The World Health Organization (WHO) classification of acute myeloid leukemia (AML) incorporates and interrelates morphology, cytogenetics, molecular genetics, and immunologic markers in an attempt to construct a classification that is universally applicable and prognostically valid. In the older French-American-British (FAB) criteria, the classification of AML is solely based upon morphology as determined by the degree of differentiation along different cell lines and the extent of cell maturation.[2,3]
Under the WHO classification, the category "acute myeloid leukemia not otherwise categorized" is morphology-based and reflects the FAB classification with a few significant modifications.[2,3] The most significant difference between the WHO and FAB classifications is the WHO recommendation that the requisite blast percentage for the diagnosis of AML be at least 20% blasts in the blood or bone marrow. The FAB scheme required the blast percentage in the blood or bone marrow to be at least 30%. This threshold value for blast percentage eliminated the category "refractory anemia with excess blasts in transformation" (RAEB-t) found in the FAB classification of myelodysplastic syndromes (MDS), where RAEB-t is defined by a marrow blast percentage between 20% and 29%. In the WHO classification, RAEB-t is no longer considered a distinct clinical entity and is instead included within the broader category "AML with multilineage dysplasia" as "AML with multilineage dysplasia following a myelodysplastic syndrome."
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Although this lowering of the blast threshold has been met with some criticism, several studies indicate that survival patterns for cases with 20% to 29% blasts are similar to survival patterns for cases with 30% or more blasts in the bone marrow.[5,6,7,8,9] The diagnosis of AML in itself does not represent a therapeutic mandate. The decision to treat should be based on other factors including patient age, previous history of MDS, clinical findings, disease progression, in addition to the blast percentage, and most importantly, patient preference.
Several groups have begun to investigate the use of gene expression profiling (GEP) using microarrays to augment current diagnostic and prognostic studies for AML. Distinct subsets can be identified using GEP that correspond to known cytogenetic and molecular abnormalities. The positive predictive value appears to be sufficiently powerful to be clinically useful only for patients with the t(8;21) and inv(16) (now referred to as core-binding factor [CBF] leukemias) and acute promyelocytic leukemia (APL) with the t(15;17). GEP identified several cases of CBF leukemias that were not diagnosed using conventional cytogenetics.[10,11,12]