Serotonin Release Assay: Functional Assay for Heparin- and Vaccine-Induced (Immune) Thrombotic Thrombocytopenia.

The serotonin release assay (SRA) has been the gold-standard assay for detection of heparin-dependent platelet-activating antibodies and integral for the diagnosis for heparin-induced thrombotic thrombocytopenia (HIT). In 2021, a thrombotic thrombocytopenic syndrome was reported after adenoviral vector COVID-19 vaccination. This vaccine-induced thrombotic thrombocytopenic syndrome (VITT) proved to be a severe immune platelet activation syndrome manifested by unusual thrombosis, thrombocytopenia, very elevated plasma D-dimer, and a high mortality even with aggressive therapy (anticoagulation and plasma exchange). While the platelet-activating antibodies in both HIT and VITT are directed toward platelet factor 4 (PF4), important differences have been found. These differences have required modifications to the SRA to improve detection of functional VITT antibodies. Functional platelet activation assays remain essential in the diagnostic workup of HIT and VITT. Here we detail the application of SRA for the assessment of HIT and VITT antibodies.

Vaccine induced thrombotic thrombocytopenia post dose 2 ChAdOx1 nCoV19 vaccination: Less severe but not void of catastrophic outcomes

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).

INFLAMMATION OVERDRIVE: A CONFIRMED DIAGNOSIS OF HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS RESULTING FROM COVID-19

SESSION TITLE: Unique Inflammatory and Autoimmune Complications of COVID-19 Infections SESSION TYPE: Rapid Fire Case Reports PRESENTED ON: 10/19/2022 12:45 pm - 1:45 pm INTRODUCTION: Coronavirus disease 2019 (COVID-19) can manifest as a severe immunologic syndrome known as hemophagocytic lymphohistiocytosis (HLH). HLH is a hyper-inflammatory state with a lethal mortality rate, especially when discovered late in the disease process. The optimal timely approach to diagnosis and treatment of secondary HLH in COVID-19 is unclear, however, risk stratification with Hscore using biomarkers can be useful to increase confidence in an HLH diagnosis. CASE PRESENTATION: A 36-year-old morbidly obese male with a history of well controlled mild intermittent asthma presented to the hospital complaining of a one week history of dyspnea and cough after failing outpatient COVID-19 treatment. Upon arrival, he was hypoxic on room air and was placed on non-invasive ventilation. He unfortunately decompensated further and was transferred to the intensive care unit where he was intubated for severe hypoxia and increased work of breathing. His course was complicated by superimposed bacterial pneumonia, vasopressor dependent septic shock, and anuric acute kidney injury requiring continuous renal replacement therapy. He remained profoundly hypoxic despite rescue therapy with multiple sessions of prone ventilation. On hospital day seventeen his platelets declined acutely and a serotonin release assay confirmed heparin-induced thrombocytopenia. His clinical status remained tenuous into the third week of admission. Notably, he developed persistent fever with associated bicytopenia and elevated lactate dehydrogenase, D-dimer, fibrinogen, triglycerides, and aspartate aminotransferase. His calculated Hscore was 189. Hematology recommended initiating HLH therapy with daily dexamethasone and etoposide, however the latter was held due to the patient's rapid hemodynamic decline. The patient succumbed to illness after a twenty-day hospitalization. His HLH was confirmed with a positive postmortem soluble-IL-2-receptor test. DISCUSSION: Proposals of routine HLH screening in critically ill patients are endorsed to promote early detection of this morbid condition. Calculating Hscore using vital signs, imaging, laboratory tests, and patient history can guide suspicion of diagnosis, since HLH-specific markers are often not feasible. Hscores more than 169 correspond to 93% sensitivity and 86% specificity in HLH diagnosis. Immunosuppression is standard therapy with hematology guidance due to the complex pathophysiology and limited research. CONCLUSIONS: This case emphasizes the importance of understanding the relationship between COVID-19 and secondary HLH. A timely diagnosis is vital in order to attempt to effectively treat a syndrome that carries a 65% mortality rate. Reference #1: Dimopoulos G, Mast Q. de, Markou N, et al. Favorable Anakinra responses in severe COVID-19 patients with secondary hemophagocytic lymphohistiocytosis. Cell Host Microbe 2020;doi: 10.1016/j.chom.2020.05.007. PubMed PMID: 32411313. Reference #2: Bauchmuller K, Manson JJ, Tattersall R, et al. Haemophagocytic lymphohistiocytosis in adult critical care. J Intensive Care Soc 2020;21:256–68. Reference #3: Schnaubelt, Sebastian MDa,∗;Tihanyi, Daniel MDb;Strassl, Robert MDc;Schmidt, Ralf MDc;Anders, Sonja MDb;Laggner, Anton N. MDa;Agis, Hermine MDd;Domanovits, Hans MDa Hemophagocytic lymphohistiocytosis in COVID-19, Medicine: March 26, 2021 - Volume 100 - Issue 12 - p e25170 doi: 10.1097/MD.0000000000025170 DISCLOSURES: No relevant relationships by Kristina Catania No relevant relationships by Katie Kennedy No relevant relationships by Josef Kinderwater No relevant relationships by MaryKate Kratzer no disclosure submitted for Ogugua Obi;

