Understanding the marker
Holotranscobalamin, also called active B12, measures B12 attached to the protein that delivers it to cells. It can be used as an alternative first test to total B12.
Active B12 gives information about the vitamin available for cells to take up. Symptoms and the laboratory's decision limits guide interpretation. An uncertain result may need methylmalonic acid or another follow-up test before deficiency can be established.
What does high Holotranscobalamin mean?
A high result can follow supplementation and is interpreted with treatment and clinical context.
What does low Holotranscobalamin mean?
Low active B12 can support a finding of reduced B12 availability. Symptoms and the specific laboratory limits matter. Further testing may help clarify an uncertain result rather than treating the number alone as a diagnosis.
More markers in the same area
- Vitamin D (25-OH)the blood measure used to assess vitamin D supply
- Vitamin B12needed for nerves and red cells
- Folate (B9)helps your body make new cells
- Vitamin A (Retinol)for vision, immunity, and skin
- Vitamin E (alpha-Tocopherol)a vitamin that helps protect cell membranes
- MagnesiumFor muscle and nerve function
This is general information about the marker, not an assessment of your values and not medical advice. Last updated: October 2026.