Terminal deoxynucleotidyl transferase (TdT)-containing peripheral blood mononuclear cells during remission of acute lymphoblastic leukemia: low sensitivity and specificity prevent accurate prediction of relapse.

Serial samples of peripheral blood were obtained from 75 children with acute lymphoblastic leukemia in remission. An immunofluorescence assay was used to quantitate TdT-containing (terminal deoxynucleotidyl transferase-containing) cells in the mononuclear leukocyte fraction of these specimens. Nine relapses in 8 patients were preceded by elevations (0.12-0.70%) in peripheral blood terminal deoxynucleotidyl transferase-containing cells noted 1-33 weeks prior to relapse. No such elevations were observed prior to 6 relapses in 4 patients. Peripheral blood terminal transferase deoxynucleotidyl-containing cells were elevated (0.12-0.61%) in 22 children who did not relapse over a 5-82 week period of observation. The sensitivity (67%) and specificity (68%) of this assay are inadequate to establish which patients have minimal residual leukemia.