Combined flow cytometric assessment of cell surface antigens and nuclear TdT for the detection of minimal residual disease in acute leukaemia.

To define more precisely the immunophenotype of lymphoid blast cells, a new flow cytometric technique for the simultaneous detection of surface antigens and nuclear terminal deoxynucleotidyl transferase (TdT) was established. After staining for the cell surface marker, mononuclear cells were treated with paraformaldehyde (1%) and methanol to permeabilize the cell membrane. Then the cells were stained by indirect immunofluorescence using a rabbit anti-human TdT antibody. Dilution experiments were performed to reveal the sensitivity of the described flow cytometric assay: 0.02% leukaemic cells could reliably be detected above background level among normal peripheral blood lymphocytes. It is concluded that the double-staining procedure described here is a sensitive tool that contributes to the detection of minimal residual disease in a substantial proportion of acute leukaemias.