Fasting Plasma Glucose

Blood Tests for Diabetes Diagnosis and Management: Fasting Plasma Glucose Test


Also known as: Fasting plasma glucose test; FPG.

What is it? The fasting blood sugar test is a carbohydrate metabolism test which 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 amonts 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 fasting blood sugar test is also used to evaluate the effectiveness of medication or dietary therapy in those already diagnosed with diabetes.

How is the fasting blood sugar test performed? The American Diabetes Association recommends that the fasting blood sugar 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. The fasting blood sugar test consists of a simple blood draw, which is sent to your doctor's lab for analysis.

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

What is the "normal" range for results? These vary according to the lab procedures used. When using the glucose oxidase and hexokinase methods, normal values are typically 70 to 100 mg/dl (3.9 to 5.6 mmol/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? Levels of 126 mg/dl (7 mmol/l) or higher indicate a need for a subsequent retest on a different day to 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 which 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.

Fasting Blood Sugar Test Fast Facts from the NIH:

  • A person with normal blood glucose has a blood glucose level below 100 mg/dl (5.6 mmol/l).
  • A test result of 100 mg/dl (5.6 mmol/l) to 125 mg/dl (6.9 mmol/l) is now considered diagnostic of prediabetes (impaired fasting glucose).
  • If the fasting blood glucose level rises to 126 mg/dl (7 mmol/l) or above, a person has diabetes.

Reviewed by Jason C. Baker, MD. 4/11

Last Modified Date: May 17, 2013


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