The traditional method of diagnosing vitamin B12 deficiency has been to measure the concentration of total vitamin B12 in serum. The total vitamin B12 concentration essentially reflects vitamin B12 which is bound to its two carrier proteins, forming holohaptocorrin (holoHC) and holotranscobalamin (holoTC). Whilst holoHC accounts for 70 % – 80 % of the vitamin B12 in serum, only holoTC (active vitamin B12) can be used by human cells. Measurement of total vitamin B12 can therefore give erroneous results because it measures the circulating vitamin B12 but does not reflect the level of active vitamin B12. The Biohit Active B12 (Holotranscobalamin) ELISA test directly measures holoTC – the biochemically active form of vitamin B12 – in human serum. This test is well suited for the screening of patients with suspected vitamin B12 deficiency. It can also be used for confirming vitamin B12 status in the large number of patients receiving an inconclusive total vitamin B12 test result.