Terminal Deoxynucleotidyl Transferase Measurements in the Differential Diagnosis of Adult Leukaemias

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Abstract
Summary. Terminal deoxynucleotidyl transferase (TDT) is an unusual DNA polymerase that does not use template information to synthesize new strands of DNA. It is normally found in high concentration in thymus (50 u/108 cells) and in low concentration in bone marrow (< 5 u/108). We report TDT measurements in the marrow and/or peripheral blood of 51 adult patients, 28 of whom had leukaemia. TDT is present in very high levels (> 50 u/108 cells) in leukaemic lymphoblasts and in low levels in leukaemic myeloblasts (< 9 u/108 cells). Of two patients who developed lymphosarcoma‐cell leukaemia following treatment of poorly differentiated lymphocytic lymphoma, one had high and one low levels of TDT in the leukaemic blast cells. Leukaemic cells from three of seven patients with chronic myeloid leukaemia in blast crisis had TDT levels within the range expected of acute lymphoblastic rather than acute myeloid leukaemia. High TDT in leukaemic cells probably marks them as derivatives of lymphoid progenitor, thymic or pluripotential stem cells. Quantitative assay of TDT may provide information useful in classifying haematological neoplasms.