Anti-platelet factor 4 pathologic antibodies after chadox1 ncov-19 vaccination mauriziano hospital diagnosis and findings

Introduction.Vaccine induced immune thrombocytopenia and thrombosis (VITT) following ChAdOx1 nCOV-19 vaccine has been described, associated with unusual site thrombosis, thrombocytopenia, raised D-dimer and high titre immunoglobulin-G (IgG) class anti-Platelet Factor 4 (PF4) antibodies. Laboratory management of suspected cases begins with a sensitive anti PF4 antibodies binding assay (PF4-ELISA). If the PF4 binding assay is negative, this patient does not have Heparin induced Thrombocytopenia (HIT) or VITT. If the PF4 binding assay is positive, the positivity should be confirmed with one or multiple HIT functional assays as available, such as the serotonin release assay (SRA), heparin-induced platelet activation assay, platelet aggregation (PAT) test, flow cytometry test.Methods.We summarized clinical and laboratory findings of 7 patients in Piedmont who developed thrombosis and thrombocytopenia following AZD1222 vaccination. Plasma from all patients was used to test for anti PF4 antibodies by 2 different ELISA assays (Immucor and Stago) and by 2 different HIT functional assays, PAT and flow cytometry (HIT alert test) both performed in the presence of heparin, PF4 or both.Results.The 7 patients [6 males and 1 female, median age: 38 (range:31-76)] presented with thrombosis 2 to 17 days post vaccination: 5 males had deep vein thrombosis not in unusual sites, 1 male had stroke and the female had cerebral venous thrombosis (CVT). None had received heparin prior symptoms onset. Only 2 out of 7 patients tested positive for anti PF4 ELISA antibodies with both assays: the men with stroke showed low positivity (OD = 0,56 and 0,41) and the female with CVT strong positivity (OD = 3,2 and 3,87). Only the female patient with CVT tested positive with both HIT functional assays, PAT and HIT alert cytometry test in the presence of PF4 independently of heparin. Both assays were inhibited by high concentrations of heparin.Conclusions. In our limited experience VITT demonstrated to be an extremely rare event in the context of AZD1222 COVID-19 vaccination even in the subset of patients with thrombosis and thrombocytopenia.

Evaluation of Heparin-Induced Thrombocytopenia in Severe COVID-19 (Case Series)

Introduction: Thrombocytopenia is associated with severe coronavirus disease-2019 (COVID-19) with the reported incidence rate to be between 5-41.7%. Heparin-induced thrombocytopenia (HIT) is typically considered a minor contributor with low incidence of 0.2-3%. However, one study noted an 8% incidence of HIT in patients with severe COVID-19. Due to the potential higher risk of HIT and the baseline higher risk of thrombosis in severe COVID-19 infection, it is important to evaluate HIT prevalence in severe COVID-19 patients. We reviewed seven potential HIT cases. Case: All seven patients with positive heparin-PF4 antibodies had severe acute respiratory distress syndrome. D-dimer was elevated in four. Median duration of heparin and/or low-molecular-weight heparin exposure was 16 days. In five cases, HIT diagnosis was made greater than 10 days post exposure. All patients had intermediate to high pretest probability for HIT. Three patients had confirmed thrombosis, and one experienced multiple clotted lines (despite negative imaging for thrombosis). Argatroban was initiated in all patients. Serotonin release assay (SRA) was obtained for two patients-one resulted positive. Only two patients survived to be discharged from the intensive care unit/hospital. Discussion: HIT may be a larger contributor to thrombocytopenia in severe COVID-19 patients. When reviewing data from pre-COVID-19 years, the incidence rate of those screened for HIT ranged between 0-4.8%. During our study period, the incidence rate of those screened for HIT was 12.9%. 78% of those with positive antibodies had COVID-19. This potential increased incidence may be attributed to patient/disease specific factors or to increased doses of heparin treatment of possible thrombosis. Until it is further characterized, it is important to screen thrombocytopenic patients with severe COVID-19 for HIT. Conclusion: True incidence of HIT in severe COVID-19 is unclear, but it may be an important contributor to thrombocytopenia that can affect patients' thrombosis risk and anticoagulation choice which merits further review.

