Simultaneous evaluation of terminal deoxynucleotidyl transferase and myeloperoxidase in acute leukemias using an immunocytochemical method.

The classification of acute leukemia is important for the selection of optimal therapy. Classification often rests on morphologic, cytochemical, and immunologic criteria, and the marker enzyme terminal deoxynucleotidyl transferase (TdT) has been considered to be a reliable indicator of lymphoblastic leukemias. Because TdT-positive cells sometimes are seen in leukemias otherwise identified as myeloblastic, the authors evaluated blasts identified as myeloid by the presence of myeloperoxidase (MPO) for the simultaneous expression of TdT. The blasts in the bone marrow aspirate or peripheral blood of unselected patients with hematologic malignancies were evaluated and 60 cases are shown. The French-American-British system and, in some patients, cytochemical and immunologic studies were used to classify the leukemias. The authors demonstrated that blasts simultaneously contained MPO and TdT in 29% of patients with acute myeloblastic leukemia and 3% of patients with acute lymphocytic leukemia (ALL). This finding supports the hypothesis that TdT is an expression of cell primitivity rather than a marker for lymphoblastic cells.