ABSTRACT
Background: The putative mechanism for vaccine-induced immune thrombotic thrombocytopenia (VITT) following ChAdOx1 nCoV-19 vaccination involves pathological anti-PF4 platelet-activating antibodies driving thrombosis and thrombocytopenia. Reports suggest platelet-activating antibodies decline within 12 weeks despite persistence of anti-PF4 by ELISA (Schonborn et al). Optimal duration of anticoagulation remains unclear. The Australian-New Zealand guidance recommends repeat testing of patients with serologically-confirmed VITT at 3, 6 and 12 months. Aim(s): To determine rate of persistence of anti-PF4 ELISA compared with functional platelet-activating tests in VITT. Method(s): We conducted a prospective study of clinicopathological adjudicated VITT patients (thrombocytopenic with D-dimer levels >5x ULN and thrombosis within 4-43 days of ChAdOx1 nCoV-19 vaccination) with a positive functional assay at diagnosis who had follow-up VITT testing. Anti-PF4 antibodies were measured by IgG-specific ELISA (Asserachrom, Stago Diagnostics). Platelet-activating antibodies were assayed using whole blood procoagulant platelet flow cytometry assay (Lee et al) or PF4-serotonin release assay. Result(s): 14 confirmed VITT patients (7 females, 7 males;median age 62.5 years [range, 40-80]) underwent follow-up testing, median follow- up of 12 weeks (range, 3-24). Median time to return a negative functional test was 12 weeks (range, 4-16) with platelet-activating antibodies persistent at 24 weeks in two patients. Median time to negative ELISA was not reached. During follow-up, 9 of 14 patients became negative by functional assay while anti-PF4 antibodies persisted in 8 of 12 (Figure 1). 93% of patients received a documented second vaccination (Comirnaty, median interval 16 weeks [range, 11-28]) and 71% have received an mRNA booster with no reported adverse events. Conclusion(s): Platelet-activating antibodies were persistent in 54% of tested patients at 12 weeks and anti-PF4 ELISA remained positive in the majority. Anti-PF4 antibodies persist longer than functional platelet-activating antibodies in VITT. Whether the decline in platelet-activating antibodies allows withdrawal of anticoagulation in these patients is unclear but could be a useful guide.
ABSTRACT
Background: Clinical and pathological features of vaccine induced immune thrombotic thrombocytopenia (VITT) following first dose ChAdOx1 nCoV19 vaccination (AZD1222) are well described. VITT post 2nd dose AZD1222 is not widely recognised however its plausible undiagnosed platelet activating antibodies formed after dose 1 may be boosted upon subsequent exposure. (Greinacher et al). Aim(s): We describe the clinicopathological features of suspected VITT post 2nd dose AZD1222 in Australia. Method(s): We conducted a prospective cohort study capturing sequential requests for confirmatory testing for suspected VITT after 2nd dose AZD1222. Testing was based upon key clinicopathological features: Thrombosis within timeframe (4-42 days), thrombocytopenia, D-Dimer >5xULN. High probability VITT (all criteria) underwent immunoassay Asserachrom HPIA IgG (Diagnostica Stago) and functional assay (serotonin release assay or procoagulant flow cytometry). Confirmed VITT cases were compared with those presenting after first dose AZD1222. Descriptive and comparative statistics were performed using SAS studio version 9.4. Result(s): 35 high probability VITT cases presented at a median of 14 days (IQR 9,18) post 2nd dose with platelet count 116 x 109/L (IQR 92, 139) and 14.5 fold increase in D-dimer (IQR 9.4,28.8) were tested. Median dose interval was 84 days (range 25-100), age 70 years (IQR 62, 78) with a male predominance of 66%. Platelet count and D-dimer fold change were less severe compared to cases post 1st dose (Table 1). PF4 antibodies by ELISA were detected in 4 (11%) and antibody mediated platelet activation demonstrable in 19 (54%). These cases were classified as confirmed VITT. Three catastrophic cases including 2 fatalities occurred (Graph 2). Concomitant factors were present in all and influenced outcome severity. Conclusion(s): We describe the largest cohort of suspected VITT post 2nd dose AZD1222. Confirmed cases are similar to those post D1 but platelet count and D-Dimer changes were milder. Similarly, catastrophic cases occurred however concomitant factors were present in all including shorter dosing intervals. (Table Presented).
ABSTRACT
Background: As a severe, though rare complication of COVID-19 vaccination, vaccine induced immune thrombotic thrombocytopenia (VITT) emerged in 2021 as a public health issue affecting vaccine confidence. Australia pre-emptively instituted a nationally coordinated system for diagnosis and management incorporating state based immunoassays (ELISA) for PF4 antibodies and national centralised functional testing using three functional assays. Aim(s): To evaluate the triage strategy based upon clinical presentation, thrombocytopenia and D-Dimer used to direct VITT testing. Method(s): Consecutive cases presenting between April 1 and June 11, 2021 referred for VITT testing in Australia were assessed regarding clinical classification, immune-assay and functional assay results, thromboses and mortality according to triage category (Table 1). Functional testing proceeded for all triaged as Probable VITT , ELISA positive and unusual site thromboses positive patients if triaged Less likely , and only for quality assurance purposes if Much less likely VITT . Anti-PF4 antibodies were measured by IgG-specific ELISA (Asserachrom, Stago Diagnostics). Platelet-activating antibodies were assayed using whole blood procoagulant platelet flow cytometry assay (Lee et al), PF4-serotonin release assay or multiplate multiple electrode aggregation. Result(s): 92% of 52 patients with a final diagnosis of VITT supported by a positive antibody assay (immunological or functional) were triaged prior to VITT specific testing into the high probability category. 100% ELISA positive individuals within the high and intermediate clinical probability groups were supported by detection of a platelet activating antibody using a functional assay and 0% in the low probability group. 16% of clinical VITT patients without confirmation of anti-PF4 antibodies by ELISA had VITT diagnosis supported by functional assay in a final clinico-pathological adjudication. Conclusion(s): Triage of testing according to clinical probability of VITT based on clinical criteria and standard laboratory tests was helpful in directing testing in resource constrained period. Demonstration of platelet activating antibodies in functional assays significantly contributes to definition of the VITT syndrome. (Table Presented).