Laboratory diagnosis of heparin-induced thrombocytopenia

Heparin Induced Thrombocytopenia Testing

This article provides a comprehensive overview of Heparin-Induced Thrombocytopenia (HIT) with an emphasis on laboratory testing and advantages of automation. HIT is a critical condition arising from heparin exposure, leading to a contradictory combination of thrombocytopenia with an increased thrombosis risk. The article discusses HIT's history, clinical presentation, laboratory diagnosis, and management strategies. It highlights the importance of interdisciplinary collaboration for effective diagnosis and treatment, underscoring advancements in technology and targeted therapies that are shaping future approaches to HIT management.

Combination of 2 Quantitative Immunoassays and Clinical Score Algorithm to Reduce False-Negative Results in Heparin-Induced Thrombocytopenia: Prevalence Study of Mauriziano Hospital in Turin, Italy

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a serious adverse effect of heparin treatment caused by platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies. Accurate diagnosis of HIT is essential but remains challenging. The aim of our study was to explore the performance of our optimized diagnostic laboratory algorithm, based on Chemiluminescence (CliA) and ELISA immunoassays, on suspected HIT patients. The study compared the prevalence of HIT diagnosis in A.O. Mauriziano with the literature.

METHODS

163 consecutive patients were investigated for suspected HIT with CliA HemosIL Acustar HIT-IgG, Werfen. HIT was ruled out in all patients with CliA <0.13 U/mL. All patients with CliA >0.13 U/mL were further investigated with Zymutest-HIA anti-PF4 IgG ELISA immunoassay. In these patients, HIT was ruled out on the combination of CliA between 0.13 and 1.0 U/mL followed by ELISA assay <0.300 OD. HIT was ruled in patients whose plasma tested positive or doubtful with CliA and positive with ELISA immunoassay and confirmed positive with a platelet aggregation test (PAT). Suspicion of HIT was revealed with clinical 4Ts score or recent suggestive anamnestic history.

RESULTS

Our diagnostic algorithm ruled out HIT diagnosis in 144/163 patients (88%) and predicted a positive PAT in 5/19 (26%) of CliA positive (4/5) or ELISA positive and CliA doubtful (1/5) patients.

CONCLUSIONS

Our prevalence was 3.1%, comparable with the literature. The approach combining 2 quantitative immunoassays' (CliA and ELISA) results and 4Ts score probability was able to rule out the diagnosis within 1 h in 66% of patients with suspected HIT and within 24 h in 88% of patients. In the remaining 12% of cases, management decisions have to be based on individualized judgment while awaiting functional confirming results (48-72 h).

Key considerations in navigating ticagrelor's reported effect on heparin-induced thrombocytopenia functional assays in a landscape of limited data

PURPOSE

This article discusses key considerations regarding ticagrelor's reported effect on heparin-induced thrombocytopenia functional assays, such as literature gaps and possible management strategies.

SUMMARY

Limited data indicate that ticagrelor may induce false-negative results in functional assays used in the diagnosis of heparin-induced thrombocytopenia. False-negative functional assays for heparin-induced thrombocytopenia could have catastrophic consequences. The manufacturer labeling of ticagrelor now includes a warning for this potential drug-laboratory interaction. This article suggests areas that would benefit from further research and strategies in navigating this possible interaction.

CONCLUSION

Clinicians should exercise caution when evaluating functional assays for heparin-induced thrombocytopenia in patients receiving ticagrelor. This article offers suggestions for future areas of research and potential management strategies.

Evaluating Diagnostic Algorithms for Heparin-Induced Thrombocytopenia using Two Combined Automated Rapid Immunoassays

Heparin-induced thrombocytopenia (HIT) is an autoimmune disorder caused by antibodies against platelet factor 4 (PF4) and heparin complexes. Rapid immunoassays (IAs) for detection of these antibodies mark a milestone in HIT diagnosis, despite a higher false-positive rate compared with functional platelet-activation assays. However, combining different rapid IAs may help to improve their diagnostic specificity. Here, we compared the individual performance of the latex immunoturbidimetric assay (LIA; HemosIL HIT-Ab [PF4-H]; sensitivity 91.7%, specificity 68.4%) and chemiluminescence immunoassay (CLIA; HemosIL AcuStarHIT-Ab [PF4-H]; sensitivity 92.4%, specificity 85.8%) with their combined performance using two unique diagnostic algorithms in a single prospective cohort of suspected HIT patients. Using the simultaneous algorithm adapted from Warkentin et al, the combined LIA-CLIA had a sensitivity of 99.0% and specificity of 64.3%. The sequential algorithm adapted from Rittener-Ruff et al was applied in two theoretical scenarios to reflect real-world circumstances in diagnostic laboratories where access to clinical information is limited: (1) assuming all patients had an intermediate 4Ts score and (2) assuming all patients had a high 4Ts score. This algorithm correctly predicted HIT in 94.5% (high 4Ts) and 96.0% (intermediate 4Ts) and excluded HIT in 82.6% (high 4Ts) and 80.1% (intermediate 4Ts) of patients in either scenario, respectively. Although both combined algorithms improved diagnostic performance of individual IAs, the simultaneous algorithm showed fewer false predictions (7.9%) than the sequential algorithm (intermediate 4Ts: 37.6% and high 4Ts: 41.5%) and proved more practical as it does not rely on physician evaluations. Our findings highlight the importance of accounting for clinician and interlaboratory variability when evaluating diagnostic tests for HIT.

A career in solving clinical-pathological conundrums: Heyde syndrome, anti-platelet factor 4 disorders, and microvascular limb ischemic necrosis

Hematology is a clinical specialty with strong roots in the laboratory; accordingly, the lab can help solve perplexing clinical problems. This review highlights clinical-pathological conundrums addressed during my 35-year hematology career at McMaster University. Heyde syndrome is the association between aortic stenosis and bleeding gastrointestinal (GI) angiodysplasia where the bleeding is usually cured by aortic valve replacement; the chance reading of a neonatal study showing reversible deficiency of high-molecular-weight (HMW) multimers of von Willebrand factor (vWF) following surgical correction of congenital heart disease provided the key insight that a subtle deficiency of HMW multimers of vWF explains Heyde syndrome. The unusual immunobiology of heparin-induced thrombocytopenia (HIT)-a highly prothrombotic, antibody-mediated, anti-platelet factor 4 (PF4) disorder featuring rapid appearance and then disappearance (seroreversion) of the pathological heparin-dependent platelet-activating antibodies-permitted identification of key clinical features that informed development of a scoring system (4Ts) to aid in HIT diagnosis. Atypical clinical presentations of HIT prompted identification of heparin-independent anti-PF4 antibodies, now recognized as the explanation for vaccine-induced immune thrombotic thrombocytopenia (VITT), as well as VITT-like disorders triggered by adenovirus infection. Another unusual feature of HIT is its strong association with limb ischemia, including limb necrosis secondary to deep-vein/microvascular thrombosis (venous limb gangrene). The remarkable observation that supratherapeutic warfarin anticoagulation predisposes to HIT- and cancer-associated venous limb gangrene provided insight into disturbed procoagulant/anticoagulant balance; these concepts are relevant to microvascular thrombosis in critical illness (symmetrical peripheral gangrene), including a pathophysiological role for proximate "shock liver" (impaired hepatic synthesis of natural anticoagulants).

