Your serum vitamin B12 test measures the amount of cobalamin circulating in your blood, a vitamin your body needs to make red blood cells, DNA, and healthy nerves. A normal result falls between roughly 160 and 950 pg/mL, with levels below 160 pg/mL considered deficient.
| Status | Serum B12 | Units |
|---|---|---|
| Normal | 300 to 950 | pg/mL |
| Borderline / low | 160 to 300 | pg/mL |
| Deficient | Less than 160 | pg/mL |
Ranges vary between labs, so compare to the range on your own lab report.
What vitamin B12 measures
This test reports the total cobalamin in your serum, most of which is bound to carrier proteins. B12 is essential for red blood cell formation, DNA synthesis, and the myelin that protects your nerves. Because your body stores B12 in the liver, deficiency often develops slowly over months or years.
What high or low levels mean
Low B12 usually points to poor absorption rather than low intake. Common causes include pernicious anemia (an autoimmune loss of intrinsic factor), a strict vegan or vegetarian diet, and long-term use of metformin or proton pump inhibitors. Symptoms can include fatigue, macrocytic anemia, tingling or numbness in the hands and feet, and memory changes. When your result sits in the borderline zone, doctors often confirm true deficiency by measuring methylmalonic acid (MMA) and homocysteine, both of which rise when B12 is functionally low. High B12 is rarely a concern on its own and usually reflects supplementation, but a high result without supplements can occasionally signal liver or kidney disease and is worth discussing with your clinician.
Vitamin B12 FAQ
What is a dangerously low B12 level?
Levels below 160 pg/mL are generally considered deficient and can cause anemia and nerve damage if untreated. Values under about 100 pg/mL warrant prompt evaluation, especially if you have neurological symptoms like numbness or balance problems.
Can I have symptoms with a normal B12 result?
Yes. A serum value in the low-normal range (160 to 300 pg/mL) can still be functionally deficient. MMA and homocysteine testing help clarify whether your cells are actually getting enough B12.
Does metformin lower B12?
Long-term metformin use can reduce B12 absorption over time. If you take metformin, periodic B12 testing is reasonable, particularly if you develop fatigue or neuropathy. Read alongside your homocysteine, ferritin, and mean corpuscular volume results for a fuller picture of anemia and nerve health.
References
- MedlinePlus Medical Encyclopedia, Vitamin B12 level. U.S. National Library of Medicine.
