Active Vitamin B12, also known as holotranscobalamin (HoloTC), is the biologically available fraction of vitamin B12 that is actually bound to its transport protein transcobalamin II and can be taken up and used by cells throughout the body. It represents only about 20–30% of total circulating B12, yet it is the only portion that delivers B12 to tissues — making it a far more meaningful measure of true B12 status than total B12.

Vitamin B12 is essential for DNA synthesis, red blood cell formation, neurological function, and the methylation cycle. Deficiency leads to megaloblastic anaemia, peripheral neuropathy, cognitive decline, and elevated homocysteine. Because a large portion of total B12 circulates as biologically inactive haptocorrin-bound B12, total B12 measurements can appear normal even when functional B12 at the cellular level is insufficient. Active B12 identifies this hidden deficiency earlier and more accurately.