Test Details: Vitamin B12 deficiency occurs if the body does not absorb enough vitamin B12 from the gastrointestinal tract or when there is not enough dietary intake of the vitamin. The Active vitamin B12 assay detects levels of serum holotranscobalamin (holoTC). HoloTC is the metabolically available component of vitamin B12 (cobalamin) and is used as a marker of vitamin B12 deficiency. Low levels of holoTC can indicate vitamin B12 depletion.