Blood Glucose testing is commonly done to diagnose and manage diabetes. Maintaining a low, even level of blood sugars is critical to the management of this condition and will help prevent debilitating side-effects.

How is it used?

Glucose testing is most commonly used to diagnose and manage diabetes. Since glucose levels in the blood vary with eating patterns, the most useful testing for diabetes is done when you are fasting—meaning that you have had nothing to eat or drink except water for 8 hours before the test.

Sometimes a glucose tolerance test (GTT) will be used—testing blood glucose after a fixed amount of sugar has been deliberately consumed. Glucose testing is also done in emergency settings to determine if low or high glucose is contributing to symptoms such as fainting and unconsciousness.

When is it ordered?

Glucose testing. This test can be used to screen healthy individuals for diabetes, because diabetes is a common disease that begins with few symptoms. Screening for glucose may occur at public health fairs or as part of workplace health programs. It may also be ordered as part of routine physical exams. Screening is especially important for people at high risk of developing diabetes—those with a family history of diabetes, those who are overweight, and those who are more than 40 years old.

Glucose testing is also ordered in patients who complain of symptoms that suggest diabetes, such as excessive thirst, weight loss, and frequent urination. When these symptoms appear, a person may have already had diabetes for several years.

People who have been identified as diabetics and who take insulin test their blood for glucose levels regularly. In this way they monitor their insulin therapy. Often this type of testing is done directly by the diabetic patient, who has a home test meter.

Glucose tolerance testing (GTT). Glucose testing is also performed during pregnancy because during this time some women develop a temporary type of diabetes, called gestational diabetes. Pregnant women take a special glucose test, called a glucose tolerance test, between their 24th and 28th week of pregnancy. In this test, women drink a fixed amount (usually 50 grams) of glucose in solution, and their blood glucose is measured 1 hour later. If the blood glucose is high, they are considered at risk of developing gestational diabetes and they will undergo further testing.

People who have borderline-high fasting blood glucose also take an oral glucose tolerance test (OGTT). In this case blood glucose is measured 2 hours after consuming a drink containing 75 grams of glucose.

What does the test result mean?

High levels of glucose usually indicate diabetes. The following information summarizes the meaning of the test results. These are based on the clinical practice recommendations of the American Diabetes Association.

Fasting Blood Glucose
From 70 to 109 mg/dL normal glucose tolerance
From 110 to 125 mg/dL impaired glucose tolerance
126 mg/dL and above probable diabetes


Oral Glucose Tolerance Test (OGTT) Results


(2 hours after a 75-gram glucose drink)

Less than 140 mg/dL normal glucose tolerance
From 140 to200 mg/dL impaired glucose tolerance
Over 200 mg/dL probable diabetes


Gestational Diabetes


(screening at 1-hour after a 50-gram glucose drink)

Less than 140 mg/dL normal glucose tolerance
140 mg/dL and over abnormal, needs further testing


Low levels of blood glucose occur in a number of conditions including hypothyroidism, Addison’s disease, liver disease, insulin overdose, and starvation.

In most cases, test results are reported as numerical values rather than as "high" or "low", "positive" or "negative", or "normal". In these instances, it is necessary to know the reference range for the particular test. However, reference ranges may vary by the patient's age, sex, as well as the instrumentation or kit used to perform the test. To learn more about reference ranges, please see the article, Reference Ranges and What They Mean. To learn the reference range for your test, consult your pharmacist, doctor or lab technician.

Is there anything else I should know?

Many forms of stress, such as trauma, infections, burns, and heart attacks, can raise blood glucose levels. Caffeine consumption may also increase blood glucose. Most patients receiving intravenous fluids will have high glucose levels because most IV fluids contain dextrose, which is quickly converted to glucose.

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