The Significance of Fasting Blood Glucose Levels

What Is a Fasting Reference Interval?

A reference interval is a range of values for a critical analyte that health professionals use to guide interpretation of laboratory test results. Increases in fasting glucose levels, even within the normal range, have been linked to a higher risk of diabetes and heart disease complications.

A test result that is outside the reference interval may not indicate a problem, especially if your health care provider conducted follow-up tests later on.

What is a fasting blood glucose test?

A blood sugar test measures the amount of a type of sugar (glucose) in a sample of your blood. Glucose is a major source of energy for most cells in your body, including your brain. Hormones made by your pancreas control how much glucose is in the blood.

A fasting blood glucose test is a simple and common blood test to detect diabetes, prediabetes and gestational diabetes (diabetes during pregnancy). You should not eat or drink anything except water for 8 hours before the test. A health care professional cleans the area on the arm where the sample will be taken, pricks the skin with a needle and draws blood into a vial for testing. You may feel a slight throbbing or bruising at the site of the finger prick, but this usually goes away quickly.

If you have diabetes, a fasting blood glucose test can help you and your doctor see how well your medication is working. A random (non-fasting) blood glucose test, done 2 hours after eating a meal, can also help your doctor understand how your diabetes medications affect your blood sugar levels at different times of the day.

How to prepare for a fasting blood glucose test

A fasting blood glucose test measures your blood sugar at a time when it should be lowest. This usually happens in the morning before you eat. You should not eat or drink anything but water for eight hours before your test.

If you are scheduled for an oral glucose tolerance test, you will drink a liquid that contains glucose. You will have a blood sample drawn before you drink the solution and again one, two and three hours after you drink it.

This test can take up to four hours to complete. To help you stay occupied, bring something to read or a project to work on while you wait for the results. It’s also important to wash your hands before the procedure. Dirty hands can affect the results. Your doctor will instruct you on how to perform the test. Generally, it will be through a vein in your arm or by finger stick. The test can be done in your doctor’s office, the hospital or a lab.

What is a non-fasting blood glucose test?

Blood sugar, or glucose, is a form of energy your body uses to get the energy it needs. When you eat food or drink a beverage, your body breaks down the carbohydrates into glucose for your cells to use. Your pancreas produces insulin to help your cells absorb the glucose.

Sometimes doctors test your blood glucose without you having to fast. This is called a random glucose test. Your doctor will take a sample of your blood at any time during the day.

If your results are normal, it means you don’t have diabetes. If you have a high result, it suggests you may have prediabetes or type 2 diabetes. Your health care provider can then decide whether you need to change your diet or medications. It’s important to plan for this test by arranging to have someone drive you home afterwards, as you may feel light-headed. The glucose test can be done at any time of the day, but it’s best to do it in the morning.

What is a blood sugar target level?

People with diabetes use target blood sugar levels as a guide for managing their disease. These targets are based on individual needs and can be determined with the help of a healthcare professional. Keeping glucose levels within the target range reduces the risk of long-term complications from diabetes such as eye problems (retinopathy), kidney problems (nephropathy) and nerve problems (neuropathy).

For most healthy people, a fasting blood sugar of 99 milligrams per deciliter or lower indicates normal blood sugar levels. A reading of 100-125 milligrams per deciliter is considered prediabetes.

A more long-term measure of blood sugar is the glycated hemoglobin test, or A1C. This test shows your average blood sugar level over the past 2 to 3 months and is a good indicator of whether you have diabetes. Your A1C goal should be under 7% for optimal health. A normal A1C level is 5.7-6.4 %.

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