Explainer · updated 2026-09-09

Vitamin B12, explained in plain English

Written by the medk.AI team · Last reviewed September 2026 by the medk.AI content team

Key facts

• Typical adult reference interval: 200-800 pg/mL (ABIM)
• The same interval in pmol/L (converted from the ABIM range): 147.6-590.2 pmol/L (ABIM)
• Blood level below which a result is classed as deficiency: 148 pmol/L (KALAYCI 2025)
• What B12 does: the body cannot make it; it builds red cells and keeps the coating on nerves intact, and it is stored in the liver for years (KALAYCI 2025)
• Where the body gets it from: foods of animal origin such as meat and dairy products, or foods fortified with it (WOLFFENBUTTEL 2024)

What it is

Vitamin B12, also called cobalamin, is a vitamin the body cannot make. It is used for building red cells and for keeping the coating on nerves intact. It is stored in the liver for years, so a shortage takes a long time to appear and a long time to fix. The result on your report is the B12 in your blood.

What the number is telling you

The level reflects what the body has stored and what the diet keeps supplying. B12 comes from foods of animal origin such as meat and dairy products, or foods fortified with it, so the intake side of the number is set by what is habitually on the plate. Below 148 pmol/L is deficiency, and it is treated because the nerve damage can outlast the anaemia. Where your own level sits is for your doctor to say.

What high or low can mean

Low: Below 148 pmol/L is deficiency, and it is treated because the nerve damage can outlast the anaemia. Because the body stores B12 for years, a shortage shows up slowly, and so does the recovery.

High: A high level usually reflects B12 supplements or a condition that releases stored vitamin into the blood. In children, persistently high levels without a clear cause can signal a serious underlying condition, so they are looked into rather than shrugged off.

What a typical result looks like

Typical adult reference intervals from ABIM run from 200 to 800 pg/mL, about 147.6 to 590.2 pmol/L. The range on your own report is the one that counts.

Questions worth asking your doctor

• What do my other blood counts say beside this result?
• If my level is low, what could be causing it, and how should we fix it?
• If my level is high, is any supplement or medicine I take contributing?
• How does this compare with my previous results?

If a result worries you, ask your doctor.

Sources

• ABIM Laboratory Test Reference Ranges (2026)
• Elevated serum cobalamin levels in childhood: a retrospective analysis (2025)
• A Brief Overview of the Diagnosis and Treatment of Cobalamin (B12) Deficiency (2024)