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The significance of an elevated serum lysozyme value in acute myelogenous leukemia with eosinophilia.

According to criteria established by the French-American-British (FAB) classification, a diagnosis of acute myelomonoblastic leukemia (FAB M4) is based on the presence of 20% bone marrow monocytes or a serum lysozyme level that exceeds the reference value by three times. Reported here is a case of acute myelogenous leukemia with eosinophilia and a cytogenetic inversion of chromosome 16 (inv 16) that lacks morphologic, cytochemical, and immunophenotypic features of monocytic differentiation, but which is associated with an elevated serum lysozyme value. The authors used an immunoelectron microscope to localize lysozyme to both normal and abnormal eosinophil granules, in addition to the secondary granules of myeloid precursors and monocytes. This enzyme could not be demonstrated within the myeloblasts of the patient studied. Postfixation with osmium tetroxide greatly reduced the staining intensity within the crystalloids of normal eosinophils, but only minimally affected that of monocytes, neutrophils, normal eosinophil granule matrix, and the abnormal granules of the leukemic eosinophils. These results demonstrate that lysozyme is present in both normal and leukemic eosinophils and that elevation of serum lysozyme in patients with acute myelogenous leukemia with eosinophilia is not a reliable indicator of monocytic differentiation. Furthermore, an occasional case of acute leukemia with inv 16 is classifiable as acute myelogenous leukemia with differentiation (FAB M2).