Active B12 optimal range and reference range
Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026
Sera's bands for Active B12:
- Optimal 70 – 165 pmol/L; sufficient 25 – 200 pmol/L; out of range below 25 or above 200 pmol/L.
Evidence grade: Established clinical threshold. A band edge that corresponds to a recognised diagnostic, risk or treatment threshold from a major clinical guideline or public-health body.
All units
| Unit | Optimal | Sufficient | Out of range |
|---|---|---|---|
| pmol/L | 70 – 165 | 25 – 200 | below 25 or above 200 |
What the evidence says
The lower bands follow the NICE guideline on vitamin B12 deficiency (2024): below 25 pmol/L is confirmed deficiency, 25 to 70 is indeterminate and worth following with methylmalonic acid, and above 70 makes deficiency unlikely. Thresholds differ between assays, so a laboratory's own validated cut-offs take precedence. No upper limit is published for active B12. The upper edges here (165 and 200 pmol/L) are kept on purpose all the same: a total B12 result entered against this marker by mistake runs several times higher, and the upper edge is what catches it.
Assay: Results differ meaningfully between platforms and methods. Compare against the interval printed by the issuing laboratory before acting on a borderline value.
Sources
- Vitamin B12 deficiency in over 16s: diagnosis and management (NG239), Recommendations, Table 1 — National Institute for Health and Care Excellence (NICE), 2024 · Clinical guidelineActive B12 below 25 pmol/L is confirmed deficiency, 25 to 70 pmol/L is indeterminate (possible deficiency), above 70 pmol/L suggests deficiency is unlikely. Matches Sera's sufficient lower edge of 25 and optimal lower edge of 70 exactly.
- Active B12/HoloTC — North Bristol NHS Trust, Severn Pathology, 2026 · Laboratory / reference-interval sourceReference range in three bands: above 70 deficiency unlikely, 25 to 70 indeterminate, below 25 consistent with deficiency. The page prints the unit as pmol/mL, which appears to be a misprint for pmol/L. No upper limit is given.
What Vitamin B12 (Active - Holotranscobalamin) measures
The part of your vitamin B12 that is carried by transcobalamin, which is the only form your cells can actually take up.
It drops earlier than total B12 when your B12 status falls, so it can catch a shortfall that a normal total B12 hides. It runs on a much smaller scale than the total test, so the two numbers are not comparable.