This case shows a complex picture by flow cytometry. On review of the plot of CD45 versus log side scatter (Scatterplot 1), numerous populations can be readily identified, including lymphocytes, monocytes, “blasts” and maturing myeloid cells. It is important in diagnosis in cases such as this to look at the total picture of all markers and not just the markers associated with the “blast” cell region. This case showed varying positivity for CD34 and CD33 (Scatterplot 2), CD13 (Scatterplot 3) and myeloperoxidase (Scatterplot 4). Markers associated with monocytic differentiation, CD4, CD14 and CD64 were positive in a population of cells associated with light scatter of monocytes (Scatterplot 5 & Scatterplot 6). Additionally, CD2, a marker associated with lymphocytes, is present on a population of the monocytic cells (Scatterplot 7). This is consistent with the diagnosis of an AML FAB type M4 (myelomonocytic). When the morphology is taken into account with the cell surface markers, this would be consistent with a diagnosis of M4, eosinophilic variant (M4Eo).¹  Interestingly, this variant is associated with a characteristic translocation and is defined in the WHO classification as a recurrent translocation, inv(16)(p13q22) or t(16:16)(p13;q22).²

Cytogenetic analysis and FISH confirmed an inv(16) in this patient. The inv(16) has been shown to fuse the CBFB gene on 16q22 with the MYH11 gene on 16p13 giving rise to a chimeric protein. An image of this can be viewed at the following Web site http://www.vysis.com/ProbeImage_5081.asp Recent studies have suggested that monitoring the CBFbeta/MY11 fusion transcript by real time quantitative PCR may provide a method to discriminate between those patients destined for relapse and those who will experience continued complete remission.³

The presence of inv (16) or t(16:16) is associated with improved prognosis with a complete remission rate of 88% and a five year survival of 61%,⁴ second only to acute promyelocytic leukemia in terms of favourable outcome for AML. The morphologic, flow cytometric and cytogenetic confirmation of AML (M4Eo) is critical to clinician in management of this disease. With a favourable prognosis patients are managed with standard chemotherapy as opposed to stem cell transplantation in first remission.