Diagnosis of AML is primarily made by experienced hematopathologists on the basis of light microscopic examination of blood and BM smears stained with Romanowsky stains, such as May-Grunwald-Giemsa or Wright-Giemsa stains. Myeloid lineage of leukemic blasts can be confirmed using cytochemical reactions, such as a reaction using o-tolidine or amino-ethyl car-bazole as substrates to detect the presence of myeloperoxidase (MPO), an enzyme present in primary granules of myeloblasts and some monoblasts; a reaction employing Sudan black B (SBB) to detect intracellular lipids that has reactivity similar to MPO but with less specificity for the myeloid lineage; and reactions detecting nonspecific esterase (NSE) that employ alpha-naphthyl butyrate and alpha-naphthyl acetate (ANA). Moreover, in the case of minimally differentiated leukemia, a distinction between AML and acute lymphoblastic leukemia (ALL) can be made with the help of immunophenotypic analysis by flow cytome-try or immunohistochemical reactions on slides. Immunophenotyping is also helpful in the identification of acute megakaryoblastic leukemia (AMKL), and in suggesting or excluding particular subtypes of AML within the WHO classification.1 This classification divides AML into several entities based on morphological and cytochemical criteria, which predominated in the previous French-American-British (FAB) classification, but also takes into account cytogenetic, molecular genetic, immunophenotypic, and clinical features. The major categories and subcategories of the WHO classification are presented in Table 2.1.

One of the most important changes introduced by the WHO classification is that the blast percentage in the marrow required for the diagnosis of AML has been reduced from 30% in the FAB classification to 20% in the WHO classification. Moreover, in patients positive for t(8;21)(q22;q22) and the AML1(RUNX1)-ETO(CBFA2T1) fusion gene, inv(16)(p13q22)/t(16;16)(p13;q22) and CBFB-MYH11, and t(15;17)(q22;q12-21) and PML-RARA, AML can be diagnosed even if the percent of blasts in the marrow is less than 20.1,2

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