The Deglycosylated Form of 1E12, a Monoclonal Anti-PF4 IgG, Strongly Inhibits Antibody-Triggered Cellular Activation in Vaccine-Induced Thrombotic Thrombocytopenia, and Is a Potential New Treatment for Vιττ

[Formula presented] Introduction Vaccine-induced thrombotic thrombocytopenia (VITT) is a severe complication of recombinant adenoviral vector vaccines used to prevent COVID-19, likely due to anti-platelet factor 4 (PF4) IgG antibodies. The specificity and platelet-activating activity of VITT antibodies strikingly resemble that of antibodies detected in “autoimmune” heparin-induced thrombocytopenia (HIT), but their features remain poorly characterized. In particular, a better knowledge of these antibodies should help to understand the mechanisms leading to hypercoagulability and the particular thrombotic events observed in VITT, but rarely in typical HIT. We have recently developed a chimeric IgG1 anti-PF4 antibody, 1E12, which strongly mimics “autoimmune” HIT antibodies in terms of specificity and cellular effects. Therefore, we assessed whether 1E12 could mimic VITT antibodies. We then evaluated the capability of DG-1E12, a deglycosylated form of 1E12 unable to bind FcγR, to inhibit cellular activation induced by VITT antibodies. Methods and Results Using a PF4-sensitized serotonin release assay (PF4-SRA) (Vayne C, New Engl J Med, 2021), we demonstrated that 1E12 (5 and 10 μg/mL) strongly activated platelets, with a pattern similar to that obtained with human VITT samples (n=7), i.e. in a PF4-dependent manner and without heparin. This platelet activation was inhibited by low heparin concentration (0.5 IU/mL), an effect also observed with VITT samples. Serotonin release induced by 1E12 was also fully inhibited by IV-3, a monoclonal antibody blocking FcγRIIa, or by IdeS, a bacterial protease that cleaves IgG and strongly inhibits the binding of IgG antibodies to FcγRIIa. This inhibitory effect of IV-3 and IdeS strongly supports that interactions between pathogenic anti-PF4 IgG and FcγRIIa play a central role in VITT. Incubation of 1E12 or VITT samples with isolated neutrophils (PMN) and platelets with PF4 (10 µg/mL) strongly induced DNA release and NETosis, supporting that PMN are involved in the processes leading to thrombosis in VITT. Furthermore, when whole blood from healthy donors incubated with 1E12 or VITT plasma was perfused in capillaries coated with von Willebrand Factor, numerous large platelet/leukocyte aggregates containing fibrin(ogen) were formed. To investigate whether 1E12 and VITT antibodies recognize overlapping epitopes on PF4, we then performed competitive assays with a deglycosylated form of 1E12 (DG-1E12), still able to bind PF4 but not to interact with Fcγ receptors. In PF4-SRA, pre-incubation of DG-1E12 (50 µg/mL) dramatically reduced platelet activation induced by VITT antibodies, which was fully abrogated for 9 of the 14 VITT samples tested. Additional experiments using a whole blood PF4-enhanced flow cytometry assay recently designed for VITT diagnosis (Handtke et al, Blood 2021), confirmed that DG-1E12 fully prevented platelet activation induced by VITT antibodies. Moreover, when platelets and neutrophils were pre-incubated with DG-1E12 (100 µg/mL), NETosis and thus DNA release, nuclear rounding, and DNA decondensation induced by VITT antibodies were completely inhibited. Finally, DG-1E12 (100 µg/mL) also fully abolished VITT antibody-mediated thrombus formation in whole blood in vitro under vein flow conditions. Comparatively, DG-1E12 did not inhibit ALB6, a murine monoclonal anti-CD9 antibody, which also strongly activates platelets in a FcγRIIa-dependent manner. Conclusions Our results show that 1E12 exhibits features similar to those of human VITT antibodies in terms of specificity, affinity and cellular effects, and could therefore be used as a model antibody to study the pathophysiology of VITT. Our data also demonstrate that DG-1E12 prevents blood cell activation and thrombus formation induced by VITT antibodies, likely due to the competitive effect of its Fab fragment on antibody binding to PF4. DG-1E12 may allow the development of a new drug neutralizing the pathogenic effect of autoimmune anti-PF4 antibodies, such as those associated with VITT. Disclosures: T iele: Bristol Myers Squibb: Honoraria, Other;Pfizer: Honoraria, Other;Bayer: Honoraria;Chugai Pharma: Honoraria, Other;Novo Nordisk: Other;Novartis: Honoraria;Daichii Sankyo: Other. Pouplard: Stago: Research Funding. Greinacher: Macopharma: Honoraria;Biomarin/Prosensa: Other, Research Funding;Sagent: Other, Research Funding;Rovi: Other, Research Funding;Gore inc.: Other, Research Funding;Bayer Healthcare: Other, Research Funding;Paringenix: Other, Research Funding;BMS: Honoraria, Other, Research Funding;MSD: Honoraria, Other, Research Funding;Boehringer Ingelheim: Honoraria, Other, Research Funding;Aspen: Honoraria, Other, Research Funding;Portola: Other;Ergomed: Other;Instrument Laboratory: Honoraria;Chromatec: Honoraria. Gruel: Stago: Other: symposium fees, Research Funding. Rollin: Stago: Research Funding.

