Vitamin B12 Level Test

Written by Mara Tyler
Medically Reviewed by George Krucik, MD

What Is a Vitamin B12 Test?

A vitamin B12 level test measures the amount of B12 in your blood. B12 is an important vitamin for many basic bodily functions, such as brain health, blood cell production, and proper nerve functioning. Low B12 levels can lead to serious nerve damage and deteriorating brain functions.

New mothers who breastfeed should also watch their vitamin B12 levels. Deficiency of this vitamin in the mother puts children at a greater risk for neurological damage and developmental problems (Harvard).

The test is fairly simple—it just requires getting your blood drawn. It only takes a few minutes, but can provide extremely valuable information about the presence or absence of this critical vitamin in your body.

Why a Vitamin B12 Test Is Done

A doctor might recommend a B12 test if you have symptoms such as:

  • tingling in the hands and feet
  • problems with balance
  • a racing heart
  • confusion
  • dementia
  • weakness
  • loss of appetite

Also, if your doctor suspects you might have pernicious anemia, you might be tested for B12 deficiency. Pernicious anemia is a reduction in red blood cells that occurs when your intestines are not able to absorb vitamin B12. Symptoms of this condition include diarrhea or constipation, exhaustion, loss of appetite, pale skin, and inflamed red tongue or gums that bleed. Pernicious anemia symptoms are not often seen until age 30; the average age of diagnosis is 60 (Gersten, 2012).

Who Might Need a Vitamin B12 Test

Your risk for a B12 deficiency is much higher if you eat a vegetarian or vegan diet. This is because most food sources of B12 come from animal products, such as:

  • dairy
  • eggs
  • fish
  • meat

Vegans, vegetarians, and those who have specific medical conditions that affect the absorption of B12 in the body might need a B12 test. Talk to your doctor if you have:

  • pernicious anemia—a condition in which your intestines are not able to absorb vitamin B12.
  • celiac disease—a disease in which gluten (found in wheat, barley, rye, and sometimes oat products) causes the body’s immune system to react by damaging the lining of the intestine. The intestinal lining contains villi, which help the body to absorb nutrients. By destroying these villi, celiac disease causes the individual to become malnourished.
  • Crohn’s disease—an autoimmune disease that causes persistent inflammation of the gastrointestinal (GI) tract. This disease primarily affects the intestines, but can also affect the rectum and the mouth. Common symptoms include abdominal cramping, tiredness, fever, and diarrhea.
  • atrophic gastritis—a condition characterized by the narrowing of the stomach lining, which reduces the body’s ability to produce adequate stomach acid. Stomach acid helps break down vitamin B12 so that it can be absorbed by the stomach.

You may also need a B12 test if you are on medications to treat diabetes or acid reflux, as some of these can interfere with healthy B12 levels.

Preparing for a Vitamin B12 Test

You probably won’t be able to eat or drink for about eight to 10 hours before your test. Make sure you’re well hydrated on the days leading up to the test. It will also be important to tell your doctor about any medications or over-the-counter pills you are taking, as certain drugs may interfere with the test results.

How a Vitamin B12 Test Is Performed

Your doctor will clean a small area of your arm or elbow with an antiseptic wipe or alcohol pad. In some cases, he or she will wrap an elastic band around the top of your arm to increase the flow of blood. This makes collecting blood from your veins much easier.

Your arm will then be pricked with a needle that the doctor then inserts into your vein. A tube that will collect blood is attached to the other end of the needle.

Once enough blood is drawn, the doctor will remove the needle. He or she will then apply a cotton swab and bandage to stop any bleeding where the needle punctured your skin.

Pain and Side Effects of a Vitamin B12 Test

The test itself is relatively painless, but you might experience a pricking pain when the needle is inserted into your arm. It’s also possible to have a little soreness or bruising on your arm for a few days after the test.

If you have a bleeding disorder, such as hemophilia, or if you have had problems with blood clotting in the past, talk to your doctor before the test. You might experience excessive bleeding where the needle punctures your skin, notifying your doctor beforehand about these conditions is essential.

Though rare, it’s possible that the vein from which blood was taken will become inflammed. This condition is called phlebitis. While it’s usually not serious, be sure to alert your doctor if you have throbbing, pain, or swelling in your arm after the test.

Results of a Vitamin B12 Test

Both high and low levels of vitamin B12 may be a problem.

High levels of B12 may be present in people who have liver disease, certain types of leukemia, and diabetes. If you are obese or eat a lot of meat, your B12 levels may also be very high.

Low levels of B12 can suggest:

  • vitamin B12 deficiency anemia
  • an internal parasite
  • hyperthyroidism (overactive thyroid)
  • folic acid deficiency anemia

It’s also possible that you’re just not getting enough vitamin B12 in your diet.

Vitamin B12 Deficiency Prevention

Many cases of B12 deficiency are rooted in underlying health problems that need to be treated. But it is possible to take preventative measures to avoid deficiencies. Preventative measures include paying close attention to your diet, especially if you are vegetarian or vegan. Try to eat plenty of food-based sources of B12, such as meat, fish, poultry, eggs, and milk. Vegetarians and vegans might try fortified cereals and soymilk. If you think you’re at risk, you can also take B12 supplements, which are available at most health food stores.

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