A blood glucose test measures the amount of a type of sugar, called glucose, in your blood. Glucose comes from carbohydrate foods. It is the main source of energy used by the body. Insulin is a hormone that helps your body's cells use the glucose. Insulin is produced in the pancreas and released into the blood when the amount of glucose in the blood rises.
Normally, your blood glucose levels increase slightly after you eat. This increase causes your pancreas to release insulin so that your blood glucose levels do not get too high. Blood glucose levels that remain high over time can damage your eyes, kidneys, nerves, and blood vessels.
There are several different types of blood glucose tests.
- Fasting blood sugar (FBS). This test measures blood glucose after you have not eaten for at least 8 hours. It is often the first test done to check for prediabetes and diabetes.
- 2-hour postprandial blood sugar. This test measures blood glucose exactly 2 hours after you start eating a meal. This is not a test used to diagnose diabetes. This test is used to see if someone with diabetes is taking the right amount of insulin with meals.
- Random blood sugar (RBS). It measures blood glucose regardless of when you last ate. Several random measurements may be taken throughout the day. Random testing is useful because glucose levels in healthy people do not vary widely throughout the day. Blood glucose levels that vary widely may mean a problem. This test is also called a casual blood glucose test.
- Oral glucose tolerance test. This test is used to diagnose prediabetes and diabetes. An oral glucose tolerance test is a series of blood glucose measurements taken after you drink a sweet liquid that contains glucose. This test is commonly used to diagnose diabetes that occurs during pregnancy (gestational diabetes). Women who had high blood sugar levels during pregnancy may have oral glucose tolerance tests after pregnancy.
- Hemoglobin A1c. This test is also called glycohemoglobin. It measures how much sugar (glucose) is stuck to red blood cells. This test can be used to diagnose diabetes. It also shows how well your diabetes has been controlled in the past 2 to 3 months and whether your diabetes medicine needs to be changed. The result of your A1c test can be used to estimate your average blood sugar level. This is called your estimated average glucose, or eAG.
To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.
Why It Is Done
Blood glucose tests are done to:
- Check for prediabetes and diabetes.
- Monitor treatment of diabetes.
- Check for diabetes that occurs during pregnancy (gestational diabetes).
- Determine if an abnormally low blood sugar level (hypoglycemia) is present. A test to measure blood levels of insulin and a protein called C-peptide may be done along with a blood glucose test to determine the cause of hypoglycemia. To learn more, see the topic C-Peptide.
How To Prepare
Fasting blood sugar (FBS)
This is one of several tests that is used to diagnose diabetes. For a fasting blood sugar test, do not eat or drink anything other than water for at least 8 hours before the blood sample is taken.
If you have diabetes, you may be asked to wait until you have had your blood tested before taking your morning dose of insulin or diabetes medicine. You may have a random blood sugar test instead, which will not require an 8-hour fast.
2-hour postprandial blood sugar
Random blood sugar (RBS) and hemoglobin A1c
No special preparation is required before having a random blood sugar or A1c test.
Oral glucose tolerance test
For an oral glucose tolerance test, you'll need to follow a special diet for 3 days before the test. And do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is taken.
To learn more about how to prepare for this test, see Oral Glucose Tolerance Test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean.
To help you understand the importance of this test, fill out the medical test information form .
How It Is Done
The health professional taking a sample of your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Apply a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little risk of a problem from having blood drawn from a vein.
- You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
- In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
A blood glucose test measures the amount of a type of sugar, called glucose, in your blood.
Results are often ready in 1 to 2 hours. Glucose levels in a blood sample taken from your vein (called a blood plasma value) may differ a little from glucose levels checked with a finger stick.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
Many conditions can change your blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
You may have diabetes. To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.
Other conditions that can cause high blood glucose levels include:
- Severe stress.
- Heart attack.
- Cushing's syndrome.
- Medicines such as corticosteroids.
- Excess production of growth hormone (acromegaly).
A fasting glucose level below 40 mg/dL (2.2 mmol/L) in women or below 50 mg/dL (2.8 mmol/L) in men that is accompanied by symptoms of hypoglycemia may mean you have an insulinoma, a tumor that produces abnormally high amounts of insulin.
Low glucose levels also may be caused by:
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Eating or drinking less than 8 hours before a fasting blood test or less than 2 hours before a 2-hour postprandial test.
- Drinking alcohol on the day of the test or several days before the test.
- Illness or emotional stress, smoking, and caffeine.
- Taking a medicine. Make sure that your doctor knows about any medicines you take and how often you take them.
Current as of: December 20, 2019
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
Adam Husney MD - Family Medicine
Matthew I. Kim MD - Endocrinology
David C.W. Lau MD, PhD, FRCPC - Endocrinology