LIGHT SCATTER AND IMMUNOPHENOTYPIC CHARACTERISTICS OF BLAST CELLS IN TYPICAL ACUTE PROMYELOCYTIC LEUKEMIA AND ITS VARIANT

Acute promyelocytic leukemia (APL), defined by the French-American-Bri tish (FAB) classification as the M3 subtype of acute myeloblastic leuk emia (AML), is readily diagnosed by direct cytomorphologic examination . The potential difficulty in recognizing the microgranular variant of APL (M3v) stems from the morphological characteristics of Am. leukemi c cells, which resemble monocytes. From 109 newly diagnosed acute leuk emic patients, 48 were classified as AML and 16 of these patients (33% ) had APL. Mononuclear cells (MNC) of all APL patients were analyzed b y now cytometry for immunophenotypic features and forward and right an gle scatter (FW-SC/RT-SC) light characteristics. Two clearly different FW-SC/RT-SC distribution patterns were recognized and defined as hype rgranular (mature cells) and hypogranular (immature cells). Correlatio n between FW-SC/RT-SC patterns and the FAB system was poor: from eight patients classified as M3, six had the hypergranular pattern and two had the hypogranular pattern; from eight cases with M3, five and three patients had hypogranular and hypergranular patterns, respectively, T he most relevant cellular immunophenotypic feature was the high freque ncy of CD34+ cases (5/7) recorded exclusively in patients with the hyp ogranular pattern. An interesting finding was that MNCs of all APL pat ients changed color from beige to green in the course of 3-4 h, wherea s none of the 93 specimens of non-M3 acute leukemia cases showed this phenomenon Summarizing, the FW-SC/RT-SC characteristics of APL blast c ells along with the immunophenotype may represent an objective and rep roducible measurement system to distinguish microgranular (M3v) from t ypical (M3) APL. In addition, the present study has identified another unique feature of APL based on the green color of their blast cells i mparted by the high content in myeloperoxidase, Cytometry 32:286-290, 1998. (C) 1998 Wiley-Liss, Inc.