Leukemoid reaction is an abnormally marked granulocytic response to some bone marrow stimulus, most commonly infection. Leukemoid reaction is basically the same process as an ordinary leukocytosis except in the degree of response. The expected peripheral blood WBC count response is even more marked than usual and may reach the 50,000-100,000/mm3 (50-100 x 109/L) range in some cases. Instead of the mild degree of immaturity expected, which would center in the band neutrophil stage, the immature tendency (“shift to the left”; see Chapter 6) may be extended to earlier cells, such as the myelocyte. The bone marrow may show considerable myeloid hyperplasia with unusual immaturity. However, the number of early forms in either the peripheral blood or bone marrow is not usually as great as in classic CML. There is no basophilia, although the increased granulation often seen in neutrophils during severe infection (“toxic granulation”) is sometimes mistaken for basophilia. The bone marrow in leukemoid reaction is moderately hyperplastic and may show mild immaturity but, again, is not quite as immature as in CML. Splenomegaly and lymphadenopathy may be present in a leukemoid reaction due to the underlying infection, but the spleen is usually not as large as in classic CML.

One other phenomenon that could be confused with CML is the so-called leukoerythroblastic marrow response (Chapter 6) seen with moderate frequency in widespread involvement of the bone marrow by metastatic cancer and occasionally in diseases such as severe hemolytic anemia, severe hemorrhage, and septicemia. Anemia is present, and both immature WBCs and nucleated RBCs appear in the peripheral blood.