dl means a person has pre-diabetes. A fasting blood glucose level of 126 mg/dl or higher means a person has diabetes.
With the OGTT test, a person's blood glucose level is measured after fasting and then 2 hours after drinking a glucose-rich beverage. If the 2-hour blood glucose level is between 140 and 199 mg/dl, the person has pre-diabetes. If the 2-hour blood glucose level is at 200 mg/dl or higher, the person has diabetes.
See the chart below for the ranges of blood sugar levels and how they are used to diagnose pre-diabetes and diabetes.