Terminal deoxynucleotidyl transferase positive acute myeloid leukaemia: an association with immature myeloblastic leukaemia.

The morphology, membrane markers and ultrastructural cytochemistry of 39 cases of acute myeloid leukaemia (AML) with variable proportion (10-99%) of terminal deoxynucleotidyl transferase (TdT) positive blasts was compared with that of 134 cases of TdT negative AML. The incidence of TdT positive AML was 22.5% and this was significantly higher in poorly differentiated myeloblastic (M0 and M1) types (54%) than in all other FAB subtypes (10%; P less than 0.001). Our findings suggest heterogeneity among TdT positive cases. Whilst the majority correspond to genuine TdT positive AML in which evidence for exclusive myeloid nature was demonstrated by phenotypic, cytochemical and ultrastructural markers, a distinct minority (22%) of cases had mixtures of lymphoid and myeloid blasts. A change in phenotype occurred in three out of six cases studied in relapse. There was no difference in the incidence of immunoglobulin (Ig) gene rearrangement between TdT positive (two out of 12) and TdT negative (one out of 11) cases, although published data suggests that Ig gene rearrangement is significantly more common in TdT positive cases. The determination of TdT in AML allows the identification of cases of mixed acute leukaemia which probably represent proliferations of multipotent progenitor cells. The majority of TdT positive cases, nevertheless, correspond to immature types of myeloblastic leukaemia which may constitute a clinically distinct subgroup.