Your doctor may want to check for high glucose levels in your blood after noting warning signs in a health history or physical exam. There are several types of blood tests used to diagnose diabetes. A fasting plasma glucose test is the most common. This test involves fasting for at least eight hours (usually overnight), after which the level of glucose in the blood is checked.
An oral glucose tolerance test is the preferred method for diagnosing gestational diabetes. It can also be used to diagnose type 2 and pre-diabetes. In this test, blood samples are drawn at timed intervals after a person drinks a sweet liquid. Glucose levels will rise and then fall quickly in a person whose body is adequately processing glucose. When diabetes is present, glucose levels will rise higher and fail to come down as quickly.
What the Numbers Mean
A healthy fasting plasma glucose level is less than 100 milligrams per deciliter (mg/dL).
A level of more than 126 mg/dL usually suggests diabetes. Levels between 100 and 126 mg/dL can mean pre-diabetes is present.
A blood glucose level that is 140 to 199 mg/dL two hours after drinking the sweet liquid suggests pre-diabetes. A level of 200 mg/dL or higher indicates diabetes.
