Chan, Clarence W.

AbstractVon Willebrand factor (VWF) level and/or function is altered in von Willebrand disease (VWD), the most common heritable bleeding disorder worldwide. Laboratory assessment of VWF is continually evolving. Historically, the primary method for the assessment of VWF platelet-binding activity was the ristocetin cofactor assay (VWF:RCo). Contemporary alternative measures of VWF platelet-binding activity include VWF:GPIbR (recombinant; using ristocetin), VWF:GPIbM (recombinant; gain-of-function mutant), and monoclonal antibody. Recently, the American Society of Hematology, International Society on Thrombosis and Haemostasis, National Hemophilia Foundation, and World Federation of Hemophilia collaboration issued guidelines recommending the use of newer assays of VWF platelet-binding activity (VWF: GPIbM, VWF: GPIbR) over VWF:RCo, given known limitations of the VWF:RCo assay. Despite this recommendation, the newer VWF:GPIbM and VWF:GPIbR assays are not United States Food and Drug Administration cleared, limiting their availability in the United States. We sought to assess assay utilization trends, agreement of VWF testing methods, and imprecision of VWF testing (based on assigned sample type) from the College of American Pathologists Proficiency Testing Surveys. The analysis confirms that, while VWF antigen testing has low imprecision, the various VWF activity assays have significant interassay variability, with VWF:RCo showing greater imprecision than the newer GPIb-binding assays. The overall trends in assay utilization reflect the barriers to complete compliance with modern VWD diagnostic guidelines in North America.

AbstractVon Willebrand factor (VWF) level and/or function is altered in von Willebrand disease (VWD), the most common heritable bleeding disorder worldwide. Laboratory assessment of VWF is continually evolving. Historically, the primary method for the assessment of VWF platelet-binding activity was the ristocetin cofactor assay (VWF:RCo). Contemporary alternative measures of VWF platelet-binding activity include VWF:GPIbR (recombinant; using ristocetin), VWF:GPIbM (recombinant; gain-of-function mutant), and monoclonal antibody. Recently, the American Society of Hematology, International Society on Thrombosis and Haemostasis, National Hemophilia Foundation, and World Federation of Hemophilia collaboration issued guidelines recommending the use of newer assays of VWF platelet-binding activity (VWF: GPIbM, VWF: GPIbR) over VWF:RCo, given known limitations of the VWF:RCo assay. Despite this recommendation, the newer VWF:GPIbM and VWF:GPIbR assays are not United States Food and Drug Administration cleared, limiting their availability in the United States. We sought to assess assay utilization trends, agreement of VWF testing methods, and imprecision of VWF testing (based on assigned sample type) from the College of American Pathologists Proficiency Testing Surveys. The analysis confirms that, while VWF antigen testing has low imprecision, the various VWF activity assays have significant interassay variability, with VWF:RCo showing greater imprecision than the newer GPIb-binding assays. The overall trends in assay utilization reflect the barriers to complete compliance with modern VWD diagnostic guidelines in North America.

AbstractRare cases of COVID‐19 vaccinated individuals develop anti‐platelet factor 4 (PF4) antibodies that cause thrombocytopenia and thrombotic complications, a syndrome referred to as vaccine‐induced immune thrombotic thrombocytopenia (VITT). Currently, information on the characteristics and persistence of anti‐PF4 antibodies that cause VITT after Ad26.COV2.S vaccination is limited, and available diagnostic assays fail to differentiate Ad26.COV2.S and ChAdOx1 nCoV‐19‐associated VITT from similar clinical disorders, namely heparin‐induced thrombocytopenia (HIT) and spontaneous HIT. Here we demonstrate that while Ad26.COV2.S‐associated VITT patients are uniformly strongly positive in PF4‐polyanion enzyme‐linked immunosorbent assays (ELISAs); they are frequently negative in the serotonin release assay (SRA). The PF4‐dependent p‐selectin expression assay (PEA) that uses platelets treated with PF4 rather than heparin consistently diagnosed Ad26.COV2.S‐associated VITT. Most Ad26.COV2.S‐associated VITT antibodies persisted for >5 months in PF4‐polyanion ELISAs, while the PEA became negative earlier. Two patients had otherwise unexplained mild persistent thrombocytopenia (140‐150 x 103/µL) 6 months after acute presentation. From an epidemiological perspective, differentiating VITT from spontaneous HIT, another entity that develops in the absence of proximate heparin exposure, and HIT is important, but currently available PF4‐polyanion ELISAs and functional assay are non‐specific and detect all three conditions. Here, we report that a novel un‐complexed PF4 ELISA specifically differentiates VITT, secondary to both Ad26.COV2.S and ChAdOx1 nCoV‐19, from both spontaneous HIT, HIT and commonly‐encountered HIT‐suspected patients who are PF4/polyanion ELISA‐positive but negative in functional assays. In summary, Ad26.COV2.S‐associated VITT antibodies are persistent, and the un‐complexed PF4 ELISA appears to be both sensitive and specific for VITT diagnosis.