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Hyperleukocytosis (HL) per se is a laboratory abnormality, commonly defined by a white blood cell (WBC) count above 100,000/µL, caused by leukemic cell proliferation. Not the high blood count itself, but complications such as leukostasis, tumor lysis syndrome (TLS) and disseminated intravascular coagulation (DIC) put the patient at risk and require therapeutic intervention. The risk of complications is higher in acute than in chronic leukemias, and particularly leukostasis occurs more often in acute myeloid leukemia (AML) for several reasons. Only a small proportion of AML patients present with HL, but these patients have a particularly dismal prognosis due to i) a higher risk of early death resulting from HL complications and ii) a higher probability of relapse and death in the long run. Whereas initial high blood counts and high lactate dehydrogenase (LDH) as indicator for high proliferation are part of prognostic scores guiding risk adapted consolidation strategies, HL at initial diagnosis must be considered a hematological emergency and requires rapid action of the admitting physician in order to prevent early death. For personal use only.on May 10, 2016. by guest www.bloodjournal.orgFro