In brief: Holotranscobalamin (HoloTC) is the fraction of vitamin B12 bound to transcobalamin and available to cells. NICE explicitly accepts active B12 as an initial test for suspected vitamin B12 deficiency.[1]
How are results interpreted?
| Active B12 | NICE interpretation |
|---|---|
| <25 pmol/L | Confirmed B12 deficiency |
| 25–70 pmol/L | Indeterminate / possible deficiency |
| >70 pmol/L | Deficiency unlikely |
Local laboratory ranges can vary, and supplements can affect interpretation.
When are further tests useful?
With borderline results and compatible symptoms, methylmalonic acid (MMA) can add useful information. Homocysteine is less specific.
Symptoms still matter
B12 deficiency can cause neurological symptoms even without marked anaemia. NICE advises not to rule it out solely because anaemia or macrocytosis is absent.[1]
FAQ
Is HoloTC the same as total B12?
No.
Is HoloTC always the better test?
No. It can be used initially but may not be sufficient in every situation.
Can supplements affect the result?
Yes.
What happens with borderline results?
Symptoms, risk factors and sometimes MMA should be considered.
Conclusion
Holotranscobalamin is a useful B12 marker, but not an isolated truth value. Good diagnosis combines laboratory results, symptoms and cause.
