The biologic significance of rare peripheral blasts after hematopoietic cell transplantation is predicted by multidimensional flow cytometry

A vexing problem after hematopoietic cell transplantation (HCT)for leukemia is assessing the biologic significance of low numbers of cells "suspicious " for relapse seen in morphologic review of peripheral blood smears (PBSs). In 27 patients, in apparent hematologic remission after HCT for leukemia, we studied the nature of such cells in PBSs to the endpoint of leukemic rel apse by using multi-dimensional flow cytometry (MDF) on blood or bone marro w aspirates. Based on abnormal cytometric maturational patterns, +/- cell s orting of blasts with fluorescence in situ hybridization with informative m arkers, we differentiated benign recovering myeloid and lymphoid precursors from leukemic cells. In 17 patients, blasts were characterized by MDF as n ormal early hematopoietic precursors, lymphoblasts, or NK cells. Of these p atients, 16 remained in remission for at least 42 days. In 10 patients, bla sts were characterized by MDF as a malignant immunophenotype; 9 relapsed wi thin 10 days and I relapsed 280 days after a graft-vs-leukemia effect MDF s tatus was strongly associated with a 90Xprobability of relapse even after a djusting for other potential variables. Morphologic triggered MDF character ization of peripheral blasts is a powerful and rapid tool for distinguishin g immature regenerative forms from early leukemic relapse.