Diagnostic value of antibody-induced procoagulant platelets in heparin-induced thrombocytopenia: communication from the ISTH SSC Subcommittee on Platelet Immunology

Heparin-induced thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder characterized by a drop in platelet count and an increased risk of thromboembolic events. The accurate diagnosis of HIT involves clinical assessment and laboratory testing with well-characterized functional tests. Recent research has shown the potential of investigating procoagulant platelet formation induced by HIT antibodies. To successfully implement these assays in clinical laboratories, careful consideration of technical and preanalytical factors is crucial. In this communication from the SSC Platelet Immunology, we provide a consensus from experts on the use of flow cytometry in HIT diagnosis, highlighting the importance of standardized protocols.

High-Dose Immunoglobulin Therapy for Multiple Thromboembolism in Persisting Heparin-Induced Thrombocytopenia

This report presents a case of an 81-year-old male with acute respiratory distress syndrome secondary to aspiration pneumonia who developed heparin-induced thrombocytopenia (HIT). His platelet count remained persistently low despite discontinuing unfractionated heparin and initiating intravenous argatroban. Multiple thromboembolisms, including a new aortic mural thrombus in the descending aorta, were observed on contrast-enhanced computed tomography (CT), resulting in a diagnosis of autoimmune HIT (aHIT). Subsequent high-dose intravenous immunoglobulin (IVIG) therapy substantially improved the platelet count and resolved thromboembolisms. This case is notable owing to the improvement of aHIT complicated by multiple thromboembolisms, including an aortic mural thrombus, following high-dose IVIG therapy. In recent years, a growing number of reports have documented the effectiveness of high-dose IVIG therapy for aHIT. However, reports on whether high-dose IVIG therapy could improve an aortic mural thrombus complicating aHIT are lacking. The successful use of high-dose IVIG therapy in the current case highlights its potential efficacy in treating aHIT complicated by multiple thromboembolisms. Further studies are required to clarify the role of IVIG in the management of aHIT with thromboembolism.

Anti-PF4 antibodies and their relationship with COVID infection

Detecting anti-PF4 antibodies remains the golden diagnostic method for heparin-induced thrombocytopenia (HIT) diagnosis with high sensitivity and specificity. Various lab tests detect anti-PF4 antibodies, including immunoassays and functional assays. Even with positive detection of the anti-PF4 antibody, several factors are involved in the result. The concept of anti-PF4 disorders was recently brought to light during the COVID pandemic since the development of vaccine-induced thrombotic thrombocytopenia (VITT) with the adenovirus-vectored-DNA vaccine during the pandemic. Circumstances that detect anti-PF4 antibodies are classified as anti-PF4 disorders, including VITT, autoimmune HIT and spontaneous HIT. Some studies showed a higher percentage of anti-PF4 antibody detection among the population infected by COVID-19 without heparin exposure and some supported the theory that the anti-PF4 antibodies were related to the disease severity. In this review article, we provide a brief review of anti-PF4 disorders and summarize the current studies of anti-PF4 antibodies and COVID-19 infection.

Critical Analysis of Thrombocytopenia Associated With Glycoprotein IIb/IIIa Inhibitors and Potential Role of Zalunfiban, a Novel Small Molecule Glycoprotein Inhibitor, in Understanding the Mechanism(s)

Thrombocytopenia is a rare but serious complication of the intravenous glycoprotein IIb/IIIa (GPIIb/IIIa; integrin αIIbβ3) receptor inhibitors (GPIs), abciximab, eptifibatide, and tirofiban. The thrombocytopenia ranges from mild (50 000-100 000 platelets/μL), to severe (20 000 to <50 000/μL), to profound (<20 000/μL). Profound thrombocytopenia appears to occur in <1% of patients receiving their first course of therapy. Thrombocytopenia can be either acute (<24 hours) or delayed (up to ~14 days). Both hemorrhagic and thrombotic complications have been reported in association with thrombocytopenia. Diagnosis requires exclusion of pseudothrombocytopenia and heparin-induced thrombocytopenia. Therapy based on the severity of thrombocytopenia and symptoms may include drug withdrawals and treatment with steroids, intravenous IgG, and platelet transfusions. Abciximab-associated thrombocytopenia is most common and due to either preformed antibodies or antibodies induced in response to abciximab (delayed). Readministration of abciximab is associated with increased risk of thrombocytopenia. Evidence also supports an immune basis for thrombocytopenia associated with the 2 small molecule GPIs. The latter bind αIIbβ3 like the natural ligands and thus induce the receptor to undergo major conformational changes that potentially create neoepitopes. Thrombocytopenia associated with these drugs is also immune-mediated, with antibodies recognizing the αIIbβ3 receptor only in the presence of the drug. It is unclear whether the antibody binding depends on the conformational change and whether the drug contributes directly to the epitope. Zalunfiban, a second-generation subcutaneous small molecule GPI, does not induce the conformational changes; therefore, data from studies of zalunfiban will provide information on the contribution of the conformational changes to the development of GPI-associated thrombocytopenia.

Point-of-care haemoglobin accuracy and transfusion outcomes in non-cardiac surgery at a Canadian tertiary academic hospital: protocol for the PREMISE observational study

INTRODUCTION

Transfusions in surgery can be life-saving interventions, but inappropriate transfusions may lack clinical benefit and cause harm. Transfusion decision-making in surgery is complex and frequently informed by haemoglobin (Hgb) measurement in the operating room. Point-of-care testing for haemoglobin (POCT-Hgb) is increasingly relied on given its simplicity and rapid provision of results. POCT-Hgb devices lack adequate validation in the operative setting, particularly for Hgb values within the transfusion zone (60-100 g/L). This study aims to examine the accuracy of intraoperative POCT-Hgb instruments in non-cardiac surgery, and the association between POCT-Hgb measurements and transfusion decision-making.

METHODS AND ANALYSIS

PREMISE is an observational prospective method comparison study. Enrolment will occur when adult patients undergoing major non-cardiac surgery require POCT-Hgb, as determined by the treating team. Three concurrent POCT-Hgb results, considered as index tests, will be compared with a laboratory analysis of Hgb (lab-Hgb), considered the gold standard. Participants may have multiple POCT-Hgb measurements during surgery. The primary outcome is the difference in individual Hgb measurements between POCT-Hgb and lab-Hgb, primarily among measurements that are within the transfusion zone. Secondary outcomes include POCT-Hgb accuracy within the entire cohort, postoperative morbidity, mortality and transfusion rates. The sample size is 1750 POCT-Hgb measurements to obtain a minimum of 652 Hgb measurements <100 g/L, based on an estimated incidence of 38%. The sample size was calculated to fit a logistic regression model to predict instances when POCT-Hgb are inaccurate, using 4 g/L as an acceptable margin of error.

