We report a rare case of biclonal biphenotypic B-cell lymphocytosis with chronic lymphocytic leukaemia phenotype. Clonality was initially misjudged by immunophenotyping because of its polyphenotypic pattern. Polymerase chain reaction analysis revealed clonality of the immunoglobulin heavy chain and kappa light chain gene rearrangement, indicating the presence of a monoclonal B-lymphocyte population. Immunophenotyping was repeated after cell sorting and revealed two CD5+CD19+ populations with different light chain restriction. Different genetic abnormalities for both clones, as evidenced by fluorescence in situ hybridisation and hypermutation analysis, support the diagnosis of two independently originated but co-existing B-cell clones. This case illustrates the importance of using multiple techniques in the diagnostic work-up of haematological malignancies.

(BELG J HEMATOL 2014;5(4):143–7)