When Flow Is Turbulent: CD19 Intensity Is a Pitfall in Acute Leukemia Classification

Abstract Mixed phenotype acute leukemias (MPALs) are a rare category of acute leukemia with biologic and genetic heterogeneity and a prognosis generally inferior to those of single lineage leukemias. The diagnosis depends on immunophenotypic assessment with a comprehensive panel of monoclonal antibodies by multiparameter flow cytometry and lineage assignment by the World Health Organization criteria. CD19 assessment is required for establishing a B-cell component in MPALs, as is determination of its intensity of expression, and assessment of the additional B-cell markers CD79a, cytoplasmic CD22, and CD10. We report the case of an MPAL, initially classified as an acute myeloid leukemia with weak CD19 expression and later reclassified upon clear demonstration of a B-cell component; such cases highlight a pitfall in classification of acute leukemias, particularly given heterogeneous antigen expression between fluorochromes and subjectivity in judgment of intensity. Practical take-away points include the need for hypervigilance in marker performance patterns in flow cytometry, both between fluorochromes and over time, the benefits of repeat sampling and blast assessment between laboratories, and the potential quality improvements unlocked by expanded flow cytometry panels in diagnostic hematopathology.