A case of biphenotypic blast crisis of unclassified myeloproliferative disorder.

We report a first case of biphenotypic blast crisis of unclassified myeloproliferative disorder (MPD). A 20-year-old patient presented with fever, splenomegaly, marked leukocytosis (603 x 10(3)/ micro l), and blasts in the peripheral blood. Since Ph chromosome and bcr-abl gene rearrangement were absent, the diagnosis of an unclassified MPD in the blast crisis phase was established. Immunophenotyping confirmed a biphenotypic crisis of myeloid and T-lymphoid antigens. The patient went into a complete remission after chemotherapy, but marked granulocytic hyperplasia (M:E ratio of 5.7) and 90% cellularity remained. Blast crisis recurred during subsequent intensification chemotherapy and the patient did not go into a complete remission regardless of the intense chemotherapy. The blast crisis transformed from unclassified MPD had a grave prognosis as it responded poorly to chemotherapy. This unique blast crisis is distinguishable from the blast crisis of chronic myelogenous leukemia.