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Leukocyte fragments may interfere in the fluorescent platelet count of Sysmex XN hematology analyzers

Dear Editors, Platelet (PLT) counting in patients with thrombocytopenia continues to represent a major challenge in laboratory hematology, even despite the sophisticated technology used in modern hematology analyzers. These instruments do perform a complete blood count (CBC) with high precision and accuracy in normal blood samples, but the analytical performance in samples with extremely low cell counts and/or samples containing abnormal cells can be problematic. Not only is precision compromised in the very low concentration range, but there is also the risk of interfering substances causing spuriously increased PLT counts, which may mask a state of medically significant thrombocytopenia.1 Substances that have been implicated as interfering factors in PLT counting include small red blood cells (RBC), schizocytes and RBC fragments,1,2 white blood cell (WBC) fragments,3,4 abnormal proteins,5 and microorganisms.6,7 The type of technology used for counting PLT in a hematology analyzer is the dominant determinant of interference risk. Impedance technology is most prone to interference as it only measures particle volume: all particles that have a similar size as platelets can interfere.1 Optical technology is less sensitive to interferences as it makes use of twoor multidimensional measurement, making it more specific than simple volume measurement.8 Only immunological methods are free of interference due to the highly specific monoclonal antibodies used. For this reason, immunological technology was chosen as the international reference method for PLT counting.9 When the Sysmex Corporation (Sysmex) launched the XN-series of hematology analyzers, they presented a novel optical PLT method, called fluorescent platelets, abbreviated as PLT-F.10 This method uses a fluorescent oxazine dye that preferentially binds to nucleic acid-rich platelet organelles such as ribosomes and mitochondria.11-13 It has convincingly been demonstrated that RBC fragments do not interfere in the PLT-F method, even if they have the same size as platelets.13 In our laboratory, the Sysmex XN-2000 is operated as the routine hematology analyzer. This instrument uses impedance methodology as the default platelet count (PLT-I). Optical (PLT-O) and fluorescent (PLT-F) platelet methods are available as optional reflex tests. Recently, we have been evaluating the CELL-DYN Sapphire analyzer (Abbott Diagnostics) for PLT counting in patients who were candidates for platelet concentrate transfusion.14 During this project, we encountered several samples demonstrating discrepancies between the various PLT methods that we could attribute to circulating WBC fragments. Two of these cases are presented here. 1 | C A SE 1