About the Vitamin B12 Test

Purpose of the test

A vitamin B12 test checks whether your body has enough cobalamin to support healthy red blood cells, a functioning nervous system, and DNA production. B12 deficiency causes two distinct problems: megaloblastic anemia (a condition where bone marrow makes abnormally large, poorly functioning red blood cells) and nerve damage. Nerve damage can become permanent if the deficiency goes undetected for too long, according to MedlinePlus. B12 cannot be made by the body. Meat, fish, poultry, eggs, and dairy are the main sources of B12.

Screening: The American Diabetes Association recommends periodic B12 monitoring for adults on long-term metformin, since the drug reduces B12 absorption in the small intestine over time.
Diagnosis: Ordered when symptoms like numbness, balance problems, unexplained fatigue, cognitive changes, or large red blood cells on a CBC suggest B12 deficiency.
Monitoring: For people already diagnosed with B12 deficiency, the test tracks whether treatment (oral supplements or injections) is restoring levels.

B12 and folate tests are sometimes ordered together. High folate intake can mask a B12 deficiency by preventing anemia, even when nerve damage is quietly progressing. The test doesn’t screen for iron deficiency, thyroid disease, or other causes of fatigue and anemia. Those need separate testing.

What does the vitamin B12 test measure?

The test covers one marker from a single blood sample processed at a CLIA-certified lab.

Nutritional Markers

Vitamin B12 (Cobalamin): There are four types of cobalamins:
Methylcobalamin – naturally occurring, found in food products; supports DNA repair and other functions.
Adenosylcobalamin – naturally occurring, found in food products; used to break down certain amino and fatty acids for energy.
Cyanocobalamin – synthetic form found in supplements and fortified food; body converts to methylcobalamin and adenosylcobalamin.
Hydroxycobalamin – produced by bacteria by breaking down food products; converts to methylcobalamin and adenosylcobalamin and may be used in treating B12 deficiencies.
Vitamin B12 test measures total cobalamin circulating in your blood using a chemiluminescent immunoassay, the standard method at most CLIA-certified labs. Most labs report a normal range of roughly 200 to 900 pg/mL, though reference ranges vary by lab. Read your result alongside the range printed on your report. A result below 200 pg/mL generally points to deficiency. A borderline result between 200 and 300 pg/mL often prompts a retest or follow-up with functional markers. Some clinicians consider a B12 level around 550 pg/mL or higher desirable, and may target 1,000 pg/mL or more in older adults, though reference ranges and targets vary by lab and source.
Serum B12 level can be normal even when cells aren’t getting enough B12 to work properly, which is why your provider may add methylmalonic acid (MMA) or homocysteine testing when your result is borderline or your symptoms and test results don’t match. According to the NIH Office of Dietary Supplements, serum B12 is the most common initial test but isn’t always sufficient on its own.

When should I get a vitamin B12 test?

Consider testing if any of these apply:

Numbness, tingling, or burning in your hands or feet
Unexplained fatigue, weakness, or shortness of breath
Balance or coordination problems
Memory changes, confusion, or mood shifts
Pale or yellowish skin
A CBC showing large red blood cells (macrocytosis)
Long-term use of metformin, proton pump inhibitors (PPIs), or H2 blockers
A vegan or strict vegetarian diet
History of gastric bypass or stomach surgery
A digestive condition like celiac disease or Crohn’s disease
Recent heavy alcohol use

For routine screening, no universal cadence exists, but certain groups benefit from periodic testing:

Adults on long-term metformin, per American Diabetes Association guidance
Adults 60 and older, who are more likely to have low stomach acid that limits B12 absorption from food
People who are pregnant or breastfeeding, due to higher B12 demands
Vegans and strict vegetarians with no reliable fortified food intake
Anyone with a malabsorption condition (celiac disease, Crohn’s disease, atrophic gastritis) or a history of stomach or small intestine surgery

How It Works

How to get tested

This test is available through a healthcare provider, clinic, or hospital lab. Most providers order it through CLIA-certified labs including LabCorp and Quest Diagnostics. You give a blood sample at a partner lab or your provider’s office. Results are typically ready within one to three business days after the lab receives your sample, though timing can vary by lab.

Testing.com connects you with CLIA-certified labs for in-person blood draws. Results are typically delivered one to two business days after the lab receives the sample.

Before the test

No fasting is needed for a standalone vitamin B12 test. If your provider orders B12 alongside other tests that do require fasting (like a lipid panel or fasting glucose), follow the fasting instructions for those tests.

Don’t stop taking B12 supplements before the draw. The test measures your current serum level, and your provider reads that number knowing your supplement use. Tell your provider what you’re taking, including the dose and how long you’ve been on it.

Some medications can affect your B12 level or how your body absorbs it. Metformin, proton pump inhibitors (PPIs), histamine-2 receptor antagonists (H2 blockers- used for acid reflux, GERD, etc.), colchicine (gout attacks), certain antibiotics, and phenytoin (controls seizures) may decrease the body’s ability to absorb vitamin B12. Don’t stop any medication without talking to your provider first.

During the test

A technician draws blood from a vein in your arm. The whole draw takes less than five minutes.

