A fasting plasma glucose test (FPG) is the prefered test for diagnosing diabetes and most reliable when done in the morning. It measures blood glucose after you have gone at least eight hours without eating.

If your fasting glucose level is 100 to 125 mg/dL, you are considered to have pre-diabetes, which increases your risk of developing diabetes. If your FPG is 126 mg/dL, confirmed by repeating the test on another day, you have diabetes.

References

National Institutes of Health, NIDDK