How Is The Fasting Blood Sugar Test Used To Diagnose Pre Diabetes?

The Fasting Blood Sugar Test (also known as the Fasting Plasma Glucose test or FPG) is a blood test that measures the amount of sugar in blood after eight hours or more without food.

This test must be performed by a medical professional.

Drawing Blood for the Fasting Blood Sugar Test

A Closer Look:  The Fasting Plasma Glucose Test

The FPG is used to diagnose pre diabetes and Type 2 diabetes, as well as to determine the effectiveness of prescribed treatments for these conditions.  The test, also referred to as a carbohydrate metabolism test, measures plasma glucose levels after a fast.  Fasting, or going without food for eight hours or more, causes glucagon (a hormone produced in the pancreas) to release sugar – in the form of glucose – into the bloodstream.

At this point, people without Type 2 diabetes will produce an adequate amount of insulin to counteract the resulting increase of sugar, which will bring the sugar levels down.  People with Type 2 diabetes are unable to do this adequately.

The test does require eight hours of fasting before the test is administered and must be given by a medical professional.  Since fasting is required, most doctors prefer administering this test first thing in the morning, before breakfast is eaten.  Another reason doctors prefer earlier tests is that morning readings tend to be higher than afternoon readings.  While an afternoon read could potentially mask a pre diabetes diagnosis, a morning FPG test is more apt to detect sugar levels that warrant intervention.  

The normal blood glucose range for an FPG test is 70 milligrams per deciliter (mg/dL) to 99 mg/dL.  

Exactly how much is a deciliter, you ask?  It’s one-tenth of a liter, which is approximately 3.3 fluid ounces. 

A reading of 100 mg/dL to 126 mg/dL indicates pre diabetes with a risk of developing Type 2 diabetes. 

A reading of 126 mg/dL or higher suggests Type 2 diabetes. Glucose levels below 70 mg/dL suggest hypoglycemia – a condition in which blood sugar is too low.

For good measure, the test is generally given twice for diagnosing pre diabetes and/or diabetes, but may be ordered throughout your treatment cycle in order to determine the effectiveness of medications and/or lifestyle changes for controlling sugar levels in the blood.

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