Pure Erythroid Leukemia Mimicking Megaloblastic Anemia in a Bone Marrow: A Potential Diagnostic Pitfall

The differential diagnosis between acute erythroid leukemia and megaloblastic anemia can be very challenging. Herein we report a case of pure erythroid leukemia that was diagnosed as megaloblastic anemia on the initial bone marrow biopsy. The patient was a previously healthy 60-year-old man who had pancytopenia. His serum vitamin B12 was slightly low. A bone marrow biopsy was performed at an outside institution and was diagnosed as megaloblastic anemia. No cytogenetic study was performed. He came to our institution 2 months later with persistent pancytopenia despite normalization of the vitamin B12 levels with folic acid and vitamin B12 supplementation. A second bone marrow biopsy was performed, and the outside marrow slides were reviewed. Peripheral blood smears from both time points showed normocytic, normochromic anemia with no hypersegmented neutrophils or giant bands. Compared with the first biopsy (80% cellularity, scattered to small clusters of increased immature erythroids), the current marrow was markedly hypercellular (>95%) with large clusters of immature erythroids (highlighted by E-cadherin and glycophorin A immunohistochemical stains), representing more than 80% of marrow cellularity. A cytogenetic study showed a very complex karyotype. Based on these findings, a diagnosis of pure erythroid leukemia was made. This case demonstrates that the differential diagnosis between acute erythroid leukemia and megaloblastic anemia can be quite challenging, especially when there is a concurrent low level of vitamin B12. Although increased immature erythroids and dysplastic features can be seen in both, megaloblastic anemia usually has macrocytic anemia, hypersegmented neutrophils and/or giant bands, normal karyotype, and cytopenia that quickly responds to vitamin B12 and folic acid supplementation. Careful morphologic evaluation and cytogenetic (karyotypic) analysis are very important for a correct diagnosis.