{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,8]],"date-time":"2026-09-08T04:07:50Z","timestamp":1788840470722,"version":"build-2803163510"},"reference-count":32,"publisher":"Wiley","issue":"7","license":[{"start":{"date-parts":[[2026,7,7]],"date-time":"2026-07-07T00:00:00Z","timestamp":1783382400000},"content-version":"vor","delay-in-days":6,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0\/"},{"start":{"date-parts":[[2026,7,1]],"date-time":"2026-07-01T00:00:00Z","timestamp":1782864000000},"content-version":"tdm","delay-in-days":0,"URL":"http:\/\/doi.wiley.com\/10.1002\/tdm_license_1.1"}],"content-domain":{"domain":["onlinelibrary.wiley.com"],"crossmark-restriction":true},"short-container-title":["Pediatric Allergy Immunology"],"published-print":{"date-parts":[[2026,7]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                  <jats:sec>\n                    <jats:title>Background<\/jats:title>\n                    <jats:p>Newborn screening (NBS) for inborn errors of immunity increasingly uses T\u2010cell receptor excision circles (TREC) and, in some programs, Kappa\u2010deleting recombination excision circles (KREC) to detect early T\u2010 and B\u2010cell lymphopenia. While TREC\u2010based screening is well established, the significance and management of isolated low KREC remain unclear.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Objective<\/jats:title>\n                    <jats:p>To evaluate the implications of two different regional post\u2010screening algorithms for isolated low KREC and to characterize the clinical course, immunological profile, and follow\u2010up of term newborns with transient B\u2010cell lymphopenia.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>We performed a retrospective multicenter study of term newborns with isolated low KREC identified through NBS, confirmed B\u2010cell lymphopenia, and subsequent normalization during follow\u2010up. KREC levels, B\u2010cell counts, and serum immunoglobulins were assessed longitudinally by RT\u2010PCR and flow cytometry.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      Eighteen newborns were enrolled. At the first evaluation (V1; mean age 13.5\u2009days), all had marked peripheral B\u2010cell lymphopenia (CD19\n                      <jats:sup>+<\/jats:sup>\n                      \u2009\u2264\u20092%; mean 54 cells\/\u03bcL), although repeat dried blood spot (DBS) testing already showed KREC values above the diagnostic cutoff in 78%. By the second visit (V2; mean age 50\u2009days), B\u2010cell percentages and absolute counts normalized in all infants, with emerging IgA and IgM production, and normal KREC on whole blood. No infectious or immunological complications were recorded over 39.5 person\u2010years of follow\u2010up (mean 2.3\u2009\u00b1\u20091.7\u2009years).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusion<\/jats:title>\n                    <jats:p>Isolated low KREC at birth may identify newborns with transient B\u2010cell lymphopenia that resolves during early infancy. Repeat KREC testing on a second DBS before referral may represent a pragmatic triage step to reduce unnecessary immunological evaluations, while preserving early assessment for newborns with persistent abnormalities. Prospective studies are needed to refine post\u2010screening strategies.<\/jats:p>\n                    <jats:p>\n                      <jats:boxed-text content-type=\"graphic\" position=\"anchor\">\n                        <jats:graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" mimetype=\"image\/png\" position=\"anchor\" specific-use=\"enlarged-web-image\" xlink:href=\"graphic\/pai70421-gra-0001-m.png\">\n                          <jats:alt-text>image<\/jats:alt-text>\n                        <\/jats:graphic>\n                      <\/jats:boxed-text>\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1111\/pai.70421","type":"journal-article","created":{"date-parts":[[2026,7,7]],"date-time":"2026-07-07T11:24:36Z","timestamp":1783423476000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Transient B cell lymphopenia revealed by\n                    <scp>KRECs<\/scp>\n                    newborn screening: Post\u2010screening referral strategies, clinical course, and follow\u2010up"],"prefix":"10.1111","volume":"37","author":[{"given":"Guarnieri","family":"Valentina","sequence":"first","affiliation":[{"name":"Pediatric Immunology Unit Meyer Children's Hospital IRCCS  Florence Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0836-3610","authenticated-orcid":false,"given":"Drago","family":"Enrico","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Rehabilitation, Ophthalmology, Genetics Maternal and Child Health (DINOGMI), University of Genoa  Genoa Italy"},{"name":"Clinical and Experimental Immunology IRCCS Istituto Giannina Gaslini  Genoa Italy"},{"name":"Gene Therapy Program, Dana Farber\/Boston Children's Cancer and Blood Disorders Center Harvard Medical School  Boston Massachusetts 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