Flow Cytometric Detection of Procoagulant Properties of Plasma from Patients with Clinically Confirmed Vaccine-Induced Immune Thrombotic Thrombocytopenia

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a severe prothrombotic complication of adenoviral vaccines including ChAdOx1 nCoV-19 (AstraZeneca) vaccine. The putative mechanism involves formation of pathological anti-PF4 antibodies that activate platelets via the FcγRIIa receptor to drive thrombosis and the associated thrombocytopenia. Functional assays are important in the VITT diagnostic pathway as not all detectable PF4 antibodies are pathogenic. Detection of procoagulant platelets (platelets supporting thrombin generation) in presence of PF4 has been proposed as a diagnostic assay for VITT (Althaus et al). Procoagulant platelets are not typically generated in response to low level agonist stimulation;however, combination of ligand binding of G-protein coupled receptors (GPCR) (eg. PAR1) and ITAM linked receptors (eg. GPVI, CLEC2 and FcγRIIa) synergistically induce procoagulant platelet formation. Here, we describe an alternative flow cytometric assay to diagnose VITT. We hypothesized that priming of platelets with a PAR1 agonist at a level sufficient to release PF4, but insufficient to generate a significant procoagulant response in donor platelets, would provide a platform in which procoagulant response would be dependent on presence of FcγRIIa dependent procoagulant antibodies in patient plasma, without requirement for additional PF4. Methods: Our previously established flow cytometry-based procoagulant platelet assay (using cell death marker GSAO and P-selectin) was modified to incorporate exogenous patient plasma and performed on whole blood from healthy donors screened for FcγRIIa responsiveness (aggregation response to anti-CD9 antibody, ALB6), primed with 5 μM SFLLRN. The assay was performed on Australian patients referred for confirmatory VITT testing with probable VITT (confirmed thrombosis within 4-42 days of ChAdOx1 nCov-19 vaccination, D-Dimer > 5x ULN, platelets < 150 x 10 9/L or falling platelet count) after screening on PF4/heparin ELISA (Asserachrom HPIA IgG Assay, Stago Diagnostics). Procoagulant response was also measured in presence of 0.5 U/mL and 100 U/mL heparin, monoclonal FcγRIIa blocking antibody, IV.3, and intravenous immunoglobulin, IVIg. Some plasmas were incubated with ChAdOx1 nCoV-19 or SARS-CoV-2 spike protein. Flow cytometry positive patients were also tested by serotonin release assay (SRA) and multiplate aggregometry. Clinical correlation was obtained. Results: Citrated plasma from 49 unique patients with suspected VITT are reported. Plasma from ELISA+ve patients with clinical picture consistent with VITT (n=31), significantly increased the procoagulant platelet proportions in healthy donors in presence of 5 μM SFLLRN (p<0.0001, Figure 1A). This increase was not seen with plasma from healthy donors (n=14);or individuals exposed to ChAdOx1 nCov-19 vaccine without VITT: thrombocytopenic thrombosis patients who were ELISA-ve and SRA-ve (n=14);or low-level ELISA+ve patients without thrombocytopenia who were negative by either multiplate or SRA (n=4). The procoagulant platelet response induced by VITT positive plasma was reduced with low dose heparin (0.5 U/mL, p<0.01) except for 3 patients who showed a heparin-enhancing effect (Figure 1B). High dose heparin (100 U/mL, p<0.0001), IV.3 (10 µg/mL, p<0.0001) or IVIg (10 mg/mL, p<0.0001) abolished the procoagulant response (Figures 1C-D). The in vitro effect of IVIg was predictive of the in vivo response to IVIg therapy (Figure 1E). Addition of SARS-CoV-2 spike protein had no effect on the procoagulant platelet response. ChAdOx1 nCov-19 had an inconsistent effect on procoagulant platelet formation in presence of VITT plasma. Use of donors without a robust aggregation response to ALB6 resulted in false negative results. Conclusion: Induction of FcγRIIa dependent procoagulant response by patient plasma, suppressible by high dose heparin and IVIg, is highly indicative of VITT in the correct clinical circumstance. This assay modification of priming donor platelets from known FcγRIIa responsive donors ith a GPCR agonist to potentiate the ITAM signaling from platelet activating immune complexes, results in a sensitive and specific assay. This may represent a functional platform that can be adopted into diagnostic laboratories to identify patients with platelet-activating antibodies and potentially predict treatment responses. [Formula presented] Disclosures: No relevant conflicts of interest to declare.

