HoloTC measures only the B12 that cells can actually use, the fraction bound to its delivery protein. It detects deficiency earlier than total B12 and shrinks the gray zone of borderline results.
Standard range versus optimal range
| Framework | HoloTC value | Source |
|---|---|---|
| Deficiency suggested | below 25-35 pmol/L | Hvas et al 2007 |
| Borderline | about 35-50 pmol/L | Hvas et al 2007 |
| Sufficient | above 50 pmol/L | Hvas et al 2007 |
When it runs high
High HoloTC simply reflects good B12 supply or recent supplementation and carries no known harm.
- B12 supplementation
- Naturally strong intake and absorption
When it runs low
Low HoloTC reveals B12 deficiency at the tissue doorstep, before total B12 falls and before anemia. It is particularly useful in pregnancy and in people whose total B12 sits in the borderline band.
- Early or developing B12 deficiency
- Poor absorption from any cause
- Strict plant-based diet without supplementation
Only the delivery that counts
Most B12 in blood travels locked to a protein called haptocorrin, a dead-end carrier that cells cannot use. Only the 20 to 30 percent bound to transcobalamin, the holoTC fraction, actually reaches tissues. Measuring total B12 is like counting all the trucks on the road, measuring holoTC counts only the ones making deliveries.
The first domino to fall
When absorption falters, holoTC drops first, before total B12 leaves the normal range and long before the blood count changes. Studies comparing the markers show holoTC separates true deficiency from borderline noise better than total B12, and it tracks the response to treatment within weeks. In pregnancy, where total B12 falls physiologically and confuses everyone, holoTC keeps its meaning.
Where it fits in practice
HoloTC is the smart second step when total B12 lands in the 200 to 400 gray zone, especially with compatible symptoms. A low holoTC confirms the suspicion, a comfortably normal one makes deficiency unlikely, and the ambiguous middle still gets settled by methylmalonic acid. Not every laboratory offers it yet, but where available it saves both over-treatment and missed diagnoses.
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