Test Overview
A vitamin
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
nervous system.
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
as a
folic acid test, because a lack of either one 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
are several
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.
- Help find the cause of
dementia or other nervous system symptoms, such as
tingling or numbness of the arms or legs (peripheral neuropathy).
- See if vitamin B12 deficiency anemia is present
after a person has been diagnosed with
atrophic gastritis.
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
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.
- 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
pinch.
Risks
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
minutes.
- 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.
Results
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.
High values
- High levels of vitamin B12 can occur in liver
disease (such as
cirrhosis or
hepatitis) and some types of
leukemia. But the vitamin B12 test is not usually used
to diagnose these problems.
- In rare cases, high levels may be found
in people with
diabetes or who are
obese.
Low values
- Low levels of vitamin B12 may mean you have vitamin B12 deficiency anemia, which might be caused by problems
with the absorption of the vitamin (such as pernicious anemia).
- Low levels
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
ileum).
- Low levels may mean an infection with a
parasite called fish tapeworm is
present.
- In rare cases, low levels may mean a person is not getting
enough vitamin B12 in his or her food.
- Low levels are linked with
hyperthyroidism or
folic acid deficiency anemia.
- High levels of protein in
the blood, such as from
multiple myeloma, can falsely decrease blood vitamin
B12 levels.
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- Taking certain medicines that might affect your test results. Tell your doctor all of the medicines that you take.
- Being
pregnant or breast-feeding.
- Taking large doses of vitamin
C.
- Drinking large amounts of alcohol.
- Having a test,
such as a
computed tomography (CT) scan, that used dyes in the
past 7 days.
- Having pernicious anemia, which means you lack the
substance (intrinsic factor) needed to absorb vitamin B12.
What To Think About
- Folic acid levels can be high in people
who lack vitamin B12. A folic acid test is often done at the same time as a
vitamin B12 test. For more information, see the topic
Folic Acid.
- Methylmalonic
acid is a substance in the blood that increases when vitamin B12 levels
decrease. A methylmalonic acid (MMA) blood test may be done to help evaluate
vitamin B12 test results.
- A change in homocysteine levels may also
affect a change in vitamin B12 levels because of
metabolism changes. For more information, see the
topic
Homocysteine.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th 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.
Credits
| By | Healthwise Staff |
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| Primary Medical Reviewer | E. Gregory Thompson, MD - Internal Medicine |
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| Specialist Medical Reviewer | Brian Leber, MDCM, FRCPC - Hematology |
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| Last Revised | December 14, 2010 |
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