Fasting and Non-Fasting Bloodwork for Diabetes

If you have diabetes or your doctor thinks you might, certain blood tests can tell a lot about how your body processes blood sugar.

Sometimes you'll need to fast. That means not eating or drinking anything except water for a specific amount of time before a blood sample is taken. This will ensure all the food you've eaten has been digested. But you don't need to fast for all blood sugar tests.

Fasting Blood Tests

Fasting Blood Glucose Test
Doctors use this test to diagnose diabetes. You'll be asked to fast for 8 hours beforehand. The test is usually done early in the morning so you don't have to go too long without eating. A blood glucose level of 126 milligrams per deciliter or higher is a sign of diabetes. A healthy score is below 100 milligrams per deciliter.

Oral Glucose Tolerance Test
This is another test used for diagnosis. You'll also need to fast for 8 hours for this one before it's done. A nurse will start the test by taking a sample of your blood. Then you'll drink a sugary liquid and stay in the doctor's office. Two hours later, a nurse will take another blood sample for testing. If that shows a blood glucose level of 200 milligrams per deciliter or higher, you'll be diagnosed with diabetes. A level of 140 to 199 milligrams per deciliter is a sign of prediabetes. That's when your blood sugar is higher than normal but you might be able to keep diabetes at bay with some lifestyle changes.

This test is also used to diagnose a condition called gestational diabetes. That's a type some women develop when they're pregnant. It's done the same way except a third sample is taken after 3 hours.

Non-Fasting Blood Tests

Random Blood Glucose Test
Doctors use this test to diagnose people who have serious diabetes symptoms. It can be given at any time on a moment's notice. So there's no need to fast before the blood sample is taken. If your blood glucose level is 200 milligrams per deciliter or higher, it's a sign of diabetes.

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Oral Glucose Challenge Test
Most pregnant women have this test to check for gestational diabetes between 24 and 28 weeks. Your doctor will ask you to have a sugary drink, stay in the office for an hour, then have a blood sample taken. There's no need to fast. You can have this test done any time of the day.

If your result is 130 milligrams per deciliter or higher, your doctor may ask you to come back another day to take an oral glucose tolerance test to confirm that you have gestational diabetes.

A1C test
This test measures your average blood glucose level over the past 3 months. That timeframe means it doesn't matter if you eat before the test. The result is given as a percentage. The higher the percentage, the higher your blood sugar levels have been.

For diagnosis, if your blood glucose level is 6.5% or higher, you probably have diabetes. A normal score is below 5.7%. After your diagnosis, your doctor will use this test to keep an eye on how you're doing. The goal is for your A1C level to be less than 7%.

Blood Glucose Meter
If you have diabetes, you'll use this test to check your blood glucose levels at home. Your doctor will tell you how often you'll need to test and if you should do it at specific times. For example, you may need to test your levels before snacks or meals. But there's no specific need to fast beforehand.

WebMD Medical Reference Reviewed by Minesh Khatri, MD on April 30, 2017

Sources

SOURCES:

American Diabetes Association: "Diagnosing diabetes and learning about prediabetes."

Mayo Clinic: "Blood sugar testing: Why, when and how," "Diabetes: Symptoms," "Diabetes: Tests and diagnosis," "Glucose challenge test: Results," "Glucose tolerance test: Results." 

National Institute of Diabetes and Digestive and Kidney Diseases: "Gestational diabetes," "Diabetes tests and diagnosis," "Diabetes and prediabetes tests."

Diabetes Action Research and Education Foundation: "General information: Questions and answers." 

American Family Physician: "Screening, diagnosis, and management of gestational diabetes mellitus."

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