Diagnosis of acute leukemia is based on the assessment of morphological features of bone marrow cells and peripheral blood. The diagnosis is established only when so-called blast cells in the bone marrow or peripheral blood are characterized by a delicate net structure of nuclear chromatin. The number of blast cells in this case should be 20% or more.

If it is less in the bone marrow, but its content in the blood is 20% or more, then the diagnosis of acute leukemia is also established. Determining the belonging of tumor cells to the myeloid or lymphoid hematopoietic lines when using the usual Romanovsky – Giemsa staining is possible only in 70% of cases. For a more precise definition, other diagnostic approaches are needed: immunophenotyping, cytochemical, cytogenetic, molecular biological and cultural studies.

A new WHO classification can be called a list of diseases characterized by a specific set of features, since it lacks a single classification feature that existed, for example, in developing the FAB classification (morphology and cytochemistry of blast cells). As a result, some forms of OL can simultaneously be assigned to different categories.

Thus, acute leukemias with Hq23 translocations are assigned to the first classification category, but the 11q23 segment anomaly is often detected in patients with secondary acute leukemia. At the same time, secondary acute myeloblastic leukemias are separated into a separate category, so it is not clear how to classify AML, which arose after previous chemotherapy of a tumor and is characterized by anomalies of the 11q23 segment, as a disease of the first category or the third.

There is also a question regarding other consistently detectable chromosomal aberrations: for example, why in the first classification category such anomalies as t (6; 9) and anomalies of chromosome 3 that have clear morphological and clinical characteristics are not considered.

In those situations when it is not possible to analyze the karyotype of blast cells, the only principle by which acute leukemia, morphological, will be classified. Again, it is unclear to which classification category AML is classified, which, for example, according to the FAB classification, is characterized as myelomonoblastic, but arose after prior chemotherapy and in which signs of myelodysplasia are determined in the bone marrow.

The presence of myelodysplasia is the basis for the selection of the second classification category. As noted earlier, morphological signs of dysplasia are detected in patients with different types of AML (myeloblastic, myelomonous and monoblastic, erythroblastic, rarely promyelocytic, etc.), with AML developed from MDS, secondary AML. This pattern was previously noted by other international experts, so it is not quite clear why a special category was selected.

Previously, it was believed that the presence of signs of myelodysplasia determines an unfavorable prognosis, and therefore, it seems that leukemias with these symptoms were considered as a separate category. However, it has now been established that the symptoms of myelodysplasia in the debut of the disease are observed quite often and do not affect the results of therapy.

In the new WHO classification, acute lymphoblastic leukemia is considered in the section of tumors arising from the precursor cells of T and B lymphocytes. In the section of lymphatic tumors from the early progenitor cells are presented:
1) lymphoblastic leukemia / lymphoma from B-lymphocyte progenitor cells (synonym: acute lymphoblastic leukemia from B-cell precursors);
2) lymphoblastic leukemia / lymphoma from T-lymphocyte progenitor cells (synonym: acute lymphoblastic leukemia from T-lymphocyte progenitor cells).

Perhaps the equivalent use of the presented definitions, the authors of the classification only believe that when the content of blast cells in the bone marrow is 25% and it is more expedient to speak of acute leukemia, less than 25% of lymphoblastic lymphoma. However, most often these terminological difficulties are speculative, since the therapy is the same in either case.

In accordance with the FAB classification, the described forms of acute lymphoblastic leukemia were defined as variants L1 and L2, however, at present, these names are practically not used. According to the FAB classification, the third form of ALL, in the modern classification, is assigned to a large section of tumors from mature (determined by immunophenotype) B-cells as Berkitt-like leukemia / lymphoma. Unfortunately, the new classification does not give clear immunophenotypic characteristics of specific sub-variants of B-and T-cell lymphoblastic leukemias, which often causes difficulties in interpreting the results of flow fluorocytometry.