Spontaneous Heparin-Induced Thrombocytopenia and COVID-19 Infection

Background and Objective: Heparin-induced thrombocytopenia (HIT) can develop if immune responses to infections become pathologic in the presence of heparins. Low molecular weight heparin or unfractionated heparin are recommended for prophylaxis and treatment of venous thromboembolic disease in hospitalized patients with Covid-19 infection but may trigger HIT. Our aim is to alert clinicians that HIT occurs in association with Covid-19 infections even in the absence of prior exposure and may not be easily recognized without a high index of suspicion. Case Summary: A 33-year-old previously healthy male was initially evaluated for low grade fever, dyspnea without hypoxia and cough. A Covid-19 PCR swab was negative despite a recent exposure. He was treated with azithromycin. However, his symptoms did not improve, he then developed right leg swelling and hypoxia, so he was re-evaluated. CTA of the chest showed bilateral pulmonary emboli and ground-glass opacities at the lung bases. Venous Duplex Ultrasound showed non-occlusive thrombus in the deep veins of right lower extremity. He was hospitalized and placed on oxygen and heparin. Covid-19 swab was negative again. Laboratory tests before heparin showed a decreased platelet count of 64,000 k/ul, elevated prothrombin time of 16.4 seconds, normal aPTT at 30.8 seconds, decreased serum fibrinogen at 120 mg/dl and markedly elevated D-dimer at 59,966 ng/ml. Lupus anticoagulant and anti-phospholipid antibody tests were negative. On heparin at the desired therapeutic aPTT target range, the right leg became significantly swollen and painful by day five. Platelet count had decreased further to 39,000 k/ul. Repeat doppler examination of the right leg now showed more severe and extensive deep venous thrombosis. D-dimer had increased to 125,133 ng/ml. The HIT 4T score was 4, suggesting intermediate probability. Rapid HIT immunoassays on 2 separate samples were positive. Heparin was discontinued and he was placed on argatroban. Serotonin release assays on 2 separate samples came back positive. Suspicion for Covid-19 infection remained high and so a Covid-19 serology sample was obtained which was positive for IgG. A repeat nasopharyngeal swab at this time turned positive. He did not receive any COVID specific treatments. As viability of his leg appeared threatened, he underwent right iliofemoral vein thrombectomy with arteriovenous fistula creation. He improved on argatroban and was transitioned to apixaban with gradual normalization of hemostasis laboratory parameters, improvement in hypoxemia and fading clinical symptoms, he was discharged home on day 15. Conclusion: Current consensus guidelines for thromboprophylaxis and treatment of thromboembolism in hospitalized patients with Covid-19 infection recommend heparins as primary therapy to reduce morbidity and mortality. However, our report in addition to the two previous reports of HIT in Covid-19 patients illustrate that HIT can be a complication in the setting of Covid-19 infection. Further, our report also highlights that HIT with thrombosis can occur in a spontaneous manner in the absence of prior heparin exposure, which has been so far studied only in bacterial infection with the hypothesis that Platelet factor 4 (PF4) can bind to negatively charged polysaccharides on the surface of bacteria, triggering an immune response. Disclosures: No relevant conflicts of interest to declare.

