Temporary blast reduction after immunoglobulin administration for congenital cytomegalovirus infection masking infant leukemia with cryptic MLL rearrangement

Differentiation between reactive bone marrow suppression due to viral infection and early stages of leukemia can be difficult particularly in young infants. We report on a 2-month-old girl presenting with pancytopenia and positive markers for congenital cytomegalovirus (CMV) infection. Definitive diagnosis of coexisting pro-B cell infant leukemia with cryptic MLL rearrangement was delayed by the transient regeneration of normal hematopoiesis and reduction of abnormal blastoid cells in the bone marrow following immunoglobulin administration. Molecular diagnosis could only be established using interphase fluorescence in situ hybridization (FISH) analysis which may be considered as a valuable additional diagnostic tool in similar cases.