Platelet dysfunction in patients with vitamin B12 deficiency.

10 cases of pernicious anaemia are reported in which 6 had abnormal aggregation with epinephrine, 3 with fibrinogen, 2 with ADP, and 2 with ristocetin. 5 patients had thrombocytopenia and 3 of these had a prolongation of the bleeding time. These abnormalities were normalized after vitamin B12 treatment. After treatment, platelet size changed toward a smaller diameter, but platelet size, however, was not significantly different from the normal platelet size distribution.