Leukocytosis, Leukopenia, and Other Reactive Changes of Myelopoiesis

Abstract

Most changes in the white blood cell count are reactive and due to an increase or decrease of cells of the myeloid series. By definition, a leukocytosis is present if leukocytes are increased to more than 10,000/μL; in leukopenia leukocytes are below 4000/μL. Infants and small children have slightly different normal values (for the normal values of blood counts, see Appendix 3). A neutropenia is present if granulocytes are below 2000/μL, agranulocytosis if granulocytes are below 500/μL. By definition, granulocytes are increased if they are greater than 8000/ μL. In leukemoid reactions, leukocytes are increased to more than 30,000/μL. A left shift in the differential count means that the number of band forms (and other precursors such as metamyelocytes) is increased to more than 5%. In a pathological left shift, more immature precursors such as promyelocytes can be seen in the peripheral blood; this is almost always a sign of a hematological disorder.