Prognostic significance of very low levels of MRD. In ∼10% of cases that were considered MRD negative (at 0.01%), a few events with an abnormal phentoype were detected, though quantitation of such a small population is not accurate. The EFS of such subjects was slightly poorer than that of those who were completely negative by flow cytometry (5-year EFS, 81% ± 3% vs 88% ± 1%).