Transcobalamin-bound vitamin B(12), or holotranscobalamin (HoloTC), a biologically active form, is believed to be a sensitive marker of vitamin B(12) deficiency. We investigated the prevalence of vitamin B(12) deficiency in gastrectomized patients using HoloTC and total vitamin B(12) to determine the diagnostic utility of HoloTC. We enrolled 128 gastrectomized patients and measured serum HoloTC, total vitamin B(12), homocysteine (Hcy), and complete blood count (CBC). HoloTC values were also obtained from 100 healthy controls. The precision of HoloTC measurement was good and the normal range of HoloTC was set at >or=42.48 pmol/L. Among the 128 gastrectomized subjects, HoloTC was low (<42.48 pmol/L) in 32 patients (25.0%) and total vitamin B(12) was low (<189 pg/ml) in 10 patients (7.8%). Among the patients who had total vitamin B(12) concentrations in the borderline range (189 approximately 400 pg/ml), 44% had low HoloTC concentrations. Clinical findings as well as Hcy concentrations suggested vitamin B(12) deficiency in this subset of patients. The patients with both low total vitamin B(12) and low HoloTC had significantly higher Hcy concentrations than those with either normal total vitamin B(12) or normal HoloTC. Although a quarter of gastrectomized patients had low HoloTC, some of these had normal total vitamin B(12) concentrations. Our study suggests that serum HoloTC is a more sensitive marker than total vitamin B(12) in diagnosing vitamin B(12) deficiency.