Top of the page Blood Glucose Test OverviewA 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 DoneBlood 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
C-Peptide.

How To PrepareFasting 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 DoneThe 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
alcohol.
Put the needle into the vein. More than one needle stick
may be needed.
Attach a tube to the needle to fill it with
blood.
Remove the band from your arm when enough blood is
collected.
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
bandage.
How It FeelsThe 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.RisksThere 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
daily.
ResultsNormal 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 test
Guide 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:

Oral Glucose Tolerance Test.

Glycohemoglobin (HbA1c, A1c).
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: Severe stress.

Heart attack.

Stroke.

Cushing’s syndrome.
Medicines such as
corticosteroids.
Excess production of growth
hormone (acromegaly).
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:
Addison’s disease.
Decreased thyroid
hormone levels (hypothyroidism).
A tumor in the
pituitary gland.
Liver disease, such as
cirrhosis.
Kidney
failure.
Malnutrition or an eating disorder, such as
anorexia.
Medicines used to treat
diabetes.
What Affects the TestReasons 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
them.
What To Think AboutGlucose
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.

ReferencesCitations American Diabetes Association (2018). Standards of medical care in diabetes—2018. Diabetes Care, 41(Suppl 1): S1–S159. http://care.diabetesjournals.org/content/41/Supplement_1. Accessed December 8, 2017. 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.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.CreditsByHealthwise 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 Top of PageNext Section:
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February 26, 2018Author:
Healthwise Staff Medical Review:
E. Gregory Thompson, MD – Internal Medicine & Kathleen Romito, MD – Family Medicine & Adam Husney, MD – Family Medicine & Matthew I. Kim, MD – Endocrinology & David C.W. Lau, MD, PhD, FRCPC – EndocrinologyAmerican Diabetes Association (2018). Standards of medical care in diabetes-2018. Diabetes Care, 41(Suppl 1): S1-S159. http://care.diabetesjournals.org/content/41/Supplement_1. Accessed December 8, 2017. 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.