2014 British Committee for Standards in Haematology (BCSH) guideline suggests using serum cobalamin (vitamin B12) cutoff < 148 pmol/L (200 pg/mL) or cutoff derived from local reference range in patient with strong clinical suspicion for vitamin B12 deficiency:
Vitamin B12 concentrations <200 pg/ml in patients with clinical manifestations have a sensitivity of 90%–95% and an estimated specificity of less than 80%. Learn more here.