Serum vitamin B12-binding proteins in a case of eosinophilic leukemia

A patient with subacute eosinophilic leukemia is presented, with full recognition of the controversy surrounding that entity. Serum vitamin B₁₂ and B₁₂ -binding protein studies and simultaneous complete blood counts were done before and during 6 months of high-dose, intermittent combination chemotherapy. The patient presented with extremely high levels of serum vitamin B₁₂, unsaturated B₁₂-binding capacity, and transcobalamin I, all of which resembled the highest values seen in chronic myelogenous leukemia. Serial studies, during and after remission induction, showed a precipitous fall of serum vitamin B₁₂ and unsaturated B₁₂-binding capacity to normal levels. The data show that transcobalamin I levels, which eventually reached low-normal range, correlate best with the level of circulating and bone marrow eosinophils. Transcobalamin II and serum third binder appeared to be normal throughout the patient's course. The B₁₂-binding protein abnormalities are not considered diagnostic of eosinophilic leukemia.