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Glucose Tests


Diabetes mellitus is a disorder characterized by abnormally high blood glucose levels (hyperglycemia). Glucose, a type of sugar, is the body's primary source of energy. In type 1 diabetes, blood glucose is elevated because the pancreas produces little or no insulin, the hormone that permits cells to remove glucose from the blood. In type 2 diabetes, high blood glucose develops because the body's cells are resistant to insulin.

The following glucose tests are performed to diagnose or monitor diabetes:

A single blood sample taken anytime after eating that day may be sufficient for a diagnosis. A blood glucose level above 200 milligrams per deciliter (mg/dL) associated with the classic symptoms of hyperglycemia (thirst, frequent urination, and weight loss) indicates that diabetes is present.

The fasting blood glucose test measures blood glucose levels after a 12- to 14-hour fast. While levels normally decrease during fasting, they remain persistently high in people with diabetes. A fasting glucose value above 125 mg/dL on at least two tests indicates diabetes.

The postprandial blood glucose test measures blood glucose levels two hours after eating a meal. This test is usually done in people who have symptoms of hyperglycemia, or when the results of a fasting glucose test suggest possible diabetes but are inconclusive. Values of 200 mg/dL or more indicate diabetes.

The oral glucose tolerance test is not necessary in most cases but is the method of choice to detect diabetes when results from the fasting and postprandial tests are borderline or inconclusive. In this test, glucose levels in the blood and urine are measured periodically for several hours following the ingestion of a beverage containing a specified dose (usually 75 grams) of glucose.

Hemoglobin A1c (HbA1c), also known as the glycosylated hemoglobin or glycohemoglobin test, is used to monitor the effectiveness of therapy in people already diagnosed with diabetes. This test measures the amount of glucose attached to hemoglobin (the oxygen-carrying protein in red blood cells), which increases as blood glucose levels rise. Since hemoglobin circulates in the blood until the red blood cells die (half the red blood cells are replaced every 12 to 16 weeks), the HbA1c test is a useful tool for measuring average blood glucose values over the previous two to three months.

Purpose of the Test

The fasting, postprandial, and oral glucose tolerance tests are used to diagnose type 1 or type 2 diabetes mellitus.

HbA1c is used to monitor the effectiveness of dietary or drug therapy in the management of diabetes mellitus.

Who Performs It

A lab technician or nurse.

Special Concerns

A wide variety of factors—including medications, some herbs and supplements, diet, recent illness, pregnancy, infection, stress, smoking, caffeine, and strenuous exercise—may affect blood glucose levels and interfere with the accuracy of the results.

Before the Test

Report to your doctor any medications, herbs, or supplements you are taking. You may be advised to discontinue some of these agents before all of these tests, except HbA1c.

Fasting glucose test:
Fast for 12 to 14 hours before the test (but no longer than 16 hours). Water is permitted.

Postprandial glucose test:
A fasting blood glucose test is performed to establish the pre-test blood glucose level.
You will be instructed to eat a balanced meal containing at least 75 grams of carbohydrates, and then fast for two hours.
Do not smoke or perform any strenuous activities after the meal.

Oral glucose tolerance test:
Your doctor will advise you to maintain a high-carbohydrate diet for three days before the test.
You should fast for 12 to 14 hours before the test. Water is permitted.
Do not smoke, exercise strenuously, or drink coffee or alcohol for eight hours before the test.
Your doctor may weigh you to determine the appropriate amount of glucose to include in the test beverage.
You may want to bring something to read during the test.

HbA1c test:
No advance preparation is necessary.

What You Experience

Fasting, postprandial, and HbA1c tests:
A sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.

Oral glucose tolerance test:
Blood will be drawn using venipuncture and a urine specimen will be obtained to determine fasting glucose levels.
You will be given a glucose-laden beverage. It will be very sweet. (It may be diluted with a small amount of lemon juice and water.)
You won't be allowed to eat anything until the test is complete, but you may drink water.
Additional blood samples are taken at 30 minutes, 60 minutes, and hourly intervals after you drink the glucose beverage.
Urine specimens are taken at hourly intervals.
The total test time is usually three hours but may last up to six hours.

Risks and Complications

The fasting, postprandial, and HbA1c tests are associated with no risks.

You may experience symptoms of hypoglycemia (weakness, restlessness, hunger, sweating, nervousness) during the oral glucose tolerance test. Tell your doctor immediately if this happens. If these symptoms persist, you will be given orange juice, and the test will be discontinued.

After the Test

After blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.

You may be given a snack or some orange juice after the oral glucose tolerance test.

You may resume your normal diet, activities, and any medications withheld before the test.

Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses.


Chemical tests are performed on the blood and urine samples to measure the level of glucose. These results and the presence of risk factors for diabetes will help your doctor in making a diagnosis.
o If you test positive for diabetes, the condition can be treated with dietary measures, exercise, and, if necessary, oral glucose-lowering medications or insulin injections.


From The Johns Hopkins Consumer Guide to Medical Tests. You can order this book now on our secure server.



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The Diabetes White Paper from The Johns Hopkins White Papers series is an annual, in-depth report written by Hopkins physicians.

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Johns Hopkins Consumer Guide to Medical Tests
Look up the latest information on a wide variety of preventive screening and diagnostic tests in The Johns Hopkins Consumer Guide to Medical Tests.


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