Leukemoid Reaction--A Tale of 50 Years.

Historical background and past knowledge: The term ‘leukemoid reaction’ was coined by Krumbhaar, in 1926, to describe the leukemia-like blood picture that was found in several non-leukemic conditions [3]. The diagnostic criteria included a total leukocyte count (TLC) of more than 50000/mm3 and/or the presence of immature leukocytes (mostly myelocytes or their equivalents) in the peripheral blood smear. These would usually be granulocytic or lymphoid, according to the predominant cell lineage. It was recognized that leukemoid reaction could be mistaken for chronic myeloid leukemia (CML). However, in those days CML was differentiated from leukemoid reaction based on clinical manifestations (rareness in children, slow progression, pallor, bleeding, bone tenderness, lymphadenopathy and massive splenomegaly), hematological smear (mostly immature leukocytes and blasts with abnormal nuclei and cytoplasm with additional thrombocytopenia, eosinophilia and basophilia) and leukocyte alkaline phosphatase (LAP) levels (low or even absent in CML in contrast to increased in leukemoid reaction).