Enzyme and immunohistochemical studies on acute monocytic leukemia (FAB M5): proposal for a new immunohistochemical subclassification.

Using only morphological criteria as proposed by the French-American-British (FAB) Study Group, the subclassification of acute monocytic leukemia (FAB M5) into the categories M5a and M5b can be difficult. We therefore investigated 13 cases of well-established M5 leukemias. The results show that immunohistochemical techniques allow a better subdivision of the acute monocytic leukemias. The less-mature types are characterized by a focal lysozyme and a negative CD68 reaction, whereas the more differentiated types express a diffusely positive lysozyme and also a positive CD68 phenotype: a staining pattern also found in the rare true histiocytic lymphomas. We regard these results as a useful addition to the FAB classification.