Cases of acute leukemia showing both terminal deoxynucleotidyl transferase (TdT) and myeloperoxidase (MPO) activities are usually classified as acute myelogenous leukemia (AML). Although some reports have implied overlap of TdT and MPO based on population percentages, direct evidence for simultaneous expression of TdT and MPO by a leukemic blast is lacking. By use of a simple new technique developed in our laboratory for identifying TdT and MPO in individual cells by light microscopy, we examined three cases of acute leukemia with both TdT and MPO positivity and found that the incidence of cells positive for both TdT and MPO was 0%, 1%, and 23%. Cytogenetic analysis showed a single leukemic clone in all patients, providing additional evidence that these leukemias arose from a single cell capable of expressing both MPO and TdT. These findings have implications for understanding the relation between MPO and TdT expression in leukemia.

Immunoperoxidase (IP) and immunofluorescence (IF) technics for the detection of terminal deoxynucleotidyl transferase (TdT) were applied to 102 cases of acute leukemia to compare their relative usefulness in the diagnosis and classification of acute leukemia. Two different IP technics were used, peroxidase-antiperoxidase (PAP) on 50 cases, avidin-biotin-peroxidase complex (ABC) on 42 cases, and both PAP and ABC on ten cases. Using 40% TdT+ cells to define positivity, 71 of 102 cases were IP+/IF+, 12 were IP+/IF-, and 19 were IP/IF-. The finding of 12 IP+/IF- cases suggests greater sensitivity of the IP method in detecting TdT+ acute leukemia. This greater sensitivity was demonstrated by both PAP and ABC technics. Direct comparison of ABC and PAP technics in ten cases showed the results by both methods to be similar. In addition, previous reports utilizing the IF method found approximately 10% of cases of AML to have greater than 10% TdT+ cells. The authors' findings for the IF method were similar (15%), but using the IP methods, the authors were able to detect greater than 10% TdT+ cells in 46% of AML cases and greater than 40% TdT+ cells in 31% of AML cases. The authors conclude that the IP technics for the detection of TdT offer several advantages and may be more sensitive relative to the IF method. Further, either the PAP or the ABC method is suitable for routine use in hematopathology laboratories.

The authors observed Auer rods in mature neutrophils, bands, and metamyelocytes in two children with AML with maturation (FAB M2). In order to determine if this rare finding was characteristic of AML M2, the authors reviewed the bone marrows and peripheral blood smears of 50 children with AML M2 and 50 children with other AML subtypes (FAB M1, M3, M4, and M5). They found Auer rods in mature neutrophils and band forms in 10 of 50 patients (20%) with AML M2 but in none of 50 patients with non-M2 AML. This finding was not related to the frequency of mature cells in the specimen or to the frequency of Auer rods in blasts. Cytogenetics did not show a consistent abnormality. The presence of Auer rods in mature granulocytes is unique to FAB M2 AML in the authors' series and supports the concept that in AML at least some of the mature myeloid cells are involved in the leukemic process.