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A blood glucose test measures the amount of a type of sugar, called glucose, in your blood. Glucose comes from carbohydrate foods. It is the main source of energy used by the body. Insulin is a hormone that helps your body's cells use the glucose. Insulin is produced in the pancreas and released into the blood when the amount of glucose in the blood rises.
Normally, your blood glucose levels increase slightly after you eat. This increase causes your pancreas to release insulin so that your blood glucose levels do not get too high. Blood glucose levels that remain high over time can damage your eyes, kidneys, nerves, and blood vessels.
There are several different types of blood glucose tests.
To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.
Blood glucose tests are done to:
This is one of several tests that is used to diagnose diabetes. For a fasting blood sugar test, do not eat or drink anything other than water for at least 8 hours before the blood sample is taken.
If you have diabetes, you may be asked to wait until you have had your blood tested before taking your morning dose of insulin or diabetes medicine. You may have a random blood sugar test instead, which will not require an 8-hour fast.
For a 2-hour postprandial test, start eating a meal exactly 2 hours before the blood sample is taken. A home blood sugar test is the most common way to check 2-hour postprandial blood sugar levels.
No special preparation is required before having a random blood sugar or A1c test.
For an oral glucose tolerance test, you'll need to follow a special diet for 3 days before the test. And do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is taken.
To learn more about how to prepare for this test, see Oral Glucose Tolerance Test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean.
To help you understand the importance of this test, fill out the medical test information form( What is a PDF document? ).
The health professional taking a sample of your blood will:
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little risk of a problem from having blood drawn from a vein.
A blood glucose test measures the amount of a type of sugar, called glucose, in your blood.
Results are often ready in 1 to 2 hours. Glucose levels in a blood sample taken from your vein (called a blood plasma value) may differ a little from glucose levels checked with a finger stick.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Type of test
Guide for normal values
Fasting blood glucose footnote 1
Less than or equal to 100 milligrams per deciliter ( mg/dL) (5.6 millimoles per liter, or mmol/L).
2 hours after eating ( postprandial) footnote 1
Less than 140 mg/dL (7.8 mmol/L) for people age 50 and younger; less than 150 mg/dL (8.3 mmol/L) for people ages 50–60; less than 160 mg/dL (8.9 mmol/L) for people age 60 and older.
Random (casual) footnote 2
Levels vary depending on when and how much you ate at your last meal. In general: 80–120 mg/dL (4.4–6.6 mmol/L) before meals or when waking up; 100–140 mg/dL (5.5–7.7 mmol/L) at bedtime.
Many conditions can change your blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
For more information on results from an oral glucose tolerance test or hemoglobin A1c test, see:
You may have diabetes. To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.
Other conditions that can cause high blood glucose levels include:
A fasting glucose level below 40 mg/dL (2.2 mmol/L) in women or below 50 mg/dL (2.8 mmol/L) in men that is accompanied by symptoms of hypoglycemia may mean you have an insulinoma, a tumor that produces abnormally high amounts of insulin.
Low glucose levels also may be caused by:
Reasons you may not be able to have the test or why the results may not be helpful include:
Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Current as ofJuly 25, 2018
Author: Healthwise StaffMedical Review: E. Gregory Thompson, MD - Internal MedicineKathleen Romito, MD - Family MedicineAdam Husney, MD - Family MedicineMatthew I. Kim, MD - Endocrinology, Diabetes and MetabolismDavid C. W. Lau, MD, PhD, FRCPC - Endocrinology
Current as of:
July 25, 2018
Medical Review:E. Gregory Thompson, MD - Internal Medicine & Kathleen Romito, MD - Family Medicine & Adam Husney, MD - Family Medicine & Matthew I. Kim, MD - Endocrinology, Diabetes and Metabolism & David C. W. Lau, MD, PhD, FRCPC - Endocrinology
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