ETHICS AND DISSEMINATION

Institutional ethics approval has been obtained by the Ottawa Health Science Network-Research Ethics Board prior to initiating the study. Findings from this study will be published in peer-reviewed journals and presented at relevant scientific conferences. Social media will be leveraged to further disseminate the study results and engage with clinicians.

Thrombotic anti-PF4 immune disorders: HIT, VITT, and beyond

Antibodies against the chemokine platelet factor 4 (PF4) occur often, but only those that activate platelets induce severe prothrombotic disorders with associated thrombocytopenia. Heparin-induced thrombocytopenia (HIT) is the prototypic anti-PF4 disorder, mediated by strong activation of platelets through their FcγIIa (immunoglobulin G [IgG]) receptors (FcγRIIa). Concomitant pancellular activation (monocytes, neutrophils, endothelium) triggers thromboinflammation with a high risk for venous and arterial thrombosis. The classic concept of HIT is that anti-PF4/heparin IgG, recognizing antigen sites on (cationic) PF4 that form in the presence of (anionic) heparin, constitute the heparin-dependent antibodies that cause HIT. Accordingly, HIT is managed by anticoagulation with a nonheparin anticoagulant. In 2021, adenovirus vector COVID-19 vaccines triggered the rare adverse effect "vaccine-induced immune thrombotic thrombocytopenia" (VITT), also caused by anti-PF4 IgG. VITT is a predominantly heparin-independent platelet-activating disorder that requires both therapeutic-dose anticoagulation and inhibition of FcγRIIa-mediated platelet activation by high-dose intravenous immunoglobulin (IVIG). HIT and VITT antibodies bind to different epitopes on PF4; new immunoassays can differentiate between these distinct HIT-like and VITT-like antibodies. These studies indicate that (1) severe, atypical presentations of HIT ("autoimmune HIT") are associated with both HIT-like (heparin-dependent) and VITT-like (heparin-independent) anti-PF4 antibodies; (2) in some patients with severe acute (and sometimes chronic, recurrent) thrombosis, VITT-like antibodies can be identified independent of proximate heparin exposure or vaccination. We propose to classify anti-PF4 antibodies as type 1 (nonpathogenic, non- platelet activating), type 2 (heparin dependent, platelet activating), and type 3 (heparin independent, platelet activating). A key concept is that type 3 antibodies (autoimmune HIT, VITT) require anticoagulation plus an adjunct treatment, namely high-dose IVIG, to deescalate the severe anti-PF4 IgG-mediated hypercoagulability state.

Immunologic Effects of Heparin Associated With Hemodialysis: Focus on Heparin-Induced Thrombocytopenia

Intermittent hemodialysis (HD) is almost invariably performed with heparin, and thus HD patients are at risk of developing the immune-mediated adverse effect heparin-induced thrombocytopenia (HIT), caused by anti-platelet factor 4/heparin IgG, which strongly activates platelets. HIT patients develop hypercoagulability with greatly increased risk of thrombosis, both venous and arterial. Certain HIT-associated complications are more likely to develop among HD patients, including hemofilter thrombosis despite heparin, intravascular catheter and/or arteriovenous fistula-associated thrombosis, post-heparin bolus anaphylactoid/anaphylactic reactions, and thrombotic stroke and acute limb artery thrombosis (reflecting the high frequency of underlying arteriopathy in many patients with renal failure). Management of HIT in HD usually requires use of an alternative (non-heparin) anticoagulant; for example, danaparoid sodium (outside the USA) or argatroban (USA and elsewhere). Whether heparin-grafted hemodialyzers (without systemic heparin) can be used safely in acute HIT is unknown. The HIT immune response is remarkably transient and usually not retriggered by subsequent heparin administration. Accordingly, since renal failure patients often require long-term HD, there may be the opportunity-following seroreversion (loss of platelet-activating HIT antibodies)-to restart heparin for HD, a practice that appears to have a low likelihood of retriggering HIT.

Heparin-Induced Thrombocytopenia in Hemodialysis Patients- The First Study from India

Heparin-induced thrombocytopenia (HIT), a rare complication of heparin therapy, presents with thrombocytopenia. It leads to paradoxical thromboembolism and has high mortality if untreated. It is less recognized, especially in hemodialysis (HD) patients who are frequently exposed to heparin during dialysis because patients with renal failure may have many other causes of thrombocytopenia. We describe the clinical presentation, diagnosis, and treatment of five cases of confirmed HIT in hemodialysis (HD) patients at our center. The initial suspicion was made based on a high 4T score and positive gel card test followed by confirmation using the functional assay with heparin-induced platelet aggregation. These patients were treated according to the recent American Society of Hematology guidelines 2018 for HIT.

Laboratory Testing for Heparin-Induced Thrombocytopenia and Vaccine-Induced Immune Thrombotic Thrombocytopenia Antibodies: A Narrative Review

Heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombotic thrombocytopenia (VITT) are highly prothrombotic (thrombosis frequency ≥50%). Both are caused by platelet-activating anti-platelet factor 4 (PF4) antibodies, forming PF4/IgG-containing immune complexes that engage platelet FcγIIa receptors, producing strong platelet activation. In HIT, heparin crosslinks several PF4 molecules, whereas in VITT, anti-PF4 antibodies alone crosslink PF4. Sufficient levels of circulating anti-PF4 antibodies are needed to create the pathogenic immune complexes on platelet surfaces; this explains why certain serum (plasma)-based assays are highly sensitive for detecting HIT/VITT antibodies. Accordingly, HIT and VITT are "clinical-pathological" disorders, that is, positive testing for such antibodies-together with a compatible clinical picture-is integral for diagnosis. Heparin (low concentrations) enhances HIT antibody-induced platelet activation, but platelet activation by VITT sera is usually inhibited by heparin. For both HIT and VITT, high sensitivity (>99% and >95%, respectively) characterizes PF4-dependent enzyme immunoassays (EIAs) and PF4-enhanced platelet activation assays; in contrast, certain rapid immunoassays have high sensitivity for HIT (>90-97%) but poor sensitivity (<25%) for VITT. HIT and VITT antibodies are directed at distinct sites on PF4: solid-phase EIAs and platelet activation assays are indifferent to these distinct antigen targets, but rapid immunoassays are not. We discuss a conceptual model where PF4 is viewed as a "globe," with the heparin-binding site the "equator"; in this model, HIT antibodies are primarily directed at antigen site(s) at the north and south "poles" of PF4 (formed when PF4 binds to heparin), whereas VITT antibodies recognize sites on the equator.

