Blood Glucose: About This Test

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What is it?

A blood glucose test measures the amount of a type of sugar, called glucose, in your blood. Several different types of blood glucose tests are used.

  • Fasting blood sugar (FBS) measures blood glucose after you have not eaten for at least 8 hours. It is often the first test done to check for prediabetes and diabetes. 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.
  • Random blood sugar (RBS) measures blood glucose regardless of when you last ate.
  • Oral glucose tolerance test (OGTT) may be used to diagnose prediabetes and diabetes. This test is a series of blood glucose measurements taken after you drink a sweet liquid that contains glucose. This test is mostly used for pregnant women to check for gestational diabetes.

The A1c is a different type of blood test that can be used to diagnose diabetes or to check your average blood sugar level over the past 2 to 3 months.

Why is this test done?

Blood glucose tests are done to:

  • Check for diabetes.
  • See how well treatment for diabetes is working.

How can you prepare for the test?

Be sure to tell your doctor about all the non-prescription and prescription medicines and herbs or other natural health products you take. There are many medicines and natural health products that can affect the results of these tests.

Fasting blood sugar (FBS)

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, 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.

Random blood sugar (RBS)

No special preparation is needed before having a random blood sugar test.

Oral glucose tolerance test

In the 3 days before this test, eat the way you are used to eating. In general, this means a well-balanced diet that contains at least 150 grams of carbohydrate a day. Good foods to eat are fruits, breads, cereals, grains, rice, crackers, potatoes, beans, and corn.

In the 8 hours right before the test, do not eat or drink anything (except water). Do not smoke, drink alcohol, or exercise.

What happens during the test?

A health professional takes a sample of your blood.

For a home glucose test, you may have a blood sample taken from your fingertip. This is done by:

  • Putting a special test strip into a device called a blood glucose meter.
  • Pricking your finger with a small needle (lancet) to collect a drop of blood.
  • Placing the blood on the test strip.

For the oral glucose tolerance test:

  • A blood sample will be taken when you arrive. This is your fasting blood glucose value. It will be compared with other samples during the day.
  • You will drink a sweet glucose liquid.
  • Depending on the reason for this test, blood samples may be taken 1, 2, or 3 hours after you drink the glucose. Blood samples may also be taken as soon as 30 minutes after you drink the glucose.

What happens after the test?

  • You will probably be able to go home right away.
  • You can go back to your usual activities right away.

When should you call for help?

Watch closely for changes in your health, and be sure to contact your doctor or nurse call line if you have any problems.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse call line if you are having problems. It's also a good idea to keep a list of the medicines you take. Ask your doctor when you can expect to have your test results.

Where can you learn more?

Go to http://www.healthwise.net/ed

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