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Diabetes & Fasting Blood Sugar: Testing, Normal Levels & More

Reviewed by: Melissa Herrmann Dierks , RDN, LDN, CDCES
Diabetes & Fasting Blood Sugar: Testing, Normal Levels & More

There are several different types of diabetes tests. Some are performed by a healthcare professional to diagnose diabetes while others are a management tool for people who have already been diagnosed. Fasting blood sugar tests ordered by a healthcare professional may be used to diagnose diabetes, while at-home fasting blood glucose testing can help people with diabetes get a baseline for their day. Read on to learn more about the test, fasting requirements, and more.

What is Fasting Blood Sugar?

Fasting blood sugar refers to your blood glucose level after a period of not eating. According to the American Diabetes Association (ADA), fasting means not having anything to eat or drink (except water) for at least eight hours.

After you eat, your blood sugar level rises — the extent to which it rises will depend on what and how much you ate. Blood sugar levels typically peak about an hour after you’ve finished eating. By fasting and then testing your blood sugar, you can gather insights into how your body manages blood sugar. For example, a high blood sugar level after an eight-hour fast could indicate that your body is unable to lower the amount of sugar in your blood. This could be a sign of insulin resistance.

What is a Fasting Blood Sugar Test?

A fasting blood glucose (FBG) test, also known as a fasting plasma glucose (FPG) test or fasting blood sugar test, is a method for measuring the amount of glucose in the blood after a period of fasting. Most commonly, fasting occurs overnight and the test is performed first thing in the morning.

There are two primary purposes of a fasting blood glucose test:

  • To diagnose diabetes
  • To understand how different factors affect your blood sugar throughout the day

A fasting blood glucose measurement can be taken by a healthcare professional by blood draw and sent to a lab, or as a fingerstick test performed at home. The two processes are testing the same thing, so they are often referred to in the same way. A healthcare professional may order a lab test to eliminate other variables that may come into play with a fingerstick test, such as substances on the hand or not using a large enough blood drop.

If your healthcare team suspects that you may have prediabetes or type 2 diabetes, they may perform a fasting plasma glucose test or an A1C test. If you have already been diagnosed with diabetes, checking your fasting blood sugar level in the morning gives you a baseline blood sugar reading for your day. These tests can be performed at home as recommended by your healthcare team, using a blood glucose meter. However, the at-home blood glucose meter testing method is not intended for the diagnosis of diabetes.

Related Reading: Blood Sugar Levels: What is Normal, Low or High, Target Ranges & More [w/ Downloadable Chart]

What a fasting blood sugar test shows

A fasting blood sugar test performed by a healthcare professional will return results that will fall into one of three different ranges: Normal range, prediabetes, or diabetes. The Centers for Disease Control and Prevention (CDC) and the ADA categorize these ranges with the following results:

Result Fasting Blood Sugar
Normal Range Under 100 mg/dL
Prediabetes 100 – 125 mg/dL
Diabetes 126 mg/dL or higher

A fasting blood sugar test that is performed by a healthcare professional and sent to a lab can help in the diagnosis of diabetes. An at-home fasting test should not be used to diagnose diabetes because there are many factors of a finger stick test that can affect accuracy, such as environmental conditions, expired or damaged test strips, residue on unwashed fingers, insufficient blood drop, and testing site location. You can read more about the factors that affect accuracy in this article: The Truth Behind the Numbers: What Affects Blood Glucose Meter Accuracy?

At-home fasting blood sugar tests have a different purpose in diabetes management. They provide a baseline blood sugar reading that, when compared to other readings throughout the day, allows people with diabetes to learn what factors affect their blood sugar and by how much. For instance, if you started the day within a normal range and then saw higher levels later on, there is likely a certain trigger that caused your levels to increase — maybe it is due to taking a certain medication, eating a big meal, or other factors.

Your healthcare professional will help you determine your personalized target fasting blood glucose range to determine if your reading is high, low, or in-range. It is important to record all of your test results in a blood sugar logbook or app so that your healthcare team can observe trends. Without checking your fasting blood sugar level in the morning, you do not have the baseline necessary to understand if and why your levels may be out of range later in the day.

What a fasting blood sugar test doesn’t show

A fasting blood sugar test taken at home with a blood glucose meter should not be used to diagnose diabetes. If a healthcare professional is trying to determine whether a person has type 1 or type 2 diabetes, they will perform other panels of tests that will be sent to a lab to get a clearer picture of your overall health.

Fasting blood sugar levels alone cannot tell you how your body handles food throughout the day. To get the best understanding of how food affects blood sugar, people with diabetes should also test throughout the day, before meals, and two hours after meals.

Factors That Can Affect Blood Sugar Levels

There are many factors that can affect your blood glucose levels. Since a fasting blood glucose test provides a baseline, it’s important to understand how certain factors can raise or lower your levels throughout the day.

The National Center for Biotechnology Information (NCBI) notes that the following factors can raise blood sugar levels:

  • Dehydration
  • Being inactive
  • Not taking enough insulin or oral diabetes medications
  • Side effects from other medications, such as steroids or antipsychotics
  • Consuming too much food that contains more carbohydrate than usual
  • Being sick, which causes your body to release hormones to fight illness
  • Stress, which can produce hormones that raise blood glucose levels
  • Short- or long-term pain (such as pain from a sunburn) that causes your body to release hormones that raise blood glucose levels
  • Menstrual periods, which cause changes in hormone levels

Alternatively, NCBI lists the following as factors that can cause low blood sugar levels:

  • Not eating enough food, such as a meal or snack with fewer carbohydrate than usual or missing a meal or snack
  • Consuming alcohol, especially on an empty stomach
  • Taking too much insulin or oral diabetes medications
  • Side effects from other medications
  • More physical activity or exercise than usual, which makes your body more sensitive to insulin and can lower blood glucose levels

If you have diabetes, testing your fasting blood sugar level in the morning can offer helpful insights into how your body is managing your blood sugar. Your healthcare team will use the results of your fasting blood glucose tests and tests throughout the day to get a clear understanding of your overall health and create custom target ranges for your own “normal” blood sugar levels.

Blood Sugar Levels Chart
Download this free handy chart on blood sugar level guidelines and helpful information on high and low blood sugar levels.
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Reviewed by: Melissa Herrmann Dierks , RDN, LDN, CDCES

Melissa Herrmann Dierks RDN, LDN, CDCES is a Registered Dietitian Nutritionist, Certified Diabetes Care and Education Specialist, and Licensed Dietitian Nutritionist in Huntersville, NC with over twenty-five years of professional experience in the field of nutrition and diabetes education. In addition to providing nutrition solutions for adults and children, she provides nutrition communication services to the food and beverage industry and is the owner of Supermarket Savvy. Melissa has held various sales and marketing positions for leading companies in the diabetes space including insulin pump, blood glucose monitor, and diabetes supply distribution companies.

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