Cloned antibodies from patients with HIT provide new clues to HIT pathogenesis

Heparin-induced thrombocytopenia (HIT) is a serious adverse drug reaction characterized by antibodies that recognize platelet factor 4/heparin complexes (PF4/H) and activate platelets to create a prothrombotic state. Although a high percentage of heparin-treated patients produce antibodies to PF4/H, only a subset also makes antibodies that are platelet activating (PA). A close correlation between PA antibodies and the likelihood of experiencing HIT has been demonstrated in clinical studies, but how PA (presumptively pathogenic) and nonactivating (NA) (presumptively benign) antibodies differ from each other at the molecular level is unknown. To address this issue, we cloned 7 PA and 47 NA PF4/H-binding antibodies from 6 patients with HIT and characterized their structural and functional properties. Findings showed that PA clones differed significantly from NA clones in possessing 1 of 2 heavy chain complementarity-determining region 3 (HCDR3) motifs, RX1-2R/KX1-2R/H (RKH) and YYYYY (Y5), in an unusually long complementarity-determining region 3 (≥20 residues). Mutagenic studies showed that modification of either motif in PA clones reduced or abolished their PA activity and that appropriate amino acid substitutions in HCDR3 of NA clones can cause them to become PA. Repertoire sequencing showed that the frequency of peripheral blood IgG+ B cells possessing RKH or Y5 was significantly higher in patients with HIT than in patients without HIT given heparin, indicating expansion of B cells possessing RKH or Y5 in HIT. These findings imply that antibodies possessing RKH or Y5 are relevant to HIT pathogenesis and suggest new approaches to diagnosis and treatment of this condition.

Acute pulmonary thromboembolism after messenger RNA vaccination against coronavirus disease 2019: A case report

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is defined as thrombosis after inoculation of adenovirus vector vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). VITT rarely occurs with messenger RNA vaccines, and the use of heparin for VITT is also controversial. A 74-year-old female patient with no risk factors for thrombosis was brought to our hospital after loss of consciousness. Nine days before admission, she had received the third vaccine against SARS-CoV-2 (mRNA1273, Moderna). Immediately after transport, cardiopulmonary arrest occurred, prompting extracorporeal membrane oxygenation (ECMO). Pulmonary angiography showed translucent images of both pulmonary arteries, resulting in the diagnosis of acute pulmonary thromboembolism. Unfractionated heparin was administered, but D-dimer subsequently became negative. Pulmonary thrombosis remained in large volume, indicating that heparin was ineffective. Treatment was shifted to anticoagulant therapy using argatroban, which increased D-dimer level and improved respiratory status. The patient was successfully weaned from ECMO and ventilator. Anti-platelet factor 4 antibody examined after treatment initiation showed negative results; however, VITT was considered as an underlying condition because of the time of onset after vaccination, the ineffectiveness of heparin, and the absence of other causes of thrombosis. In case heparin is not effective, argatroban can be an alternative therapy against thrombosis.

Learning objective

During the coronavirus disease 2019 pandemic, treatment with vaccine against severe acute respiratory syndrome coronavirus 2 has been widely performed. Vaccine-induced immune thrombotic thrombocytopenia is the most common thrombosis after adenovirus vector vaccines. However, thrombosis can also occur after messenger RNA vaccination. Though commonly used for thrombosis, heparin may be ineffective. Non-heparin anticoagulants should be considered.

Unique features of vaccine-induced immune thrombotic thrombocytopenia; a new anti-platelet factor 4 antibody-mediated disorder

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a highly prothrombotic disorder caused by anti-PF4 antibodies that activate platelets and neutrophils, leading to thrombosis. Heparin-induced thrombocytopenia (HIT) is a related anti-PF4 mediated disorder, with similar pathophysiology and clinical manifestations but different triggers (i.e., heparin vs adenoviral vector vaccine). Clinically, both HIT and VITT typically present with thrombocytopenia and thrombosis, although the risk of thrombosis is significantly higher in VITT, and the thromboses occur in unusual anatomical sites (e.g., cerebral venous sinus thrombosis and hepatic vein thrombosis). The diagnostic accuracy of available laboratory testing differs between HIT and VITT; for VITT, ELISAs have better specificity compared to HIT and platelet activation assays require the addition of PF4. Treatment of VITT and HIT is anticoagulation non-heparin anticoagulants; however, heparin may be considered for VITT if no other option is available.

Multiple Electrode Aggregometry (Multiplate): Functional Assay for Vaccine-Induced (Immune) Thrombotic Thrombocytopenia (VITT)

Vaccine-induced immune thrombotic thrombocytopenia (VITT) was first described in 2021 and represents an adverse reaction to adenoviral vector COVID-19 vaccines AstraZeneca ChAdOx1 nCoV-19 (AZD1222) and Johnson & Johnson Ad26.COV2.S vaccine. VITT is a severe immune platelet activation syndrome with an incidence of 1-2 per 100,000 vaccinations. The features of VITT include thrombocytopenia and thrombosis within 4-42 days of first dose of vaccine. Affected individuals develop platelet-activating antibodies against platelet factor 4 (PF4). The International Society on Thrombosis and Haemostasis recommends both an antigen-binding assay (enzyme-linked immunosorbent assay, ELISA) and a functional platelet activation assay for the diagnostic workup of VITT. Here, the application of multiple electrode aggregometry (Multiplate) is presented as a functional assay for VITT.

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.

Off-the-shelf cryopreserved platelets for the detection of HIT and VITT antibodies

Heparin-induced thrombocytopenia (HIT) is suspected much more often than it is confirmed. Technically simple platelet factor 4 (PF4)-polyanion enzyme-linked immunosorbent assays (ELISAs) are sensitive but nonspecific. In contrast, accurate functional tests such as the serotonin release assay, heparin-induced platelet activation assay, and PF4-dependent P-selectin expression assay require fresh platelets and have complex assay end points, limiting their availability to specialized reference laboratories. To enable broad deployment of functional testing, we sought to extend platelet viability significantly by optimizing storage conditions and developed a simple functional assay end point by measuring the release of a platelet α-granule protein, thrombospondin-1 (TSP1), in an ELISA format. Platelet cryopreservation conditions were optimized by freezing platelets at controlled cooling rates that preserve activatability. Several-month-old cryopreserved platelets were treated with PF4 or heparin and were evaluated for their ability to be activated by HIT and vaccine-induced immune thrombotic thrombocytopenia (VITT) antibodies in the TSP1 release assay (TRA). HIT and spontaneous HIT patient samples induced significantly higher TSP1 release using both PF4-treated (PF4-TRA) and heparin-treated cryopreserved platelets relative to samples from patients suspected of HIT who lacked platelet-activating antibodies. This latter group included several patients that tested strongly positive in PF4-polyanion ELISA but were not platelet-activating. Four VITT patient samples tested in the TRA activated PF4-treated, but not heparin-treated, cryopreserved platelets, consistent with recent data suggesting the requirement for PF4-treated platelets for VITT antibody detection. These findings have the potential to transform the testing paradigm in HIT and VITT, making decentralized, technically simple functional testing available for rapid and accurate in-hospital diagnosis.

