Serum B12 and folate levels

At a glance

• Total serum B12 measures all circulating cobalamin, most of it bound to haptocorrin and unavailable to cells. Active B12 (holotranscobalamin, holoTC) measures the 20 to 30% bound to transcobalamin, the fraction cells can take up.
• Both have a wide grey zone. Functional markers resolve it: methylmalonic acid (MMA) is specific for B12 deficiency among the vitamin deficiencies, but also rises in renal impairment, hypovolaemia, bacterial overgrowth and inborn errors; homocysteine rises in both B12 and folate deficiency.
• Test before treating, and treat neurological signs the same day whatever the level.