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
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.
- 2-hour postprandial blood sugar 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)
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 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, or glycohemoglobin, 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.
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