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Blood sugar test - blood

A blood glucose test measures the amount of a sugar called glucose in a sample of your blood.

Glucose is a major source of energy for most cells of the body, including brain cells. Carbohydrates are found in fruit, cereal, bread, pasta, and rice. They are quickly turned into glucose in your body. This raises your blood glucose level.

Hormones made in the body help control blood glucose levels.

Alternative Names

Random blood sugar; Blood sugar level; Fasting blood sugar; Glucose test

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

The test may be done in two ways:

  • After you have not eaten anything for at least 8 hours (fasting)
  • At any time of the day (random)

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test Is Performed

Your doctor may order this test if you have signs of diabetes . More than likely, the doctor will order a fasting blood sugar test.

The blood glucose test is also used to monitor patients who already have diabetes.

The test may also be done if you have:

  • An increase in how often you need to urinate
  • Blurred vision
  • Confusion or a change in the way you normally talk or behave 
  • Fainting spells
  • Seizures (for the first time)

SCREENING FOR DIABETES

This test may also be used to screen a person for diabetes.

High blood sugar and diabetes may not cause symptoms in the early stages. A fasting blood sugar test is almost always done to screen for diabetes.

If you are over age 45, you should be tested every 3 years.

If you have any of the risk factors below, ask your health care provider about getting tested at an earlier age and more often:

  • Overweight (body mass index, or BMI, of 25 or higher) and other risk factors
  • Blood pressure of 140/90 mmHg or higher, or unhealthy cholesterol levels
  • Member of a high-risk ethnic group (African American, Hispanic American, Native American, Asian American, or Pacific Islander)
  • Woman who has delivered a baby weighing 9 pounds or more, or who had gestational diabetes
  • Polycystic ovary disease
  • Close relative with diabetes (such as a parent, brother or sister)

Children age 10 and older who are overweight and have at least two of the risk factors listed above should be tested for type 2 diabetes every 3 years, even if they have no symptoms.

Normal Results

If you had a fasting blood glucose test, a level between 70 and 100 milligrams per deciliter (mg/dL) is considered normal.

If you had a random blood glucose test, a normal result depends on when you last ate. Most of the time, the blood glucose level will be below 125 mg/dL.

The examples above show the common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or may test different specimens. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

If you had a fasting blood glucose test:

  • A level of 100 to 125 mg/dL means you have impaired fasting glucose, a type of prediabetes. This increases your risk of developing type 2 diabetes.
  • A level of 126 mg/dL and higher usually means you have diabetes.

If you had a random blood glucose test:

  • A level of 200 mg/dL or higher often means you have diabetes.
  • Your health care provider will order a fasting blood glucose, HbA1c test , or glucose tolerance test , depending on your random blood glucose test result.
  • In someone who has diabetes, an abnormal result on the random blood glucose test may mean that the diabetes is not well controlled. 

Other medical problems can also cause a higher-than-normal blood glucose level, including:

A lower-than-normal blood glucose level (hypoglycemia ) may be due to:

  • Hypopituitarism (a pituitary gland disorder)
  • Underactive thyroid gland or adrenal gland
  • Tumor in the pancreas (insulinoma - very rare)
  • Too little food
  • Too much insulin or other diabetes medications
  • Liver or kidney disease
  • Weight loss after weight loss surgery
  • Vigorous exercise

Some medicines can raise or lower your blood glucose level. Before having the test, tell your health care provider about all the medicines you are taking.

Risks

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

References

American Diabetes Association. Standards of medical care in diabetes -- 2014. Diabetes Care. 2014:37 Suppl 1:S14-80.

Buse JB, Polonsky KS, Burant CF. Type 2 diabetes mellitus. In: Melmed S, Polonsky KS, Larsen PR, et al., eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 31.

Khan MI, Weinstock RS. Carbohydrates. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 16.

Updated: 8/5/2014

Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial Team.


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