The diagnosis of acute leukemia with undifferentiated or minimally differentiated blasts.

A small group of acute leukemias can be classified by morphological, cytochemical, or immunological marker analysis neither as acute lymphatic leukemia nor as acute myeloid leukemia. These leukemias are referred to as acute undifferentiated leukemia (AUL) and make up 2%-7% of the acute nonmyeloid leukemias. These leukemias are poorly defined in the literature and are also sometimes referred to as AML-MO. Most of the definitions include in the morphological and cytochemical criteria the expression of myeloid antigens. Here the value of these markers and of other techniques used in the diagnosis of undifferentiated or minimally differentiated leukemia is discussed. More than half of the leukemias that are undifferentiated by morphology and cytochemistry on the light-microscopic level show a positive reaction for myeloperoxidase by electron microscopy, which points to an early myeloid differentiation of those leukemias. Immunological marker analysis in most cases is inconclusive. Most show a positive reaction for CD 13, CD 33 or other myeloid-associated markers. However, in about half of these leukemias co-expression of lymphatic markers is seen. In a small minority, only lymphatic markers are expressed. Cytogenetic abnormalities which are found in these leukemias vary in type, and antigen receptor rearrangements are not lineage specific. Receptor studies, gene expression, and in vitro culture studies may, in the near future, contribute substantially to our knowledge about the commitment of these undifferentiated or minimally differentiated blasts. Recent definitions for AUL and AML-MO based on these different techniques are discussed.