High-frequency Contactless Sensor for the Detection of Heparin-Induced Thrombocytopenia Antibodies via Platelet Aggregation

Heparin-induced thrombocytopenia (HIT), a severe autoimmune disorder, occurs in patients undergoing heparin therapy. The presence of platelet-activating antibodies against platelet factor 4/Heparin in the blood confirms patients suffering from HIT. The most widely used methods for HIT diagnosis are immunoassays but the results only suit to rule out HIT as the assays provide only around 50% specificity. To confirm HIT, samples with positive results in immunoassays are retested in functional assays (>98% specificity) that track platelet-activating antibodies via platelet aggregation. However, the protocols in functional assays are either time-consuming (due to the requirement of the detection of serotonin release) or require highly trained staff for the visualization of platelets. Here, we applied a cheap and easy-to-use contactless sensor, which employs high-frequency microwaves to detect the changes in the resonant frequency caused by platelet aggregation/activation. Analysis of change in conductivity and permittivity allowed us to distinguish between HIT-like (KKO) and non-HIT-like (RTO) antibodies. KKO caused a stronger reduction of conductivity of platelet samples than RTO. Our results imply that the high-frequency contactless sensor can be a promising approach for the development of a better and easier method for the detection of HIT.

Application of a clinical decision rule and laboratory assays in pediatrics: Adult heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is rare among pediatric patients. The diagnosis of HIT depends upon clinical decision tools to assess its pretest probability, supported by laboratory evidence of anti-platelet factor 4 (anti-PF4)/heparin antibodies.

AIMS

To compare the use of the 4Ts score clinical decision tool, clinical characteristics, and laboratory findings between pediatric and adult patients with suspected HIT.

METHODS

We compiled all pediatric patients in our center for whom HIT testing was performed during the years 2015-2021. These were compared with a cohort of consecutive adult patients. Laboratory diagnosis of HIT was performed with particle gel immunoassay (PaGIA) as screening test and confirmed by an automated latex-enhanced immunoturbidimetric assay (LIA) and/or by functional flow cytometry assay (FCA).

RESULTS

The cohort included 34 children (under 18 years) and 105 adults. Adults mostly received heparins for thromboembolism prophylaxis and treatment (72.4%, n = 76), and were more frequently treated with low-molecular-weight heparin (LMWH). Children were mostly exposed during cardiopulmonary bypass and extracorporeal membrane oxygenation (ECMO, 61.8%, n = 21), and were more frequently treated with unfractionated heparin (UFH). Compared with adults, children had significantly higher 4Ts scores. Nevertheless, adults had a slightly higher rate of a positive diagnosis of HIT. Six out of 16 adults with confirmed HIT presented with thrombosis (37.5%), whereas all three pediatric patients with HIT presented with thrombosis (p = .087).

CONCLUSIONS

4Ts scores are higher in children compared with adult patients for whom laboratory tests for HIT were obtained. A potentially higher incidence of thrombosis in children with HIT may be attributable to the severity of underlying illness.

Vaccine-induced immune thrombotic thrombocytopenia: Updates in pathobiology and diagnosis

Coronavirus disease 2019 (COVID-19) is a viral respiratory infection caused by the severe acute respiratory syndrome virus (SARS-CoV-2). Vaccines that protect against SARS-CoV-2 infection have been widely employed to reduce the incidence of symptomatic and severe disease. However, adenovirus-based SARS-CoV-2 vaccines can cause a rare, thrombotic disorder termed vaccine-induced immune thrombotic thrombocytopenia (VITT). VITT often develops in the first 5 to 30 days following vaccination and is characterized by thrombocytopenia and thrombosis in unusual locations (e.g., cerebral venous sinus thrombosis). The diagnosis is confirmed by testing for anti-PF4 antibodies, as these antibodies are capable of platelet activation without any cofactor. It can be clinically challenging to differentiate VITT from a similar disorder called heparin-induced thrombocytopenia (HIT), since heparin is commonly used in hospitalized patients. VITT and HIT have similar pathobiology and clinical manifestations but important differences in testing including the need for PF4-enhanced functional assays and the poor reliability of rapid immunoassays for the detection of anti-platelet factor 4 (PF4) antibodies. In this review we summarize the epidemiology of VITT; highlight similarities and differences between HIT and VITT; and provide an update on the clinical diagnosis of VITT.

Anti-platelet factor 4/heparin antibodies in patients with Hantaan virus infection

Background

Hemorrhagic fever with renal syndrome (HFRS) induced by Hantaan virus infection and heparin‐induced thrombocytopenia (HIT) are associated with symptoms such as thrombocytopenia and thrombosis. However, related molecules, such as anti–platelet factor 4 (PF4)/heparin antibodies, in patients with HFRS have not been evaluated.

Objectives

To test plasma levels of anti‐PF4/heparin antibodies and study the possible role of these antibodies in HFRS pathogenesis.

Methods

Indirect ELISA was used to determine plasma levels of anti‐PF4/heparin antibodies in 75 patients with HFRS and 20 normal controls. The 4Ts (thrombocytopenia, timing of platelet count fall, thrombosis or other sequelae, and other causes of thrombocytopenia) scoring system was used to determine the probability of HIT occurrence. A PF4‐enhanced platelet activation assay was used to detect the pathological effects of anti‐PF4/heparin antibodies. The laboratory/clinical features and viral load of all the patients were also assessed.

Results

Of the 75 patients with HFRS enrolled in this study, 69 had thrombocytopenia. Platelet count was negatively correlated with Hantaan viral load. Moreover, the optical density (OD) values of plasma antibodies against PF4/heparin in normal controls were less than 0.65, 4 patients tested strongly positive for anti‐PF4/heparin antibodies (OD values, 1.51–3.87), 21 patients were weakly positive (OD values, 0.66–0.74), and 50 patients were negative (OD values, 0.16–0.65). Moreover, all 4 patients who tested strongly positive for anti‐PF4/heparin antibodies showed a low probability of HIT (4Ts score of 3 or less) and had negative results in the PF4‐enhanced platelet activation assay.

Conclusions

Hantaan virus infection produces nonpathogenic antibodies against PF4/heparin; however, the generation mechanism of these antibodies requires further study.

Monoclonal and oligoclonal anti-platelet factor 4 antibodies mediate VITT

Kanack and colleagues analyze anti-platelet factor 4 antibodies from 5 patients with vaccine-induced thrombotic thrombocytopenia (VITT) secondary to COVID-19 adenoviral vaccination and antibodies from patients with spontaneous heparin-induced thrombocytopenia (HIT) and classical HIT. VITT antibodies are monoclonal or oligoclonal, similar to spontaneous HIT, whereas classical HIT antibodies are polyclonal. Heparin inhibits antibody-induced platelet activation in VITT, suggesting that heparin should be considered for the treatment of VITT.

