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Understanding the Fasting Plasma Glucose Test

A Call to Action


Updated June 02, 2014

The fasting plasma glucose (FPG) test, also known as the fasting blood sugar test, measures blood sugar levels and is used to diagnose diabetes. Relatively simple and inexpensive, the test exposes problems with insulin functioning.

Prolonged fasting triggers a hormone called glucagon, which is produced by the pancreas. It causes the liver to release glucose (blood sugar) into the bloodstream. If a person doesn’t have diabetes, his or her body reacts by producing insulin, which prevents hyperglycemia (high blood sugar). However, if one’s body cannot generate enough insulin or cannot appropriately respond to insulin, fasting blood sugar levels will stay high.

How the Fasting Plasma Glucose Test Is Done

The test consists of a simple, noninvasive blood test. Prior to being tested, a person must not to eat for 12 to 14 hours. Because of this fast, the test is usually done in the morning.

Understanding the Results of the Fasting Plasma Glucose Test

Doctors interpret test results by looking at glucose levels in the blood. Diagnosis categories include the following, measured in milligrams per deciliter (mg/dL):
  • In the fasting plasma glucose test, 70 mg/dL to 99 mg/dL is considered within the normal range.
  • A reading of 100 mg/dL to126 mg/dL suggests prediabetes, indicating an increased risk in developing full-blown diabetes.
  • A reading above 126 mg/dL is the threshold at which diabetes is diagnosed.
  • Blood glucose levels lower than 70 mg/dL imply an episode of hypoglycemia, in which blood sugar is dangerously low.
If the results are borderline, other tests might be done, including the oral glucose tolerance test or the postprandial plasma glucose test. Even if results are within the normal range, additional testing may be ordered to determine if a person has additional risk factors for diabetes, such as high body mass index, or if he or she exhibits other symptoms of diabetes.

What Can Affect Test Results?

Results may vary from lab to lab, or -- in the same lab -- from day to day. As a result, two abnormal results from tests taken on two different days are required to confirm a diagnosis.

Results may be lower if blood is drawn in the afternoon rather than in the morning. And a glucose level sometimes can be “falsely low” if too much time passes between when the blood is drawn and the lab processes the sample. The results can also be affected by previous or current medical conditions or by personal habits, such as smoking and exercise.

Of course, abnormal test results may also indicate diabetes. A health care professional should consider a person’s full medical history when conducting this test and interpreting the results.

After the Results

Whatever the results, a person should consult his or her health care team -- a doctor, nutritionist, etc. Keep in mind that this blood test is used not only to diagnose diabetes, but also to prevent it. Higher values are likely to reflect diet and lifestyle issues as well as insulin functioning.

Whether a person has type 1, type 2 or gestational diabetes, a healthy lifestyle helps insulin to work better. In this sense, the fasting plasma glucose test is a signal for action, not a cause for despair.


Gagnon, Claudia, and Jean-Patrice Baillargeon. "Suitability of Recommended Limits for Fasting Glucose Tests in Women with Polycystic Ovary Syndrome." Canadian Medical Association Journal 176.7(2007): 933-938. <http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1828174>

Nathan, ed., David M. Diabetes: A Handbook for Living. Boston: Harvard Health Publications, 2004.

"Diagnosis of Diabetes." National Diabetes Information Clearinghouse. Jan. 2005. National Institute of Diabetes and Digestive and Kidney Diseases. 10 Sep. 2007. <http://diabetes.niddk.nih.gov/dm/pubs/diagnosis/diagnosis.pdf>

Youngerman-Cole, Sydney. "Blood Glucose." Mount Auburn Hospital. 26 Aug. 2005. Healthwise. 10 Sep. 2007. <http://www.mountauburn.caregroup.org/body.cfm?xyzpdqabc=0&id=6&action=detail&AEProductID=hw_knowledgebase&AEArticleID=hw8252&AEArticleType=MedicalTest>

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