Blood Glucose

Test Overview

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.

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 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.
  • 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). For more information, see the medical test Oral Glucose Tolerance Test. This test is not commonly used to diagnose diabetes in a person who is not pregnant.

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 insulin and a protein called C-peptide may be done along with a blood glucose test to determine the cause of hypoglycemia. For more information, see the medical test C-Peptide.

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 medicine.

2-hour postprandial blood sugar

For a 2-hour postprandial test, start eating 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.

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(What is a PDF document?) .

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.
  • Apply pressure to the site and then 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.

Risks

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.
  • Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.

Results

Normal

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 than glucose levels checked with a finger stick.

Blood glucose

Fasting blood glucose:

70–99 milligrams per deciliter (3.9–5.5 mmol/L)

2 hours after eating (postprandial):

70–145 mg/dL (3.9–8.1 mmol/L)

Random (casual):

70–125 mg/dL (3.9–6.9 mmol/L)

 

Normal results may vary from lab to lab. 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.

High values

You may have diabetes. But your doctor will not use just one test result to diagnose you with the condition.

  • 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. For more information, see the medical test Oral Glucose Tolerance Test.
    • 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 measures in the range of 100 mg/dL (5.6 mmol/L) to 125 mg/dL (6.9 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, medicines such as corticosteroids, or excess production of growth hormone (acromegaly).

Low values

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.
  • Illness or emotional stress, smoking, and caffeine.

Taking a medicine, such as birth control pills, medicines used to treat high blood pressure, phenytoin (Dilantin), furosemide (Lasix), triamterene (Dyrenium, Dyazide), hydrochlorothiazide (Esidrix, Oretic), niacin, propranolol (Inderal), or corticosteroids (prednisone), can cause changes in your test results. Make sure that your doctor knows about any medicines you take and how often you take them.

What To Think About

  • Other tests are needed to accurately diagnose diabetes. A blood glucose test may not identify some people with prediabetes or early diabetes. Many experts recommend using a glucose tolerance test if your fasting blood glucose level measures in the range of 100 mg/dL (5.6 mmol/L) to 125 mg/dL (6.9 mmol/L). This range is above the normal range but below the range that indicates diabetes. For more information, see the medical test Oral Glucose Tolerance Test.
  • Glucose levels in urine can also be measured. Many people with diabetes have glucose in their urine. But the level in the blood must be very high before glucose can be detected in the urine. For this reason, tests for glucose in urine are not used to diagnose or monitor diabetes. For more information, see the medical test Urine Test.
  • If you have diabetes, you will be able to measure your blood glucose levels at home. For more information, see the medical test Home Blood Glucose Test.
  • A glycohemoglobin test can help monitor the long-term control of blood glucose levels in people with diabetes. This test is the preferred method of monitoring long-term control of blood sugar levels. For more information, see the medical test Glycohemoglobin (GHb).
  • 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 most commonly done to screen pregnant women for gestational diabetes. For more information, see the medical test Oral Glucose Tolerance Test.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits

Author Christine Wendt, R.D., L.D.
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Maria Essig
Primary Medical Reviewer Caroline S. Rhoads, MD - Internal Medicine
Specialist Medical Reviewer David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
Last Updated July 14, 2009

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