Detection of Platelet-Activating Antibodies Associated with Vaccine-Induced Thrombotic Thrombocytopenia by Flow Cytometry: An Italian Experience

Rare cases of thrombocytopenia and thrombosis after anti-COVID-19 adenovirus-associated mRNA vaccines (VITT) due to platelet-activating anti-platelet-factor 4 (PF4)/polyanion antibodies have been reported. VITT laboratory diagnosis, similarly to heparin-induced thrombocytopenia (HIT) diagnosis, requires immunoassays for anti-PF4/polyanion antibodies identification, such as ELISA assays and platelet-activating functional tests, such as heparin-induced platelet activation test (HIPA), to confirm their pathogenicity. We compared the flow cytometry (FC) measurement of platelet p-selectin exposure to the gold standard functional test HIPA for diagnosis confirmation in 13 patients with a clinical VITT syndrome (6M/7F; median age 56 (33–78)) who resulted positive to anti-PF4/polyanion antibodies ELISA assays (12/13). FC and HIPA similarly identified three different patterns: (1) a typical non-heparin-dependent VITT pattern (seven and six patients by FC and HIPA, respectively); (2) low/no platelet activation in patients under IvIg therapy (five out of five and two out of four patients by FC and HIPA, respectively); (3) a HIT pattern. Antibodies investigated by FC became negative after 7, 17, and 24 days of therapy in three patients. FC measurement of P-selectin exposure was as sensitive as HIPA but simpler to detect anti-PF4/polyanion antibodies in VITT patients. FC could reliably discriminate VITT from HIT, thus helping for the choice of the anticoagulant.

Vaccine‐induced prothrombotic immune thrombocytopenia without thrombosis may not require immune modulatory therapy: A case report

Background

Vaccine‐induced immune thrombotic thrombocytopenia (VITT) is a rare complication of the ChAdOx1 nCoV‐19 and Ad26.COV2.S COVID‐19 vaccines. It presents most commonly with severe thrombocytopenia and thrombotic complications with extremely high D‐dimer levels 5–30 days after vaccination. We report a patient who presented with mild thrombocytopenia and minimally elevated D‐dimer levels without thrombosis, but who tested positive for antiplatelet factor 4 (PF4) platelet‐activating antibodies on a PF4‐enhanced serotonin‐release assay.

Key Clinical Question

Is immunomodulation necessary in patients who present without thrombosis?

Clinical Approach and Conclusions

Treatment with rivaroxaban alone was followed by platelet normalization despite persistence of anti‐PF4 antibodies. This case provides support that vaccination for COVID‐19 can induce a broad, heterogeneous prothrombotic disorder characterized by anti‐PF4 platelet‐activating antibodies that shares features with classical heparin‐induced thrombocytopenia (HIT) and autoimmune HIT syndromes and that immunomodulation may not be required in those without thrombosis.

How to dose and monitor argatroban for treatment of HIT

Dosing and monitoring of argatroban for treatment of heparin-induced thrombocytopenia (HIT) remain uncertain. Marchetti and colleagues critically and systematically reviewed their institutional experience using this direct thrombin inhibitor to treat patients with laboratory-confirmed HIT, and have formulated several practical recommendations for managing this challenging clinical situation. Commentary on: Marchetti et al. Managing argatroban in heparin-induced thrombocytopenia: A retrospective analysis of 729 treatment days in 32 patients with confirmed heparin-induced thrombocytopenia. Br J Haematol. 2022 (Online ahead of print). https://doi.org/10.1111/bjh.18120.

The COVID Complex: A Review of Platelet Activation and Immune Complexes in COVID-19

Coronavirus disease 2019 (COVID-19) is a highly prothrombotic viral infection that primarily manifests as an acute respiratory syndrome. However, critically ill COVID-19 patients will often develop venous thromboembolism with associated increases in morbidity and mortality. The cause for this prothrombotic state is unclear but is likely related to platelet hyperactivation. In this review, we summarize the current evidence surrounding COVID-19 thrombosis and platelet hyperactivation. We highlight the fact that several studies have identified a soluble factor in COVID-19 patient plasma that is capable of altering platelet phenotype in vitro. Furthermore, this soluble factor appears to be an immune complex, which may be composed of COVID-19 Spike protein and related antibodies. We suggest that these Spike-specific immune complexes contribute to COVID-19 platelet activation and thrombosis in a manner similar to heparin-induced thrombocytopenia. Understanding this underlying pathobiology will be critical for advancement of future research and therapeutic options.

Cost-effectiveness analysis of alternative anticoagulation in suspected heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a life-threatening complication associated with high medical costs. Factor Xa inhibitors have gradually replace approved treatment with intravenous direct thrombin inhibitors despite their off-label indication, because of easier management and favorable economic profile.Whether they are cost-effective remains unclear. We evaluated the cost-effectiveness of approved and off-label anticoagulants in patients with suspected HIT, based on census data from the largest Swiss hospital between 2015 and 2018. We constructed a decision tree model that reflects important clinical events associated with HIT. Relevant cost data were obtained from the finance department or estimated based on the Swiss-wide cost tariff. We estimated averted adverse events (AE) and incremental cost-effectiveness ratio as primary outcome parameters. We performed deterministic and probabilistic sensitivity analyses with 2000 simulations to assess the robustness of our results. In the base-case analysis, the total cost of averting one AE was 49,565 CHF for argatroban; 30,380 CHF for fondaparinux, and 30,610 CHF for rivaroxaban; after adjusting for 4Ts score: 41,152 CHF (argatroban); 27,710 CHF (fondaparinux) and 37,699 CHF (rivaroxaban). Fondaparinux and rivaroxaban were more clinically-effective than argatroban with AE averted of 0.820; 0.834 and 0.917 for argatroban, fondaparinux and rivaroxaban, respectively. Treatment with fondaparinux resulted in less cost and more AE averted, hence dominating argatroban. Results were most sensitive to AE rates and prolongation of stay. Monte Carlo simulations affirmed our base-case analysis. This is the first cost-effectiveness analysis comparing argatroban with fondaparinux and rivaroxaban using primary data. Fondaparinux and rivaroxaban resulted in more averted AE but fondaparinux had greater cost savings. Fondaparinux could be a viable alternative to argatroban.

A Multicenter Retrospective Evaluation of Direct Oral Anticoagulants for the Treatment of Heparin-Induced Thrombocytopenia

INTRODUCTION

Direct oral anticoagulants (DOACs) represent an off-label but potential alternative to traditional therapies for heparin-induced thrombocytopenia (HIT).

OBJECTIVE

The objective of this study was to evaluate the efficacy and safety of DOACs in patients with a diagnosis of laboratory-confirmed HIT.

METHODS

A multicenter retrospective cohort study of adult patients with HIT treated with apixaban, rivaroxaban, or dabigatran between 1 January 2013 and 1 January 2020 was performed. Patients with an intermediate or high pre-test probability for HIT and a positive antiplatelet factor 4/heparin complex assay, latex immunoturbidimetric assay, or serotonin release assay were included for analysis. The primary outcome was the composite of newly diagnosed venous or arterial thromboembolism, gangrene, or severe limb ischemia requiring amputation at 3 months following DOAC initiation. This study was approved by local institutional review boards, and the requirement for informed consent was waived.

RESULTS

A total of 77 patients from four health systems were included. The median 4Ts score was 5 (interquartile range 4.5-6), and 38 patients (49.4%) had a diagnosis of HIT with thrombosis. The most frequently used DOAC was apixaban (n = 51), followed by rivaroxaban (n = 24) and dabigatran (n = 2). In total, 63 (81.8%) patients received parenteral non-heparin anticoagulation prior to DOAC initiation. Nine patients (11.7%) experienced the primary outcome of HIT-related thrombotic events. Of the 14 patients who exclusively received DOAC therapy, none experienced the primary outcome. Major bleeding occurred in five (6.5%) patients.

