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-British (FAB) classification as the M3 subtype of acute myeloblastic leukemia (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 APL leukemic cells, which resemble monocytes. From 109 newly diagnosed acute leukemic patients, 48 were classified as AML and 16 of these patients (33%) had APL Mononuclear cells (MNC) of all APL patients were analyzed by flow cytometry for immunophenotypic features and forward and right angle scatter (FW-SC/RT-SC) light characteristics. Two clearly different FW-SC/RT-SC distribution patterns were recognized and defined as hypergranular (mature cells) and hypogranular (immature cells). Correlation 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. The most relevant cellular immunophenotypic feature was the high frequency of CD34+ cases (5/7) recorded exclusively in patients with the hypogranular pattern. An interesting finding was that MNCs of all APL patients changed color from beige to green in the course of 3-4 h, whereas none of the 93 specimens of non-M3 acute leukemia cases showed this phenomenon. Summarizing, the FW-SC/RT-SC characteristics of APL blast cells along with the immunophenotype may represent an objective and reproducible measurement system to distinguish microgranular (M3v) from typical (M3) APL In addition, the present study has identified another unique feature of APL based on the green color of their blast cells imparted by the high content in myeloperoxidase.