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.