Blood Tests for Diabetes: Fasting Plasma Glucose Test

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By : dLife Editors

Also known as: FPG; fasting blood glucose test; fasting blood sugar test

What is it? The fasting blood sugar test is a carbohydrate metabolism test that measures plasma (or blood) glucose levels after a fast. Fasting (no food for at least 8 hours) stimulates the release of the hormone glucagon, which in turn raises plasma glucose levels. In people without diabetes, the body will produce and process adequate amounts of insulin to counteract the rise in glucose levels. In people with diabetes, this does not happen, and the tested glucose levels will remain high.

Why is the fasting blood sugar test performed? Generally, as a screening test for diabetes. The American Diabetes Association (ADA) recommends that everyone over age 45 take this test every three years. People with symptoms of diabetes or multiple risk factors should also have the test. The fasting blood sugar test is also used to evaluate the efficacy of medication or dietary therapy in those already diagnosed with diabetes.

How is the fasting blood sugar test performed? The fasting blood sugar test consists of a simple blood draw, which is sent to your doctor’s lab for analysis. The ADA recommends that the test be administered in the morning, because afternoon tests tend to give lower readings. It is also usually more convenient to take the test in the morning, because you must fast for at least 8 hours beforehand.

How frequently should the fasting blood sugar test be performed? Up to two times for diagnostic purposes, or as required while monitoring a treatment regime.

What is the “normal” range for results? These can vary from lab to lab, and according to the lab procedures used. When using the glucose oxidase and hexokinase methods, normal values are typically 70 to 99 mg/dl (3.9 to 5.5mmol/l). Medications, exercise, and recent illnesses can impact the results of the fasting blood sugar test, so an appropriate medical history should be taken before it is performed.

What do abnormal results mean? A reading of 100 mg/dl to 125 mg/dl (5.6 mmol/l to 6.9 mmol/l) indicates prediabetes (impaired fasting glucose). Levels of 126 mg/dl (7 mmol/l) or higher indicate a need for a subsequent retest on a different day.  Two abnormal results from two separate tests confirm a diagnosis of diabetes. Results that measure only slightly above the normal range may require further testing, including the oral glucose tolerance test or the postprandial plasma glucose test, to confirm a diabetes diagnosis.

Other conditions which may result in an elevated result include pancreatitis, Cushing’s syndrome, liver or kidney disease, eclampsia, and other acute illnesses, such as sepsis and myocardial infarction (heart attack).

A lab result that measures below the normal range can indicate problems such as reactive hypoglycemia, renal or hepatic insufficiency, hypopituitarism, or malabsorption syndrome, and will usually require further diagnostic testing.


American Association of Clinical Endocrinologists. “Diagnosis of Type 2 Diabetes Mellitus.” AACE Diabetes Resource Center. Accessed August 30, 2017.

American Diabetes Association. “Diagnosing Diabetes and Learning About Prediabetes.” September 22, 2014. Accessed August 30, 2017.

Joslin Diabetes Center. “Diagnosing Impaired Glucose Tolerance.” August 2001. Accessed 2017.

MedlinePlus. National Library of Medicine. “Blood Sugar Test—Blood.” Accessed August 30, 2017.

Mayo Clinic. “Tests and Diagnosis.” Accessed August 30, 2017.

National Institute of Diabetes and Digestive and Kidney Diseases.  “Diabetes Tests & Diagnosis.” November 2016. Accessed August 30, 2017.