B12 test measures the amount of
vitamin B12 in the blood. The body needs this B
vitamin to make blood cells and to maintain a healthy
Vitamin B12 is found in
animal products such as meat, shellfish, milk, cheese, and eggs. Most people
who eat animal products are not likely to develop
vitamin B12 deficiency anemia unless their bodies
can't absorb it from food. Strict vegetarians (vegans) who do not eat animal
products and babies of mothers who are strict vegetarians are at increased risk
for developing anemia and should take a supplement containing vitamin B12.
Vitamin B12 is stored in the liver for a year or more, which reduces a person's
risk of anemia.
Vitamin B12 is usually measured at the same time
folic acid test, because a lack of either one or both can lead
to a form of anemia called
megaloblastic anemia. Lack of vitamin B12 also affects
the nervous system.
Why It Is Done
A vitamin B12 test is used to:
Check for vitamin B12 deficiency anemia. There
risk factors for this anemia, such as those who have had stomach or
intestinal surgery, small intestine problems, or people with a family history
of this anemia.
Diagnose the cause of certain types of anemia, such
as megaloblastic anemia.
See if vitamin B12 deficiency anemia is present
after a person has been diagnosed with
How To Prepare
Do not eat or drink (other than water)
for 10 to 12 hours before the test.
How It Is Done
Your health professional drawing blood
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
Put a gauze pad or cotton ball over the needle site as
the needle is removed.
Put pressure to the site and then a bandage.
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 chance of a problem from
having a blood sample taken from a vein.
You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
In rare cases, the vein may become swollen after the
blood sample is taken. This problem is called phlebitis. A warm compress can be
used several times a day to treat this.
Ongoing bleeding can be a
problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and
other blood-thinning medicines can make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your doctor before your blood sample is taken.
A vitamin B12 test measures the amount of
vitamin B12 in the blood.
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.
may also occur following removal of part or all of the stomach (gastrectomy),
gastric bypass surgery, or gastric stapling surgery, or following surgery to
remove part of the small intestine where this vitamin is absorbed (terminal
Low levels may mean an infection with a
parasite called fish tapeworm is
In rare cases, low levels may mean a person is not getting
enough vitamin B12 in his or her food.
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
How this information was developed to help you make better health decisions.