You’ll feel a brief sting when the needle goes in. The technician ties a band around your upper arm to make the vein easier to find, cleans the skin, and then inserts a small needle attached to a collection tube.

The technician removes the needle and covers the spot with a small bandage. Keep it on for at least 15 minutes to ensure bleeding stops. Some people get a small bruise. That’s harmless and clears up on its own. Call your provider if you notice lasting pain, swelling, or redness.

After the test

Results are typically ready within one to three business days after the lab receives your sample, though timing can vary by lab and provider system. You’ll get them through the ordering provider’s patient portal or office.

What do my results mean?

The test reports your B12 level in picograms per milliliter (pg/mL). Here’s how most labs categorize results, though reference ranges vary from lab to lab:

Result

B12 Level

Normal

200–900 pg/mL (range varies by lab)

Borderline

200–300 pg/mL

Deficient

Below 200 pg/mL

Elevated

Above 900 pg/mL

Based on the results, your provider will ask about your supplement use, diet, medications, and discuss whether your symptoms match the result.

If your vitamin B12 levels are normal

Your blood has enough circulating B12. But a normal serum level doesn’t always rule out a functional deficiency at the cellular level. If your symptoms still point to B12 problems (numbness, fatigue, cognitive changes), your provider may order other tests to check whether B12 is actually being used by your cells. Those markers are more sensitive for tissue-level deficiency than serum B12 alone.

Methylmalonic acid (MMA) – regulated by B12 and may be elevated with B12 deficiency
Homocysteine – may be elevated causing damage to blood vessels and cardiac issues
Folate (B9) – a deficiency causes similar symptoms including megaloblastic anemia

If your vitamin B12 levels are low

A result below 200 pg/mL points to deficiency. A borderline result between 200 and 300 pg/mL often warrants a retest or follow-up testing. The next step is figuring out why.

Two main causes exist: not getting enough B12 from food, or not absorbing it properly. Dietary deficiency responds well to oral supplements. Absorption problems (from pernicious anemia, post-gastric surgery, or atrophic gastritis) often need B12 injections because oral B12 can’t get past the absorption block, as described by MedlinePlus on pernicious anemia.

Your provider may order an intrinsic factor antibody test to check for pernicious anemia, or MMA and homocysteine to confirm the deficiency. According to the NIH Office of Dietary Supplements, MMA testing is generally indicated when serum B12 falls between 150 and 399 pg/mL. Treatment depends on the cause, not just the number.

If your vitamin B12 levels are high

An elevated B12 level is generally harmless unless extremely high doses of B12 are taken. Even the ingestion of a large amount of B12 foods do not cause issues because your body can absorb only what is needed. Because it’s a water-soluble vitamin, excess B12 normally leaves the body in urine or feces.

A high result is often explained by recent B12 supplements or injections. When there’s no obvious supplement explanation, high B12 can suggest liver disease, kidney disease, or certain blood disorders like chronic myelogenous leukemia (CML) or polycythemia vera (PV), according to Mayo Clinic’s overview of vitamin B12 testing. Your provider will look at your full picture to decide whether follow-up testing is needed.

FAQs

What are normal vitamin B12 levels on a blood test?

Most labs set the normal range at roughly 200 to 900 pg/mL, but ranges vary between labs. Read your result alongside the reference range printed on your specific report. A result between 200 and 300 pg/mL is often considered borderline and may prompt a retest.

Do you need to fast for a vitamin B12 blood test?

No. Fasting isn’t required for a standalone B12 test. If your provider orders B12 alongside a lipid panel or fasting glucose, follow the fasting instructions for those companion tests.

Should I stop taking B12 supplements before the test?

No. Don’t stop your supplements. The test measures your current serum level, and your provider interprets that number knowing what you take. Tell your provider the dose and how long you’ve been taking it.

What does it mean if my vitamin B12 is very high (over 1,000 or 2,000 pg/mL)?

A very high result is often from recent ingestion of supplements or a B12 injection. That’s generally not a concern on its own. When there’s no supplement explanation, high B12 can point to liver disease, kidney disease, or certain blood disorders. Your provider can review your result in context and decide whether follow-up is needed.

What is a vitamin B12 and folate blood test?

It’s a test that measures both B12 and folate (vitamin B9) from the same blood sample. The two are often ordered together because both deficiencies can cause megaloblastic anemia, and high folate can mask a B12 deficiency by preventing anemia even while nerve damage progresses. If your provider suspects a nutritional deficiency behind your symptoms, both markers together give a clearer picture.

What is vitamin B12 called on a lab report?

Lab reports may identify the test for vitamin B12 level as “Vitamin B12,” “Cobalamin,” or “Cobalamin, Serum.” The name varies slightly between LabCorp and Quest Diagnostics, but all refer to the same test.

Can a normal B12 result still mean I have a deficiency?

Yes. Serum B12 can look normal even when your cells aren’t getting enough to function properly. If your symptoms suggest deficiency but your serum level is in range, ask your provider about MMA and homocysteine testing. According to the NIH Office of Dietary Supplements’ vitamin B12 fact sheet, MMA is the most sensitive marker of B12 status and can help confirm deficiency when serum B12 alone is inconclusive.