OBJECTIVES: The development of External Quality Assessment Schemes (EQAS) for clinical flow cytometry (FCM) is challenging in the context of rare (immunological) diseases. Here, we introduce a novel EQAS monitoring the primary immunodeficiency Orientation Tube (PIDOT), developed by EuroFlow, in both a 'wet' and 'dry' format. This EQAS provides feedback on the quality of individual laboratories (i.e., accuracy, reproducibility and result interpretation), while eliminating the need for sample distribution. METHODS: In the wet format, marker staining intensities (MedFIs) within landmark cell populations in PIDOT analysis performed on locally collected healthy control (HC) samples, were compared to EQAS targets. In the dry format, participants analyzed centrally distributed PIDOT flow cytometry data (n=10). RESULTS: We report the results of six EQAS rounds across 20 laboratories in 11 countries. The wet format (212 HC samples) demonstrated consistent technical performance among laboratories (median %rCV on MedFIs=34.5 %; average failure rate 17.3 %) and showed improvement upon repeated participation. The dry format demonstrated effective proficiency of participants in cell count enumeration (range %rCVs 3.1-7.1 % for the major lymphoid subsets), and in identifying lymphoid abnormalities (79.3 % alignment with reference). CONCLUSIONS: The PIDOT-EQAS allows laboratories, adhering to the standardized EuroFlow approach, to monitor interlaboratory variations without the need for sample distribution, and provides them educational support to recognize rare clinically relevant immunophenotypic patterns of primary immunodeficiencies (PID). This EQAS contributes to quality improvement of PID diagnostics and can serve as an example for future flow cytometry EQAS in the context of rare diseases.

Cancer Research Centre Salamanca Spain

CLIP Cytometry Department of Pediatric Hematology and Oncology 2nd Faculty of Medicine Charles University and University Hospital Motol Prague Czech Republic

Department of Diagnostic Sciences Ghent University Ghent Belgium

Department of Haematology University Hospital Ghent Ghent Belgium

Department of Laboratory Medicine University Hospital Ghent Ghent Belgium

Department of Laboratory Medicine University Hospital Leuven Leuven Belgium

Department of Microbiology Immunology and Transplantation KU Leuven Leuven Belgium

Department of Pediatric Pulmonology and Immunology and PID Research Laboratory University Hospital Ghent Ghent Belgium

Translational and Clinical Research Program Centro de Investigación del Cáncer and Instituto de Biología Molecular y Celular del Cáncer Consejo Superior de Investigaciones Científicas Department of Medicine IBSAL and CIBERONC University of Salamanca Salamanca Spain

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