When one stops eating vitamin B-12 (cobalamins), one passes through four stages of negative cobalamin balance: serum depletion [low holotranscobalamin II, ie, low vitamin B-12 on transcobalamin II (TCII)], cell depletion (decreasing holohaptocorrin and low red cell vitamin B-12 concentrations), biochemical deficiency (slowed DNA synthesis, elevated serum homocysteine and methylmalonate concentrations), and, finally, clinical deficiency (anemia). Serum vitamin B-12 is on two proteins: the circulating vitamin B-12 delivery protein, TCII, and the circulating vitamin B-12 storage protein, haptocorrin. Because TCII is depleted of vitamin B-12 within days after absorption stops, the best screening test for early negative vitamin B-12 balance is a measurement of vitamin B-12 on TCII (holoTCII). HoloTCII falls below the bottom of its normal range long before total serum vitamin B-12 (which is mainly vitamin B-12 on haptocorrin) falls below the bottom of its normal range.
