Holotranscobalamin (Active B12)
Holotranscobalamin, often called active B12, measures the portion of vitamin B12 that is bound to a carrier protein and actually available for your cells to use. It represents the biologically usable fraction of your total B12.
Because only a share of the B12 in your blood can enter your cells, this test can flag an early shortfall that total B12 sometimes misses. It is used to sharpen the picture when B12 status is uncertain or when symptoms suggest deficiency despite a borderline total level. Think of it as a closer look at the B12 your body can truly access.
A high active B12 usually reflects recent supplementation or high intake rather than a health concern, since the body does not store water-soluble B12 to harmful levels. Occasionally it accompanies other conditions and is interpreted alongside your total B12. On its own it is rarely worrying.
A low active B12 suggests less usable vitamin is reaching your cells and can be an early sign of deficiency. It is often followed up with MMA or homocysteine to confirm whether cells are actually short of B12.
Labs provide a reference range, and results near the lower edge may warrant confirmatory testing. A clinician can help interpret this alongside your total B12 and symptoms.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.