The Importance of Holotranscobalamin MeasurementClinically significant vitamin B12deficiency can occur even with total vitamin B12levels apparently within normal range. There is an indeterminate zone between approximately 154 and 300 pmol/L of vitamin B12where there is likely misclassification of B12status if relying on total serum B12. The aim of the present study was to assess the usefulness of holotranscobalamin in diagnosis of B12deficiency. Blood samples were collected and subjected to assays for vitamin B12and holotranscobalamin. We examined the levels of holotranscobalamin in 32 subjects (n=32, f=18, m=14) with vitamin B12values within interval 154-300 pmol/L. These subjects were compared with control group with vitamin B12>300 pmol/L (n=31, f=17, m=14). 25% of subjects with vitamin B12of 154 to 300 pmol/L had low levels of holotranscobalamin. Holotranscobalamin levels of patients with vitamin B12of 154 to 300 pmol/L were significantly lower than those of control subjects (38.55 ± 23.0 vs. 61.35 ± 31.81 pmol/L, p<0.01). The obtained results also show a positive significant relationship between levels of holotranscobalamin and vitamin B12(r=0.53, p<0.01). In conclusion, holotranscobalamin is a better indicator of early vitamin B12deficiency than total serum cobalamins and it is a useful tool in the diagnosis of vitamin B12deficiency.