What’s a Vitamin B12 Test?

Vitamin B12 is vital for good health. Your body needs steady levels of this nutrient to make enough red blood cells and keep your nervous system working.

For most people who eat a balanced diet, low B12 levels are rare. But there are reasons why they may dip below normal. A simple blood test can show whether your levels are healthy, low or somewhere in between.

Do I Need the Test?

Your doctor may recommend you have one for a few different reasons, such as:

  • You’ve been diagnosed with anemia.
  • He suspects you have a medical condition that affects how well your body absorbs B12.
  • You’re taking medications that may interfere with B12 absorption.
  • You have symptoms linked to low B12 levels.

The main health problem associated with poor B12 absorption is a condition doctors call “pernicious anemia.” It develops if you lack intrinsic factor, a type of protein made in the stomach. Without it, you can’t absorb enough vitamin B12 from food.

Crohn’s disease and celiac disease may also cause your B12 levels to go down.

Several types of medications might affect your vitamin B12 levels. Among the most common are drugs to help treat gastroesophageal reflux disease (GERD). Medications called proton-pump inhibitors reduce the amount of acid in the stomach, which is needed for food to release B12. Metformin, a drug for diabetes, also may interfere with B12 absorption.

If you’re not taking any of these medications or haven’t been diagnosed with pernicious anemia or other health problems that might cause lower B12 levels, your doctor may order the test based on your symptoms and health history.

Common symptoms of low vitamin B12 include:

  • Fatigue
  • Fuzzy thinking
  • Numbness or tingling in your hands and feet
  • Headaches
  • Moodiness

These could be signs of many conditions. A blood test that checks your B12 levels can rule out or confirm that your low B12 levels may be the issue.

What Does the Test Involve?

It’s a simple blood test. You can get it anytime, and you don’t need to go without food (fasting) before you do. Your doctor can add it to the order for your standard fasting blood test that checks your levels of cholesterol, glucose, and other markers of health.

You should tell your doctor about all of the medications and supplements you take before the test. Some of them may affect the results.


Understanding the Results

A normal level of vitamin B12 in your bloodstream is between 190 and 900 nanograms per liter (ng/L). If you’re at the low end of that range, or below 190 ng/L, your doctor may order additional tests.

If your test shows B12 levels of less than 150 ng/L, your doctor may want you to be tested for methylmalonic acid. If you have vitamin B12 deficiency, you’ll probably have higher levels of this substance. Another common test to help diagnose pernicious anemia is an Intrinsic Factor Blocking Antibody (IFBA) blood test.

You may also have low levels of another B vitamin called folate, which could affect your B12 levels, too.

Moving Forward

If your vitamin B12 test shows that your levels are healthy, you don’t need to do anything but continue eating a balanced diet. Sources of vitamin B12 include fish, meat, dairy, and other foods fortified with B12, like cereal and milk.

If you’re still concerned about your levels, talk with your doctor about whether B12 supplements or dietary changes make sense.

But if you have low vitamin B12 levels, you’ll likely need regular injections of B12. Doctors prefer these over oral supplements, especially when you have a condition that limits how much of the vitamin your stomach can absorb.

Vitamin B12 deficiency can lead to problems with brain function, your nervous system, and other aspects of your health. It’s important to check your levels if there are any signs that they are low.

If you’re over age 50, a vitamin B12 test makes sense. It’s also a good time to talk to your doctor about whether your diet provides enough B12, or if taking a supplement would help.

WebMD Medical Reference Reviewed by Laura J. Martin, MD on April 02, 2017



University of Rochester Medical Center: “Vitamin B12 and Folate.”

National Institutes of Health Office of Dietary Supplements: “Vitamin B12.”

National Heart, Lung, and Blood Institute: “What is pernicious anemia?”

Mayo Clinic: “Vitamin Deficiency Anemia: Symptoms and Causes,” “Vitamin B12 Assay, Serum.”

Oregon State University Micronutrient Information Center: “Vitamin B12.”

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