Blood Glucose Test
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) 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.
- Random blood sugar (RBS) 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.
- A 2-hour postprandial blood sugar test measures blood sugar exactly 2 hours after you start eating a meal. This test is most often done at home when you have diabetes. It can see if you are taking the right amount of insulin with meals.
- The hemoglobin A1c test and the oral glucose tolerance test (OGTT) are other tests used to look at blood sugar levels. The A1c test can be used to estimate your average blood sugar level over the past 2 to 3 months. The OGTT is commonly used to diagnose diabetes that occurs during pregnancy (gestational diabetes).
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.
How To Prepare
Fasting blood sugar (FBS)
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 you take your morning dose of insulin or diabetes medicine.
Random blood sugar (RBS)
No special preparation is needed before having a random blood sugar test.
How It Is Done
A health professional uses a needle to take a blood sample, usually from an arm.
How It Feels
You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of having a problem from this test. A small bruise may form at the site.
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.
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:
Current as of: April 13, 2022
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