Top of the page
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 criteria from Diabetes Canada.
Blood glucose tests are done to:
If you do not have diabetes, do not eat or drink anything other than water for at least 8 hours before the blood sample is taken.
If you have diabetes, check with your doctor to find out how long you need to fast. You may be asked to wait until you have had your blood tested before taking your morning dose of insulin or diabetes medicine.
For a 2-hour post-prandial 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 post-prandial blood sugar levels.
No special preparation is required before having a random blood sugar 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 the topic 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.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
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.
You may have diabetes. Your doctor will make a diagnosis of type 2 diabetes based on a physical examination and the results of blood tests and any symptoms you may have.
Other conditions that can cause high blood glucose levels include:
A fasting glucose level below 2.2 mmol/L in women or below 2.8 mmol/L in men that is accompanied by symptoms of hypoglycemia may mean you have an insulinoma, a tumour 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:
Current as ofJuly 25, 2018
Author: Healthwise StaffMedical Review: E. Gregory Thompson MD - Internal MedicineBrian D. O'Brien MD - Internal MedicineKathleen Romito MD - Family MedicineAdam Husney MD - Family MedicineAlan C. Dalkin MD - EndocrinologyMatthew I. Kim MD - EndocrinologyDavid C.W. Lau MD, PhD, FRCPC - Endocrinology
Current as of: July 25, 2018
Author: Healthwise Staff
Medical Review:E. Gregory Thompson MD - Internal Medicine & Brian D. O'Brien MD - Internal Medicine & Kathleen Romito MD - Family Medicine & Adam Husney MD - Family Medicine & Alan C. Dalkin MD - Endocrinology & Matthew I. Kim MD - Endocrinology & David C.W. Lau MD, PhD, FRCPC - Endocrinology
To learn more about Healthwise, visit Healthwise.org.
© 1995-2019 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.