Blood Glucose

Blood Glucose

Test Overview

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.

  • Fasting blood sugar (FBS). This test 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.
  • 2-hour postprandial blood sugar. This test measures blood glucose exactly 2 hours after
    you start eating a meal. This is not a test used to diagnose diabetes. This test is used to see if someone with diabetes is taking the right amount of insulin with meals.
  • Random blood sugar (RBS). It measures blood glucose regardless of when you last ate. Several random
    measurements may be taken throughout the day. Random testing is useful because
    glucose levels in healthy people do not vary widely throughout the day. Blood
    glucose levels that vary widely may mean a problem. This test is also
    called a casual blood glucose test.
  • Oral glucose tolerance test. This test is
    used to diagnose prediabetes and diabetes. An oral glucose
    tolerance test is a series of blood glucose measurements taken after you drink
    a sweet liquid that contains glucose. This test is commonly used to diagnose
    diabetes that occurs during pregnancy (gestational diabetes). Women who had high blood sugar levels during pregnancy may have oral glucose tolerance tests after pregnancy.
  • Hemoglobin A1c. This test is also called glycohemoglobin. It measures how much sugar (glucose) is stuck to red blood cells. This test can be used to diagnose diabetes. It also shows how well your diabetes has been controlled in the past 2 to 3 months and whether your diabetes medicine needs to be changed.
    The result of your A1c test can be used to estimate your average blood sugar level. This is called your estimated average glucose, or eAG.

To make a diagnosis of type 2 diabetes, your doctor will use the American Diabetes Association's criteria.

Why It Is Done

Blood glucose tests are done to:

  • Check for prediabetes and diabetes.
  • Monitor
    treatment of diabetes.
  • Check for diabetes that occurs during
    pregnancy (gestational diabetes).
  • Determine if an abnormally low
    blood sugar level (hypoglycemia) is present. A test to
    measure blood levels of insulin and a protein called C-peptide may be done
    along with a blood glucose test to determine the cause of hypoglycemia.
    To learn more, see the topic

How To Prepare

Fasting blood sugar (FBS)

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.

2-hour postprandial blood sugar

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.

Random blood sugar (RBS) and hemoglobin A1c

No special preparation is
required before having a random blood sugar or A1c test.

Oral glucose tolerance 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? ).

How It Is Done

The health professional taking a sample
of your blood will:

  • Wrap an elastic band around your upper arm to stop the flow of
    blood. This makes the veins below the band larger so it is easier to put a
    needle into the vein.
  • Clean the needle site with
  • Put the needle into the vein. More than one needle stick
    may be needed.
  • Attach a tube to the needle to fill it with
  • Remove the band from your arm when enough blood is
  • Apply a gauze pad or cotton ball over the needle site as
    the needle is removed.
  • Put pressure on the site and then put on a

How It Feels

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


There is very little risk of a problem from
having blood drawn from a vein.

  • You may develop a small bruise at the puncture site. You can
    reduce the risk of bruising by keeping pressure on the site for several minutes
    after the needle is withdrawn.
  • In rare cases, the vein may become
    inflamed after the blood sample is taken. This condition is called phlebitis
    and is usually treated with a warm compress applied several times



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.

Blood glucose tests and normal values
Type of testGuide for normal values

Fasting blood
glucosefootnote 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 2

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 3

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:

High values

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:

Low values

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:

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Eating or drinking less than 8 hours before a
    fasting blood test or less than 2 hours before a
    2-hour postprandial test.
  • Drinking alcohol on the day of the test or several days before the test.
  • Illness or
    emotional stress, smoking, and caffeine.
  • Taking a medicine. Make sure
    that your doctor knows about any medicines you take and how often you take

What To Think About

  • Glucose
    levels in urine can also be measured. Many people with diabetes have glucose in
    their urine. But the level in the blood must be very high before glucose can be
    detected in the urine. For this reason, tests for glucose in urine are not used
    to diagnose or monitor diabetes. To learn more, see Urine Test.
  • If you have diabetes, you will
    be able to measure your blood glucose levels at home. To learn more, see Home Blood Glucose Test.



  1. American Diabetes Association (2018). Standards of medical care in diabetes—2018. Diabetes Care, 41(Suppl 1): S1–S159. Accessed December 8, 2017.
  2. Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
  3. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.


ByHealthwise StaffPrimary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine Kathleen Romito, MD - Family Medicine Adam Husney, MD - Family Medicine Specialist Medical Reviewer Matthew I. Kim, MD - Endocrinology David C.W. Lau, MD, PhD, FRCPC - Endocrinology

Current as ofFebruary 26, 2018

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