Fasting Blood Glucose

Reviewed by: HU Medical Review Board | Last reviewed: October 2020.

The fasting blood glucose test is the quickest and simplest test used to diagnose diabetes. The test measures the level of glucose (sugar) in your blood. For the most reliable results, it is best to have the test done in the morning, after fasting (nothing to eat or drink except water) for at least 8 hours. If your fasting blood glucose level is above 126 mg/dL, you likely have diabetes.1

Why is a fasting blood glucose test important?

The fasting blood glucose test helps you and your doctor see how well your body manages your blood sugar in the absence of food. Your doctor might have you take a fasting blood glucose test if you have any of the common signs of high blood sugar, including:1

  • Blurred vision
  • Frequent infections
  • Numbness or tingling, especially in the legs, feet, arms, and hands
  • Sores or cuts that heal slowly or do not heal
  • Excessive thirst and increased urination
  • Sudden, unexplained weight loss
  • Fatigue
  • Constant hunger

You might also be asked to take a random blood sugar test if you have any of the risk factors for type 2 diabetes, including high blood pressure and being overweight.1

When you do not eat or drink anything but water for several hours, your body releases glucose into your blood. In people who do not have diabetes, the body’s insulin works to stabilize blood glucose levels.2

However, if you have diabetes, your body is resistant to insulin. At later stages of diabetes, your body does not make enough insulin. Insulin is a hormone that helps glucose (sugar) get into cells so it can be used for energy. Since you do not have enough insulin to help this process, too much glucose stays in your blood.1

Even though many people who have type 2 diabetes do not have symptoms, the condition can cause serious, long-term complications if left untreated. Over time, high blood sugar can affect many of your organs, including your heart, nerves, eyes, and kidneys. The fasting blood glucose test is an important tool to help you and your doctor monitor your blood sugar levels and keep them under control to improve your health.3

How is a fasting blood glucose test done?

A fasting blood glucose test is a quick procedure that is usually done in your doctor’s office or a lab. You will need to fast (not eat or drink anything but water) for 8 to 12 hours before your test. Your doctor will give you specific directions before your test. If possible, schedule your test for early in the morning so you can fast overnight.3

Before your test tell your doctor about any medicines you are taking or if you are sick, since these can affect your results.3

At the time of your test, your doctor or a member of your healthcare team will take a sample of your blood, usually from your finger. They will then measure the amount of glucose that is in your blood. The results are usually available quickly.3

What do the test results mean?

Your fasting blood glucose test results will help you and your doctor know if you have type 2 diabetes. The American Diabetes Association uses the following numbers to diagnose diabetes:4

  • Normal: A blood glucose level that is 99 mg/dL or lower
  • Prediabetes: A blood glucose level between 100 mg/dL and 125 mg/dL
  • Diabetes: A blood glucose level of 126 mg/dL or higher

If your test result indicates diabetes, your doctor will probably have you take another fasting blood glucose test again on a different day to recheck your blood sugar levels. If 2 or more of your test results are above normal, you will be diagnosed with prediabetes or type 2 diabetes. Talk to your doctor about ways to manage your condition and live a healthy life with type 2 diabetes.4

How was your type 2 diabetes diagnosed?

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