Fasting Blood Sugar Test
What is fasting blood sugar?
When does a person need to have their blood glucose measured with this test?
Prediabetes, Type 2 diabetes and gestational diabetes often have no symptoms at first. A person can have the condition and not know it. Healthcare providers usually order a fasting blood sugar test:
- As part of a standard annual physical examination to monitor a person’s blood glucose over time.
- For pregnant women to ensure that pregnancy hormones are not causing diabetes.
- When a person has symptoms of diabetes, a family history of diabetes or risk factors for diabetes (such as obesity).
- When a person has had a previous blood glucose level that was higher than normal.
Who performs a fasting blood sugar test?
Fasting blood sugar tests occur in a doctor’s office, lab or hospital.
How does blood glucose work?
Blood sugar, or glucose, is the main sugar found in your blood. When you eat or drink, your body breaks down carbohydrates in the food or liquid into sugar for energy.
Then your pancreas produces a hormone called insulin, which helps blood sugar enter your body’s cells. It also helps your body store blood sugar for later use. As the cells use the blood sugar, glucose and insulin levels in the blood decrease.
But with diabetes, your body has trouble regulating glucose. Either your pancreas doesn’t make enough insulin, or your body doesn’t process insulin effectively. Therefore, the amount of sugar in your blood is higher than normal. And a lot of glucose in your blood is bad for your body.
A fasting blood glucose test measures the amount of glucose in your blood when it should be at its lowest. That happens in the morning and when you haven’t had anything to eat or drink for a while.
How do I prepare for a fasting blood sugar test?
To prepare for a fasting glucose test, you should have nothing to eat or drink (except water) for 8 to 12 hours. Your healthcare provider will tell you how long to fast.
What can I expect on the day of the test?
A fasting blood glucose test is often done with a common blood draw. A healthcare provider will:
- Clean the area inside your elbow to kill any germs.
- Tie a band around your upper arm, which will help the veins in your arm fill with blood.
- Insert a clean needle into a vein (which feels like a brief pinch).
- Draw blood into a vial that’s attached to the needle and labeled with your personal information.
- Remove the band and then the needle.
- Put pressure on the needle insertion site to stop the bleeding.
- Place a bandage on the area.
After the test, the healthcare provider will send the blood sample to a lab for testing.
In some cases, your healthcare provider may be able to test your blood sugar with a prick on the finger instead of a needle in the vein. The provider uses a blood glucose monitor and test strip to measure your blood glucose level in the office.
Are there any risks with a fasting blood sugar test?
A blood test is a very common and safe test, with almost no chance of any complications.
Results and Follow-Up
When will I get the results of the test?
Results of a fasting blood glucose test are usually available quickly, within days or hours.
What is normal fasting blood sugar, and what is a diabetes blood sugar level?
The results of the fasting blood sugar test will come back as a number:
- 99 mg/dL or lower: This is a normal fasting blood sugar level.
- 100–125 mg/dL: Fasting blood sugar in this range typically indicates prediabetes. This means your blood sugar levels are higher than normal but not high enough to be classified as diabetes.
- 126 mg/dL or above: This indicates high blood sugar, the main sign of diabetes.
If you have a high fasting blood sugar level, your healthcare provider may repeat the test to make sure. If the test reveals that you have prediabetes, your healthcare provider will recommend you repeat the fasting blood sugar test every year or two. The results will help you know whether you are progressing to Type 2 diabetes.
Can I do a fasting blood sugar test at home instead?
There are kits that allow you to test your blood sugar at home. But they should not be used to diagnose diabetes.
The results are often not as accurate as tests by a healthcare provider. Several factors can affect accuracy, such as:
- Environmental conditions.
- Unclean skin on the finger being pricked.
- Expired or damaged test strips.
- Not enough blood in the testing sample.
At-home blood testing kits are intended to help people who have already been diagnosed. They help people with diabetes manage the condition and understand what factors affect their blood sugar.
A note from Cleveland Clinic
Fasting blood sugar is a simple, common blood test to diagnose prediabetes, diabetes or gestational diabetes. Before the test, you shouldn’t have anything to eat or drink (except water) for 8 to 12 hours. Talk to your healthcare provider about whether you need a fasting blood glucose test.
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy