What is a Fasting Blood Sugar Test?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 21 December 2019
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There are many reasons to test blood sugar levels, which are also called glucose levels. One of these is to determine if glucose levels are too high, which could indicate conditions like diabetes. A number of tests exist to measure blood sugar, but when screening for diabetic conditions, one of the preferred tests may be the fasting blood sugar test. This test can be more revealing because the body has had time to process any food consumed, which means the test won’t be as affected by remaining sugars the body has produced from recent food consumption.

The standard recommendations when getting a fasting blood sugar test are that people abstain from food for at least eight hours. Eight to ten hours is the desirable window. When folks already have diabetes and takes medicines like insulin, they are usually directed to not use insulin until after the test has been performed.

People should check with their doctors to be certain the latter recommendation applies to them. The easiest time for most people to fast is at night during sleep. It helps to find laboratories open early in the morning, especially for early risers.

A total fast means abstention from all foods, but there might be some flexibility on consumption of certain liquids. Plain water, and black coffee or tea without milk/sugar/creamers might be okay. Verifying this with the lab is advised.


The lab fasting blood sugar test is withdrawal of blood via needle aspiration. This blood will then be evaluated for blood sugar or glucose levels. Readings may fall into categories called normal, prediabetic and diabetic. A test result can show hypoglycemia or low blood sugar, too.

A normal reading is between 70-99 milligrams per deciliter (mg/dL). People are considered prediabetic and at elevated risk for developing diabetes if they have a reading that exceeds this amount by a few mg/dL. Diabetes is strongly suspected with a reading of 126 mg/dL or higher, though if this were the first test with such a result, doctors would likely perform other blood testing to confirm results.

There are different scales for determining hypoglycemia. Some suggest readings of 55 mg/dL indicate hypoglycemia. Others believe the reading has to be lower, such as 40-50 mg/dL, to meet diagnostic standards for low blood sugar conditions.

A fasting blood sugar test may be used in labs to look for evidence of glucose high/lows. It can alternately be used in a different way, at home, to evaluate treatment efficacy for people already diagnosed with diabetes. Special blood sugar monitors that collect a tiny amount of blood and test it for glucose can be used first thing in the morning so that they duplicate the fasting blood sugar test performed labs. These test machines aren’t appropriate when diabetes is only suspected, but they may play a vital role in helping to control maintenance of diabetes and determine appropriate insulin dosage for each day.



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