Improved Identification of Megakaryoblasts by Flow Cytometry Relative to Immunohistochemistry
Karen Moser, Ian Bovio, Sally Hill, Sheryl R Tripp, Sherrie L Perkins, David W Bahler. University of Utah Health Sciences Center, Salt Lake City, UT; ARUP Laboratories, Salt Lake City, UT
Background: Immunophenotyping by flow cytometry (FC) or immunohistochemistry (IHC) is an essential tool for identifying cases of acute myeloid leukemia (AML) with megakaryoblastic differentiation. Demonstration of the megakaryocytic markers CD41, CD42b, and/or CD61 on blasts is used to identify megakaryoblasts. However, it is unclear which of these markers may be most relevant. Moreover, previous studies have not compared the sensitivity of FC to IHC in detecting megakaryoblasts.
Design: We retrospectively identified cases with increased myeloblasts (CD34 and/or CD117 positive) expressing at least one megakaryocyte-associated antigen reviewed between 1/2008 and 7/2011. Cases in which CD42b and CD61 were performed initially, or those with frozen cells available for additional studies were included. When paraffin blocks were available, IHC for CD42b and CD61 was performed on core biopsies and particle clots. Five-color FC immunophenotyping was performed using an FC500 cytometer (Beckman-Coulter).
Results: A total of 13 cases were identified (12 AML and 1 RAEB-2). Of the AML cases, 2 were in patients with trisomy 21, while the others were AML NOS. CD41 was analyzed by FC on 10/13 cases, CD61 on 13/13 cases, and CD42b on 13/13 cases. All cases were positive for CD61 (13/13) and CD41 (10/13). The majority of cases were negative for CD42b (8/13). Expression of CD42b on blasts was weaker than that seen on platelets, a helpful feature in excluding platelet adhesion to blasts. Eight of 13 cases had core biopsies available for IHC studies, and 5 of these had particle clots. The blasts were negative for CD61 in 8/8 cases, in contrast to the CD61 expression observed by FC. CD42b showed weak, focal staining in 2/8 cases, which were also CD42b positive by FC. There was no difference in CD61 and CD42b staining between the cores and clots, suggesting that decalcification does not negatively affect antigen expression.
Conclusions: Immunophenotyping by FC is more sensitive than IHC for detection of CD42b and CD61 on myeloblasts. The use of FC is important to include in diagnostic testing panels for AML to identify cases with megakaryocytic differentiation that may be missed by IHC alone. Evaluation of CD42b is useful in some cases to confirm that CD41 and CD61 expression is a property of the blast population. Given that megakaryoblasts in this series either lacked or expressed weak CD42b as compared with platelets, it is unlikely that the presence of CD41 and CD61 was due to platelet adherence to blasts.
Wednesday, March 21, 2012 1:00 PM
Poster Session VI # 248, Wednesday Afternoon