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Purpose: In this study, we retrospectively reviewed the use of flow cytometry (FCM) in the diagnosis of inborn errors of immunity (IEIs) at a single center in Algeria. Sharing insights into our practical experience, we present FCM based diagnostic approaches adapted to different clinical scenarios., Methods: Between May 2017 and February 2024, pediatric and adult patients presenting with clinical features suggestive of immunodeficiency were subjected to FCM evaluation, including lymphocyte subset analysis, detection of specific surface or intracellular proteins, and functional analysis of immune cells., Results: Over a nearly seven-year period, our laboratory diagnosed a total of 670 patients (372 (55.5%) males and 298 (44.5%) females), distributed into 70 different IEIs belonging to 9 different categories of the International Union of Immunological Societies classification. FCM was used to diagnose and categorize IEI in 514 patients (76.7%). It provided direct diagnostic insights for IEIs such as severe combined immunodeficiency, Omenn syndrome, MHC class II deficiency, familial hemophagocytic lymphohistiocytosis, and CD55 deficiency. For certain IEIs, including hyper-IgE syndrome, STAT1-gain of function, autoimmune lymphoproliferative syndrome, and activated PI3K delta syndrome, FCM offered suggestive evidence, necessitating subsequent genetic testing for confirmation. Protein expression and functional assays played a crucial role in establishing definitive diagnoses for various disorders. To setup such diagnostic assays at high and reproducible quality, high level of expertise is required; in house reference values need to be determined and the parallel testing of healthy controls is highly recommended., Conclusion: Flow cytometry has emerged as a highly valuable and cost-effective tool for diagnosing and studying most IEIs, particularly in low-income countries where access to genetic testing can be limited. FCM analysis could provide direct diagnostic insights for most common IEIs, offer clues to the underlying genetic defects, and/or aid in narrowing the list of putative genes to be analyzed., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision., (Copyright © 2024 Tahiat, Belbouab, Yagoubi, Hakem, Fernini, Keddari, Belhadj, Touri, Aggoune, Stoddard, Niemela, Zerifi, Melzi, Aboura, Saad-Djaballah, Ferhani, Ketfi, Messaoudi, Bencharif Madani, Benhacine, Dehimi, Okka, Amroune, Fellahi, Bendahmane, Khoulani, Oukil, Soufane, Bourelaf, Boubidi, Boukhenfouf, Amine Ifri, Khelafi, Boudiaf, Khelifi Touhami, Meçabih, Boucelma, Zelaci, Gacem, Ladj, Mekki, Bensaadi, Benhalima, Zeroual, Bioud, Benameur, Bouhdjila, Bouzerar, Ibsaine, Maouche, Kedji, Smati, Boukari, Lambert, Rosenzweig, Notarangelo and Djenouhat.)

Background: Inborn errors of immunity (IEI) predispose patients to various infectious and non-infectious complications. Thanks to the development and expanding use of flow cytometry and increased awareness, the diagnostic rate of IEI has markedly increased in Algeria the last decade., Aim: This study aimed to describe a large cohort of Algerian patients with probable IEI and to determine their clinical characteristics and outcomes., Methods: We collected and analyzed retrospectively the demographic data, clinical manifestations, immunologic, genetic data, and outcome of Algerian IEI patients - diagnosed in the department of medical immunology of Beni Messous university hospital center, Algiers, from 2008 to 2021., Results: Eight hundred and seven patients with IEI (482 males and 325 females) were enrolled, 9.7% of whom were adults. Consanguinity was reported in 50.3% of the cases and a positive family history in 32.34%. The medium age at disease onset was 8 months and at diagnosis was 36 months. The median delay in diagnosis was 16 months. Combined immunodeficiencies were the most frequent (33.8%), followed by antibody deficiencies (24.5%) and well-defined syndromes with immunodeficiency (24%). Among 287 patients tested for genetic disorders, 129 patients carried pathogenic mutations; 102 having biallelic variants mostly in a homozygous state (autosomal recessive disorders). The highest mortality rate was observed in patients with combined immunodeficiency (70.1%), especially in patients with severe combined immunodeficiency (SCID), Omenn syndrome, or Major Histocompatibility Complex (MHC) class II deficiency., Conclusion: The spectrum of IEI in Algeria is similar to that seen in most countries of the Middle East and North Africa (MENA) region, notably regarding the frequency of autosomal recessive and/or combined immunodeficiencies., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Belaid, Lamara Mahammed, Drali, Oussaid, Touri, Melzi, Dehimi, Berkani, Merah, Larab, Allam, Khemici, Kirane, Boutaba, Belbouab, Bekkakcha, Guedouar, Chelali, Baamara, Noui, Baaziz, Rezak, Azzouz, Aichaoui, Moktefi, Benhatchi, Oussalah, Benaissa, Laredj, Bouchetara, Adria, Habireche, Tounsi, Dahmoun, Touati, Boucenna, Bouferoua, Sekfali, Bouhafs, Aboura, Kherra, Inouri, Dib, Medouri, Khelfaoui, Redjedal, Zelaci, Yahiaoui, Medjadj, Touhami, Kadi, Amireche, Frada, Houasnia, Benarab, Boubidi, Ferhani, Benalioua, Sokhal, Benamar, Aggoune, Hadji, Bellouti, Rahmoune, Boutrid, Okka, Ammour, Saadoune, Amroun, Belhadj, Ghanem, Abbaz, Boudrioua, Zebiche, Ayad, Hamadache, Ouaras, Achour, Bouchair, Boudiaf, Bekkat-Berkani, Maouche, Bouzrar, Aissat, Ibsaine, Bioud, Kedji, Dahlouk, Bensmina, Radoui, Bessahraoui, Bensaadi, Mekki, Zeroual, Chan, Leung, Tebaibia, Ayoub, Mekideche, Gharnaout, Casanova, Puel, Lau, Cherif, Ladj, Smati, Boukari, Benhalla and Djidjik.)