I was interested in the thoughts of the group about how different people are reporting percentage of blasts determined by flow cytometry. While this isn't always a problem, particularly on original presentation when blast count are very high, it can become more difficult in determining small numbers of residual blasts following therapy. This is particularly true in pediatric B-cell ALL, where the percentage of blasts following induction therapy (<5%, 5% - 25%, >25%) is used to determine subsequent therapy. While I always correlate flow findings with morphology, we all know the difficulties of definitively identifying residual lymphoblasts in a post therapy marrow which is often hypocellular and contains numerous stripped nuclei and hematogones. In my experience, it is usually not difficult to identify a distinct population of abnormal residual blasts (10/19 positive, bright CD34, TdT, etc.). The question comes in determining what to use for the denominator. Using 3 color flow with CD45 gating, the nucleated RBCs usually form a distinct CD45 negative population. However, also mixed in this group of cells are non-viable cells and other debris. In addition, the blasts in some cases of pre B ALL fall in this region. Do you use all of the events in the histogram as the denominator or some other number. Thanks in advance for any input you might have on this issue. Christopher S. Bee, M.D. Medical Director, Cellular Immunology and Flow Cytometry Wilford Hall Medical Center Lackland AFB, Texas (210) 292-5455 DSN 554-5455 christopher.bee@59mdw.whmc.af.mil
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