CONCLUSION

In this retrospective cohort study, DOACs were associated with rates of thrombotic and hemorrhagic events similar to those with other therapies currently used in the treatment of HIT.

Multicentre evaluation of 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, as an internal quality control in HIT functional assays: Communication from the ISTH SSC Subcommittee on Platelet Immunology

BACKGROUND

Functional tests for the diagnosis of heparin-induced thrombocytopenia (HIT) exhibit variable performance.

OBJECTIVES

We evaluated in a multicenter study whether 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, could be used as an internal quality control.

METHODS

5B9 was sent to 11 laboratories in seven countries, and six initial concentrations ranging from 10 to 400 μg/mL were tested by heparin-induced platelet activation assay (HIPA), serotonin release assay (SRA), platelet aggregation test (PAT), flow cytometry (FC), or heparin-induced multiple-electrode aggregometry (HIMEA). Each method was evaluated in three different laboratories using experimental procedures identical to those usually applied for the diagnosis of HIT by testing platelets from 10 different healthy donors.

RESULTS

The procedures used varied among the laboratories, particularly when platelet-rich plasma and whole blood were used. Nevertheless, positive results were obtained with at least 100 μg/ml of 5B9 for most donors tested by all centers (except one) performing HIPA, SRA, or HIMEA. FC and PAT results were more heterogeneous. FC results from one center that used washed platelets preincubated with PF4 were positive with all donors at 50 µg/ml 5B9, but at least 200 μg/ml of 5B9 were required to activate cells with most donors tested using PAT.

CONCLUSION

This study confirms that HIT functional tests are not well standardized and exhibit variable sensitivity for the detection of platelet-activating antibodies. However, 5B9 is a potentially useful tool to standardize functional tests, to select responding platelet donors, and consequently to improve the performance of these assays and comparability between laboratories.

Heparin-induced Thrombocytopenia: Perioperative Diagnosis and Management

Heparin-induced thrombocytopenia is a severe prothrombotic disease. Timely diagnosis and treatment are essential. Application of diagnostic algorithms based on validated clinical scoring tools and rapid, specific laboratory assays may improve outcomes.

COVID-19 and Venous Thromboembolism: From Pathological Mechanisms to Clinical Management

Coronavirus disease 2019 (COVID-19), which is becoming a global pandemic, is caused by SARS-CoV-2 infection. In COVID-19, thrombotic events occur frequently, mainly venous thromboembolism (VTE), which is closely related to disease severity and clinical prognosis. Compared with historical controls, the occurrence of VTE in hospitalized and critical COVID-19 patients is incredibly high. However, the pathophysiology of thrombosis and the best strategies for thrombosis prevention in COVID-19 remain unclear, thus needing further exploration. Virchow’s triad elements have been proposed as important risk factors for thrombotic diseases. Therefore, the three factors outlined by Virchow can also be applied to the formation of venous thrombosis in the COVID-19 setting. A thorough understanding of the complex interactions in these processes is important in the search for effective treatments for COVID-19. In this work, we focus on the pathological mechanisms of VTE in COVID-19 from the aspects of endothelial dysfunction, hypercoagulability, abnormal blood flow. We also discuss the treatment of VTE as well as the ongoing clinical trials of heparin anticoagulant therapy. In addition, according to the pathophysiological mechanism of COVID-19-associated thrombosis, we extended the range of antithrombotic drugs including antiplatelet drugs, antifibrinolytic drugs, and anti-inflammatory drugs, hoping to find effective drug therapy and improve the prognosis of VTE in COVID-19 patients.

VITT following Ad26.COV2.S vaccination presenting without radiographically demonstrable thrombosis

We report a case of VITT following Ad26.COV2.S COVID-19 vaccination without radiographically demonstrable thrombosis at presentation.

Early VITT recognition and treatment with nonheparin anticoagulation can prevent severe thrombotic complications.

We report a case of vaccine-induced immune thrombotic thrombocytopenia (VITT) in a young man diagnosed 13 days after Ad26.COV2.S COVID-19 (Johnson & Johnson/Janssen) vaccination. He presented to us with 5 days of progressive left leg pain, thrombocytopenia, hypofibrinogenemia, and markedly elevated d-dimers, but without radiographically demonstrable thrombosis. Despite negative imaging, we initiated treatment of presumptive VITT given the striking clinical picture that included the timing of his recent adenovirus-based COVID-19 vaccine, leg symptoms, marked thrombocytopenia, and consumptive coagulopathy. He received intravenous immune globulin, prednisone, and argatroban and was discharged 7 days later much improved. His positive platelet factor 4 enzyme-linked immunosorbent assay antibody test returned after treatment was initiated. To our knowledge, this is the first reported case of VITT following Ad26.COV2.S vaccination presenting without radiographically demonstrable thrombosis. Our patient highlights the importance of knowing vaccine status and initiating treatment as soon as possible in the right clinical setting, even in the absence of radiographic evidence of thrombus. Early VITT recognition and treatment provide an opportunity to prevent serious thrombotic complications.

Predictors of mortality in thrombotic thrombocytopenia after adenoviral COVID-19 vaccination: the FAPIC score

AIMS

The clinical manifestation and outcomes of thrombosis with thrombocytopenia syndrome (TTS) after adenoviral COVID-19 vaccine administration are largely unknown due to the rare nature of the disease. We aimed to analyse the clinical presentation, treatment modalities, outcomes, and prognostic factors of adenoviral TTS, as well as identify predictors for mortality.

METHODS AND RESULTS

PubMed, Scopus, Embase, and Web of Science databases were searched and the resulting articles were reviewed. A total of 6 case series and 13 case reports (64 patients) of TTS after ChAdOx1 nCoV-19 vaccination were included. We performed a pooled analysis and developed a novel scoring system to predict mortality. The overall mortality of TTS after ChAdOx1 nCoV-19 vaccination was 35.9% (23/64). In our analysis, age ≤60 years, platelet count <25 × 103/µL, fibrinogen <150 mg/dL, the presence of intracerebral haemorrhage (ICH), and the presence of cerebral venous thrombosis (CVT) were significantly associated with death and were selected as predictors for mortality (1 point each). We named this novel scoring system FAPIC (fibrinogen, age, platelet count, ICH, and CVT), and the C-statistic for the FAPIC score was 0.837 (95% CI 0.732-0.942). Expected mortality increased with each point increase in the FAPIC score, at 2.08, 6.66, 19.31, 44.54, 72.94, and 90.05% with FAPIC scores 0, 1, 2, 3, 4, and 5, respectively. The FAPIC scoring model was internally validated through cross-validation and bootstrapping, then externally validated on a panel of TTS patients after Ad26.COV2.S administration.

