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Essentials    FcγRIIa mediates life‐threatening heparin‐induced thrombocytopenia (HIT).  Most anti‐platelet factor (PF)4‐heparin IgGs are not pathogenic so diagnosis of HIT is challenging.  Dimeric rsFcγRIIa was used to quantify receptor‐binding activity of anti‐PF4‐heparin antibodies.  Dimeric rsFcγRIIa binding specifically correlated with occurrence of HIT.Summary   Background  Heparin‐induced thrombocytopenia ( HIT ) is a major and potentially fatal consequence of antibodies produced against platelet factor 4 ( PF 4)–heparin complexes following heparin exposure. Not all anti‐ PF 4–heparin antibodies are pathogenic, so overdiagnosis can occur, with resulting inappropriate use of alternative anticoagulation therapies that have associated risks of bleeding. However, definitive platelet functional assays are not widely available for routine analysis.    Objectives  To assess the utility of dimeric recombinant soluble Fcγ RII a (rsFcγ RII a) ectodomains for detecting  HIT  antibodies.    Patients/Methods  Plasma from 27 suspected  HIT  patients were tested for pathogenic anti‐ PF 4–heparin antibodies by binding of a novel dimeric Fcγ RII a ectodomain probe. Plasmas were also tested by the use of  PF 4–heparin IgG  ELISA , the Hemos IL  AcuStar  HIT  IgG‐specific assay, and a serotonin release assay ( SRA ).    Results  The dimeric rsFcγ RII a test produced no false positives and excluded four samples that were positive by IgG  ELISA . In this small patient cohort, the novel assay correctly assigned 93% of the suspected  HIT  patients, with two of the  HIT  patients being scored as false negatives. The improved discrimination of the novel assay over the IgG  ELISA , which scored four false positives, supports the mechanistic interpretation that binding of dimeric rsFcγ RII a detects pairs of closely spaced IgG antibodies in  PF 4–heparin immune complexes.    Conclusions  This study found the cell‐free, function‐based dimeric rsFcγ RII a assay to be convenient, simple, and potentially predictive of  HIT . The assay had improved specificity over the IgG  ELISA , and correlated strongly with the AcuStar  HIT  IgG‐specific assay, warranting further evaluation of its potential to identify  HIT  in larger patient cohorts.

Dimeric FcγR ectodomains detect pathogenic anti‐platelet factor 4–heparin antibodies in heparin‐induced thromobocytopenia