Glucose is a kind of sugar existing in fruits and many other foods. It is the main source of energy which the body uses. Most of the carbohydrates that people eat are turned into glucose that can be used for energy or stored in the liver and kidneys as glycogen.

A hormone insulin helps the body use and control the amount of glucose in the blood. Insulin is produced in the pancreas and released into the blood when the amount of glucose in the blood rises. People who do not produce enough insulin develop diabetes. People can also develop diabetes if they do not respond normally to the insulin their bodies produce. This occurs most commonly when a person is overweight.

Normally, blood glucose levels rise slightly after a person eats a meal. This rise causes the pancreas to release insulin so that blood glucose levels do not get too high. Blood glucose levels that remain high over time can cause damage to the eyes, kidneys, nerves, and blood vessels.

Blood glucose tests may be done at different times:

• Fasting blood sugar (FBS). This is a measurement of blood glucose after not eating for 12 to 14 hours. It is often the first test done to help detect diabetes.
• 2-hour postprandial blood sugar (2-hour PP). This is a measurement of blood glucose done exactly 2 hours after taking a meal.
• Random blood sugar (RBS). A random blood sugar measurement may also be called a casual blood glucose test. This is a measurement of blood glucose that is taken regardless of when the person had the last meal. Sometimes several random measurements are taken throughout a 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 indicate a problem.

Rarely, an oral glucose tolerance test may be done with a blood glucose test to confirm a diagnosis of diabetes. An oral glucose tolerance test is a series of blood glucose measurements taken after a person drinks a liquid containing a specific amount of glucose. An oral glucose tolerance test is most commonly used to diagnose gestational diabetes; it is not recommended to diagnose diabetes in a person who is not pregnant. For more information, see Oral Glucose Tolerance Test.

A blood glucose test is done on a blood sample taken from a vein or capillary.

Blood Glucose
Results

Normal

Normal results may vary from lab to lab.

Greater than normal values may mean

The American Diabetes Association (ADA) criteria for diagnosing diabetes are met when any of the following results have been repeated on at least two different days:

• A fasting blood glucose level is higher than 125 mg/dL.
• A 2-hour oral glucose tolerance test result is higher than 200 mg/dL. For more information, see Oral Glucose Tolerance Test.
• Symptoms of diabetes are present and a 2-hour postprandial or random blood glucose test is greater than 200 mg/dL.
• A person who has a fasting blood glucose level between 110 and 125 mg/dL is considered to have an impaired fasting glucose (IFG) level and may have an increased risk for developing diabetes.
• Other conditions that can result in high blood glucose levels include severe stress, heart attack, stroke, Cushing's syndrome, certain rare tumors, or excess production of growth hormone acromegaly.

Lower than normal values may mean

• A fasting glucose level below 40 mg/dL in women or below 50 mg/dL in men that is accompanied by symptoms of hypoglycemia may indicate an insulinoma, a tumor that produces abnormally high amounts of insulin.
• Lower than expected glucose levels can also indicate Addison's disease, an underactive thyroid or pituitary, liver disease (such as cirrhosis ), malnutrition, or a problem that hampers the intestines absorbing the nutrients in food (such as sprue).