Determining the Rate of Anti-PF4 Antibody Positive Results in Patients Presenting with Venous Thrombosis but a Normal Platelet Count Following ChAdOx1 Ncov-19 Astrazeneca Vaccination: An Australian Combined State Testing Centre Experience

Introduction The CHaDOx1 nCov-19 AstraZeneca (AZ) vaccination has been associated with an antibody-mediated prothrombotic syndrome, termed “Thrombosis with Thrombocytopenia Syndrome” (TTS)[1-3]. The current diagnostic criteria for TTS are thrombosis (venous or arterial) within 4-42 days of AZ vaccine, thrombocytopenia and presence of an antibody to platelet factor 4 (PF4)[4, 5]. TTS commonly presents with cerebral venous sinus thrombosis (CVST) or splanchnic vessel thrombosis (SVT), but outside of TTS, CVST and SVT are uncommon, with an overall incidence of less than 0.5 per 100,000 [5-7]. Deep vein thrombosis (DVT) and pulmonary embolism (PE) are also associated with TTS, however the background incidence of venous thromboembolism (VTE) is much higher, with 1-2 events per 1000 patients per year[7, 8]. Therefore, many patients will present with new VTE and a recent exposure to the AZ vaccine, requiring consideration of investigation for TTS. Recent data suggests that PF4 antibodies can be seen in up to 8% of patients without thrombosis but following AZ vaccination[9]. We hypothesised in patients with recent AZ vaccination, new VTE but with a normal platelet count, that the incidence of a PF4 antibody is similar to this background rate of PF4 positivity. If confirmed, then presence of a normal platelet count despite new VTE and recent vaccination may exclude TTS without the need for PF4 antibody testing. We present our preliminary data on the rates of PF4 antibody positivity amongst patients with VTE, recent AZ vaccination and a normal platelet count at presentation. Aim and Methods To assess the incidence of PF4 ELISA positive results in patients with confirmed VTE, recent vaccination (within 4-42 days) with the first dose of AZ vaccine, and platelet count greater than 150x10 9/L. A retrospective audit of cases referred with suspected TTS to Monash Pathology, Melbourne, Victoria, and New South Wales Health Pathology at Royal Prince Alfred Hospital and St George Hospital sites Sydney, New South Wales, Australia, for testing for anti PF4 antibodies from 1 st April to 31 st July 2021. Patient sera were tested for the Anti-PF4 antibody using the STAGO Asserachrom HPIA IgG ELISA (Asnières sur Seine, France). For patients with a positive PF4 antibody test additional testing was sought for either the presence of platelet activating antibodies with a flow cytometry-based assay or the presence of spontaneous serotonin release without heparin in the serotonin release assay. Results From April 1 st to July 31 st 350 tests were run on 332 patients. 91 patients met our criteria, of whom 51 were female and 40 male, with a median age of 73 years. Median platelet count at presentation was 226x10 9/L, and median D dimer values were 10 times the upper limit of normal. 86 patients had either DVT, PE or both, including 2 with upper limb DVT, and 5 patients had PE with concurrent arterial events (1 axillary artery thrombosis, 3 arterial strokes, 1 coronary artery thrombosis). Further details are presented in table 1. 82 patient samples tested negative for anti-PF4 antibodies by ELISA, 5 were positive, and were 4 weak positive/equivocal (see table 2 for further details). Of the positive results, 3 had functional testing available, of which 2 were negative, and 1 showed discordant results, with a positive SRA but negative flow cytometry. None of the weak positive/equivocal cases had functional testing results available. Of the negative ELISA results, 5 patients had functional testing results available, of which 4 were negative. One of these cases had positive testing by flow cytometry, but negative by SRA (case included in table 2). Conclusion In our Australian cohort of patients with their first dose of AZ vaccine and new VTE within 4-42days, but a normal platelet count (therefore not fulfilling the clinical criteria of TTS), the incidence of a positive PF4 antibody test was 9/91 (9.9%, 95% CI 3.7-15.9%) and only one had evidence of platelet activating antibodies. This observed rate is similar to that observed in healthy patients wi hout thrombosis who received AZ vaccination as described by Thiele et. al., 2021. Further confirmation in a larger cohort of VTE patients is required, but if confirmed, then PF4 ELISA testing in patients with VTE and normal platelet count post AZ vaccine may not be required, and should give clinicians confidence to institute routine management. [Formula presented] Disclosures: No relevant conflicts of interest to declare.

