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Blood Glucose

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 use and control the amount of glucose in your blood. 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.

Several different types of blood glucose tests are used.

Fasting blood sugar (FBS) measures blood glucose after you have not eaten for at least 8 hours. It often is the first test done to check for diabetes.
2-hour postprandial blood sugar (2-hour PC) measures blood glucose exactly 2 hours after you eat a meal.
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 indicate a problem. This test is also called a casual blood glucose test.
Oral glucose tolerance test is used to diagnose diabetes that occurs during pregnancy (gestational 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 not recommended for diagnosing diabetes in a person who is not pregnant. For more information, see the medical test Gestational Diabetes.

Why It Is Done

Blood glucose tests are done to:

Check for 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 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 taking your morning dose of insulin or diabetes medication.

2-hour postprandial blood sugar (2-hour PC)
For a 2-hour postprandial test, eat a meal exactly 2 hours before the blood sample is taken. A home blood sugar test is the most common way to check 2-hour postprandial blood sugar levels.

Random blood sugar (RBS)
No special preparation is required before having a random blood sugar test.

Results

Normal
A blood glucose test measures the amount of a type of sugar, called glucose, in your blood. Results are usually available within 1 to 2 hours.

Blood glucose normal values:

Fasting blood glucose:	70-99 milligrams per deciliter or less than 5.5 mmol/L
2 hours after eating (postprandial):	70-145 mg/dL (less than 7.9 mmol/L)
Random (casual):	70-25 mg/dL (less than 7.0 mmol/L)

Normal results may vary from lab to lab. Many conditions can change your blood glucose levels. Your health professional will discuss any significant abnormal results with you in relation to your symptoms and medical history.

High values

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 126 mg/dL (7.0 mmol/L) or higher.
A 2-hour oral glucose tolerance test result is 200 mg/dL (11.1 mmol/L) or higher.
Symptoms of diabetes are present and a random blood glucose test is 200 mg/dL (11.1 mmol/L) or higher

Symptoms of diabetes include increased thirst and frequent urination (especially at night), unexplained increase in appetite, unexplained weight loss, fatigue, erection problems, blurred vision, and tingling or numbness in the hands or feet.

If your fasting blood glucose level is between 100 mg/dL (5.5 mmol/L) and 126 mg/dL (7.0 mmol/L), you are considered to have prediabetes (impaired fasting glucose), and you have an increased chance of getting diabetes.

Other conditions that can cause high blood glucose levels include severe stress, heart attack, stroke, Cushing's syndrome, medications such as corticosteroids, cancers, or excess production of growth hormone (acromegaly).

Low glucose levels also may be caused by:

Addison's disease.
Decreased thyroid hormone levels (hypothyroidism).
A tumor in the pituitary gland.
Liver disease, such as cirrhosis.
Kidney failure.
Malnutrition or an eating disorder, such as anorexia.
Medications used to treat diabetes.

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.
Illness or emotional stress, smoking, and caffeine.

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References

Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2002). Mosby's Manual of Diagnostic and Laboratory Tests, 2nd ed. St. Louis: Mosby.

Credits
Author Sydney Youngerman-Cole, RN, BSN, RNC
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Tracy Landauer
Primary Medical Reviewer Caroline S. Rhoads, MD - Internal Medicine
Specialist Medical Reviewer David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
Last Updated August 26, 2005

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