The Fasting Plasma Glucose Test

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

The fasting glucose test is recommended as a screening test for people over age 45, tested every three years. It is also done if you have had symptoms of diabetes or multiple risk factors for diabetes.

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 you don’t have diabetes, your body reacts by producing insulin, which prevents hyperglycemia (high blood sugar). However, if your 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, you must avoid eating or drinking for at least eight hours. This is known as fasting. Because of this fast, the test is usually done in the morning.

Understanding the Results of the Fasting Plasma Glucose Test

Doctors interpret FPG test results by looking at glucose levels in the blood. Diagnosis categories include the following, measured in milligrams per deciliter (mg/dL):

  • If the fasting plasma glucose test is 70 mg/dL to 99 mg/dL, this 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 positive or borderline, the test will need to be repeated on a second day or other tests might be done, such as the hemoglobin A1c test, oral glucose tolerance test or the postprandial plasma glucose test.

What Can Affect the Fasting Glucose 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 healthcare professional should consider a person’s full medical history when conducting this test and interpreting the results.

After the Results

Whatever the results, you should consult your healthcare 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.

A Word from Verywell

The fasting blood glucose test is a non-invasive blood test can be used to determine diabetes risk, to diagnose diabetes, and to evaluate blood sugar and insulin function in those people who have diabetes. Abnormal results can help guide you to make lifestyle changes and determine medication adjustments if needed. 

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