Diagnosis and classification of the acute leukemias: recent advances and controversial issues

Although morphology and cytochemistry continue to be the mainstay of the diagnosis of acute leukemia (AL), new developments in immunophenotyping, cytogenetics, molecular biology, and in vitro assays have dramatically improved our understanding of this disease and enabled the identification of entities with distinct clinico-biologic features. Immunophenotyping is essential for diagnosing and subclassifying acute lymphoblastic leukemia (ALL) and is also very helpful in certain types of acute myeloid leukemias (AML), such as AML with minimal differentiation or acute megakaryoblastic leukemia. Cytogenetic findings are clinically relevant for diagnosis and prognosis. Nonrandom chromosomal abnormalities such as t(15;17)(q22;q12) or t(1;19)(q23;p13) have been so closely associated with distinct types of acute leukemias that their recognition can allow diagnosis independent of the other criteria. Molecular analysis is a powerful method in the assessment of the malignant potential, clonality, and classification of the ALs. It has become clear that in some leukemias a proportion of patients exhibit the biologically relevant molecular defect in the absence of a karyotypic equivalent. On the other hand apparently uniform chromosomal abnormalities such as the t(1;19), t(9;22), t(8;14), or t(15;17) may differ at the molecular level. In vitro assays can evaluate the growth pattern and cell-cycle kinetics of leukemic cells, as well as their sensitivity to therapeutic agents. All these data are relevant to the management of AL. Because the French-American-British (FAB) classification does not fully correlate with much of this new information, alternative classifications have been proposed. In this review we concentrate on recent diagnostic contributions resulting from advances in biotechnology and discuss some of the points that arouse controversy in the single classifications.