Vitamin B12 blood test: what low or high B12 means

What the B12 test measures

Vitamin B12 helps make red blood cells and keeps nerves working. The test measures how much B12 is in your blood. Labs often ask you to skip food and drink for 6 to 8 hours first.

A B12 level alone can be misleading. That is why borderline results are often checked with MMA and sometimes homocysteine. Gabriel's B12 lab guide has more detail.

What causes low B12

• Medicines. Long-term metformin and long-term acid reducers for heartburn (such as proton pump inhibitors and H2 blockers) are linked to low B12.
• Diet. B12 comes mainly from animal foods, so a strict vegetarian or vegan diet without fortified foods or supplements can run low.
• Pernicious anemia. The body lacks intrinsic factor, the protein needed to absorb B12.
• Gut conditions and surgery. Celiac disease, Crohn disease, and surgery that removes part of the stomach or small intestine, including some weight-loss operations.
• Alcohol use and nitrous oxide misuse.
• Pregnancy can lower measured levels.

Low B12 can cause tiredness, pale skin, shortness of breath with exercise and a sore, red tongue. Over time it can damage nerves, causing numbness or tingling in the hands and feet, balance problems and memory trouble. It can also raise MCV; see the MCV blood test.

MMA and homocysteine: confirming a low result

B12 is needed to break down methylmalonic acid (MMA). When B12 is truly short, MMA builds up, so a high MMA supports a real deficiency. Homocysteine also rises when B12 or folate is low, but it is less specific.

These tests are most useful when B12 is low-normal or when symptoms don't fit the number. Folate is often checked at the same time; see the folate lab guide.

What a high B12 can mean

Extra B12 is usually cleared in urine, so a high level is uncommon. If you take B12 supplements or shots, tell your clinician, since that changes how a result reads. Without supplements, a high B12 has been linked to liver disease (such as cirrhosis or hepatitis) and some blood disorders, so an unexplained high result is worth a follow-up.

Gabriel's optimal band (545 to 1,100 pg/mL) is narrower than most lab ranges. Higher is not always better.

What to do with an abnormal result

1. Bring your medication list, especially metformin and heartburn medicines. Don't stop any medicine on your own.
2. Ask about MMA if your B12 is low or borderline, plus a CBC and folate.
3. Ask why it's low, not just how to raise it. Absorption problems need a different plan from a diet gap.
4. Recheck after a change at the interval your clinician suggests.

See a clinician promptly if low B12 comes with numbness or tingling, trouble walking or balancing, confusion or memory changes, vision changes, or shortness of breath and a fast heartbeat.