Holotranscobalamin, the metabolically available fraction of vitamin B12.
What it is
Active B12 (holotranscobalamin) reflects B12 available for cells, complementary to total serum B12. Assay methods and units vary.
Why it matters
Discordant total and active B12 results are a reason for clinical review with symptoms, folate, and blood count.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
B12 is required for DNA synthesis and myelin; deficiency can affect blood and nerves.
For athletes
Plant-based athletes may track active B12 with total B12 rather than relying on one number.
