Optimizing diagnostic thresholds of total vitamin B12 (B12) for identifying cobalamin deficiency in adults with macrocytic anemia

OBJECTIVES: Identifying individuals with vitamin B12 (B12) deficiency is challenging due to poor harmonization across total B12 assays. To establish clinically meaningful thresholds for the Roche assay, we characterized B12 concentrations associated with deficiency by comparing individuals with macrocytic anemia and other anemia subtypes or no anemia. METHODS: We retrospectively analysed 10 years of laboratory data from adults tested for total B12 and folate (Roche Cobas), homocysteine (Abbott Architect), and haematological parameters (Sysmex XE/XN). Individuals receiving vitamin supplementation or with isolated folate deficiency were excluded. Anemia subtypes (normocytic, microcytic, macrocytic) were classified using red blood cell count, hemoglobin concentration, and mean corpuscular volume relative to reference intervals. RESULTS: Among 5,147 subjects (median age 65 years; 25th-75th percentile: 49-77), 36.8 % had anemia. Total B12 concentrations decreased by 2.3 ng/L for each 1 μmol/L increase in homocysteine and by 6.8 ng/L per decade of age increase (p