Thrombocytopenia with and without Thrombosis Following COVID-19 Vaccination

Introduction: Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-COV-2) were rapidly developed during the COVID-19 pandemic. There is emerging evidence of adverse hematologic effects including thrombocytopenia, for recipients of both mRNA and adenovirus-vector vaccines. We report findings in 9 patients diagnosed with thrombocytopenia following administration of an approved COVID-19 vaccine and managed according to the ASH COVID-19 Thrombosis with Thrombocytopenia Syndrome (TTS) recommendations [https://www.hematology.org/covid-19/vaccine-induced-immune-thrombotic-thrombocytopenia]. Methods: The study population included adults >18 years of age presenting to a large Canadian tertiary care centre, between April 1 st, 2021 and May 31 st, 2021, with new-onset thrombocytopenia within 31 days of receiving COVID-19 vaccination. Vaccines approved during this time period in Canada included BNT162b2 (Pfizer-BioNTech, mRNA) vaccine, mRNA-1273 (Moderna, mRNA) vaccine, and ChAdOx1-S (AstraZeneca (AZ), adenovirus vector-based) vaccine. We report on the initial presentation, management and 90-day outcomes. Results: Among 9 patients with thrombocytopenia included in this cohort, the median age was 55 years (range 24 to 73), and 5 patients (56%) were female. Seven patients received AZ and 2 had Pfizer vaccines. All events occurred after the first dose of COVID-19 vaccine with a median of 11 days between vaccination and presentation to hospital (range 2 to 31). All patients admitted to hospital tested negative for COVID-19 by PCR. Four patients developed TTS, as confirmed on both HIT ELISA and serotonin release assay, following AZ vaccination. Two patients presented with headaches and were diagnosed with cerebral vein thrombosis (CVT);and 2 presented with dyspnea and were diagnosed with venous thromboembolism (VTE). Platelet counts at presentation ranged 14-136 and D-dimer ranged 4000 to >44,000. HIT ELISA optical densities were persistently elevated. Three patients were admitted to hospital and received non-heparin parenteral anticoagulation, IVIG, and steroids. One patient had refractory thrombocytopenia with extension of CVT prompting use of therapeutic plasma exchange. Two patients had recurrent thrombocytopenia within 30 days of discharge and responded to repeat IVIG treatment. Five patients developed immune thrombocytopenic purpura (ITP), four without associated thrombosis and one patient with history of ITP and splenectomy, maintained on Revolade, presented with ITP flare and deep vein thrombosis. Presenting complaints included petechial rash and minor bleeding such as epistaxis. Platelet counts ranged from undetectable to 67;D-dimer levels were normal in all at presentation. Four patients were admitted to hospital and received IVIG +/- steroids. Two patients had recurrent severe thrombocytopenia within 14 days of discharge, requiring repeat steroid pulse. See Table for summary of all patients. Conclusion: In summary, application of the ASH TTS guidance to patients presenting with thrombocytopenia, with and without thrombosis, following COVID-19 vaccination was instrumental in the early identification and successful management of these complications. [Formula presented] Disclosures: Carrier: Sanofi: Honoraria;Pfizer: Honoraria, Research Funding;Servier: Honoraria;Bayer: Honoraria;Leo Pharma: Honoraria, Research Funding;BMS: Honoraria, Research Funding. Le Gal: BMS: Honoraria;Aspen: Honoraria;Bayer: Honoraria;LEO Pharma: Honoraria;Pfizer: Honoraria;Sanofi: Honoraria. Castellucci: BMS: Honoraria;Pfizer: Honoraria;Amag Pharmaceuticals: Honoraria;The Academy: Honoraria.