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Active B12 (Holotranscobalamin)

Chemical Pathology

Background information

  • In the circulation, the majority (70-90%) of total vitamin B12 is bound to a carrier protein called haptocorrin. Approximately 20-30% of total circulating vitamin B12 is bound to a different carrier protein, transcobalamin. Only vitamin B12 bound to transcobalamin can be taken up by cells to exert its biological effects; this is therefore referred to as ‘active vitamin B12’ or ‘active B12’.
  • Total B12 levels may fall physiologically in pregnancy, which may not represent deficiency at the cellular level. Unlike total B12, active B12 levels remain stable in pregnancy and are a better indicator of vitamin B12 deficiency. Therefore, active B12 is the recommended initial test for suspected vitamin B12 deficiency during pregnancy (NG239).
  • Active B12 testing is currently not available for any non-pregnant patients. Active B12 will only be assayed when B12 testing is requested on a pregnant patient. This will occur automatically within the laboratory; the test cannot be requested on ICE or EPR.
  • Repeat testing is not indicated within the same pregnancy (repeat samples will be blocked).

Sample requirements

Sample requirement: 5ml gold top SST (or rust top for the Acute Unit)

5ml gold tube

Patient preparation

None.

Storage/transport

Send at ambient temperature to the laboratory. If unavoidable, samples can be stored refrigerated overnight.

Required information

Please confirm that the patient is pregnant.

Turnaround times

The active B12 assay is performed at GRH in batches twice a week. The turnaround time will therefore be less than one week. The test cannot be ordered urgently without discussion with the duty biochemist.

Reference ranges

Active B12 <25 pmol/L: Active B12 result consistent with vitamin B12 deficiency (NG239)

Active B12 25 – 70 pmol/L: Active B12 result indeterminate – possible vitamin B12 deficiency (NG239). GHNHSFT maternal anaemia pathway: if Hb not rising and ferritin normal then refer to obstetrician for IM B12 and oral folate to be commenced.

Active B12 > 70 pmol/L: Active B12 result suggests vitamin B12 deficiency is unlikely (NG239). GHNHSFT maternal anaemia pathway: refer to obstetrician for IM B12 and oral folate to be commenced.

Further information

Internal requestors, please refer to the Trust Guideline Anaemia in Pregnancy (M1000) for interpretation and treatment guidelines.

Please see NICE Guideline NG239 for more information on vitamin B12 testing and deficiency.

Page last updated: 02/09/2025 | Page last reviewed: 02/09/2025

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