CONCLUSIONS

Fibrinogen levels, age, platelet count, and the presence of ICH and CVT were significantly associated with mortality in patients with TTS, and the FAPIC score comprising these risk factors could predict mortality. The FAPIC score could be used in the clinical setting to recognize TTS patients at high risk of adverse outcomes and provide early intensive interventions including intravenous immunoglobulins and non-heparin anticoagulants.

Anti-PF4 VITT antibodies are oligoclonal and variably inhibited by heparin

Background

COVID-19 vaccines have been associated with a rare thrombotic and thrombocytopenic reaction, Vaccine-induced immune thrombotic thrombocytopenia (VITT) characterized by platelet-activating anti-PF4 antibodies. This study sought to assess clonality of VITT antibodies and evaluate their characteristics in antigen-based and functional platelet studies.

Methods

Anti-PF4 antibodies were isolated from five patients with VITT secondary to ChAdOx1 nCoV-19 (n=1) or Ad26.COV2.S (n=4) vaccination. For comparative studies with heparin-induced thrombocytopenia (HIT), anti-PF4 antibodies were isolated from one patient with spontaneous HIT, another with “classical” HIT, and two patients with non-pathogenic (non-platelet activating) anti-PF4 antibodies. Isolated antibodies were subject to ELISA and functional testing, and mass spectrometric evaluation for clonality determination.

Results

All five VITT patients had oligoclonal anti-PF4 antibodies (3 monoclonal, one bi- and one tri-clonal antibodies), while HIT anti-PF4 antibodies were polyclonal. Notably, like VITT antibodies, anti-PF4 antibodies from a spontaneous HIT patient were monoclonal. The techniques employed did not detect non-pathogenic anti-PF4 antibodies. The ChAdOx1 nCoV-19-associated VITT patient made an excellent recovery with heparin treatment. In vitro studies demonstrated strong inhibition of VITT antibody-induced platelet activation with therapeutic concentrations of heparin in this and one Ad26.COV2.S-associated VITT patient. Oligoclonal VITT antibodies with persistent platelet-activating potential were detected at 6 and 10 weeks after acute presentation in two patients tested. Two of the 5 VITT patients had recurrence of thrombocytopenia and one patient had focal seizures several weeks after acute presentation.

Conclusion

Oligoclonal anti-PF4 antibodies mediate VITT. Heparin use in VITT needs to be further studied.

Platelet factor 4 polyanion immune complexes: heparin induced thrombocytopenia and vaccine-induced immune thrombotic thrombocytopenia

BACKGROUND

This is a review article on heparin-induced thrombocytopenia, an adverse effect of heparin therapy, and vaccine-induced immune thrombotic thrombocytopenia, occurring in some patients administered certain coronavirus vaccines.

MAIN BODY/TEXT

Immune-mediated thrombocytopenia occurs when specific antibodies bind to platelet factor 4 /heparin complexes. Platelet factor 4 is a naturally occurring chemokine, and under certain conditions, may complex with negatively charged molecules and polyanions, including heparin. The antibody-platelet factor 4/heparin complex may lead to platelet activation, accompanied by other cascading reactions, resulting in cerebral sinus thrombosis, deep vein thrombosis, lower limb arterial thrombosis, myocardial infarction, pulmonary embolism, skin necrosis, and thrombotic stroke. If untreated, heparin-induced thrombocytopenia can be life threatening. In parallel, rare incidents of spontaneous vaccine-induced immune thrombotic thrombocytopenia can also occur in some patients administered certain coronavirus vaccines. The role of platelet factor 4 in vaccine-induced thrombosis with thrombocytopenia syndrome further reinforces the importance the platelet factor 4/polyanion immune complexes and the complications that this might pose to susceptible individuals. These findings demonstrate, how auxiliary factors can complicate heparin therapy and drug development. An increasing interest in biomanufacturing heparins from non-animal sources has driven a growing interest in understanding the biology of immune-mediated heparin-induced thrombocytopenia, and therefore, the development of safe and effective biosynthetic heparins.

SHORT CONCLUSION

In conclusion, these findings further reinforce the importance of the binding of platelet factor 4 with known and unknown polyanions, and the complications that these might pose to susceptible patients. In parallel, these findings also demonstrate how auxiliary factors can complicate the heparin drug development.

Direct Oral Anticoagulants as Successful Treatment of Heparin-Induced Thrombocytopenia: A Parisian Retrospective Case Series

Background: Heparin-induced thrombocytopenia (HIT) is a prothrombotic life-threatening disorder caused by an adverse reaction to heparin exposure. In this context, it is imperative to stop heparin immediately and to replace it by a non-heparin anticoagulant therapy. Despite their advantages, the use of direct oral anticoagulants (DOACs) is only emerging for HIT treatment, and their use remains rare. Objective: To improve our knowledge on the emerging role of DOACs as treatment of HIT and give an overview of our local practices in this context. Patients/Methods: This is a multi-centric retrospective case series of HIT patients referred to our Parisian pharmacovigilance network and treated with DOACs. Results: We report the cases of seven patients from four healthcare centers, diagnosed with HIT (4T score ≥ 4, positive anti-PF4/heparin immunoassay and positive serotonin-release assay) and treated with DOACs. After a few days on substitutive parenteral treatment (n = 6) or directly at HIT diagnosis (n = 1), these patients were treated with either rivaroxaban (n = 6) or apixaban (n = 1) during acute HIT phase. Mean time to platelet count recovery after heparin discontinuation was 3.3 days (range 3-5). No patient experienced major or clinically relevant non-major bleeding or thrombosis that could be related to DOAC treatment during follow-up. Conclusions: Our cases studies are consistent with recent guidelines credit to the potential and safe use of DOAC during acute HIT in clinically stable patients.

HIT and VITT antibodies: fraternal-not identical-twins

Not applicable (no abstract in Invited Editorial Focus).

Diagnosing heparin-induced thrombocytopenia: The need for accuracy and speed

Heparin-induced thrombocytopenia (HIT) is a prothrombotic condition resulting from pathogenic antibodies to complexes of heparin and platelet factor 4 (PF4). The diagnosis of HIT can be challenging due to the widespread use of heparin and the frequency of thrombocytopenia in hospitalized patients. Laboratory testing for HIT typically includes an immunoassay to detect antibodies to PF4-heparin and a functional assay. Current HIT diagnostic algorithms recommend using the 4Ts score to determine the need for HIT laboratory testing. Automated calculation of HIT clinical prediction scores in the electronic health record may improve the identification of patients who should undergo HIT testing. Another challenge in the management of patients with suspected HIT is the turnaround time of the laboratory testing needed to confirm the diagnosis. Due to the high daily thrombotic risk of HIT, clinicians must treat patients with intermediate to high pretest likelihood of HIT empirically while awaiting the test results. Treatment for HIT often involves alternative anticoagulants that lack reversal agents, which may increase bleeding risk, prolong hospital stays, and increase costs for patients suspected of having HIT. Rapid immunoassays hold promise to improve the speed of HIT diagnosis. These assays must retain a very high sensitivity for this "can't miss" diagnosis, yet have sufficient specificity to be of diagnostic value. A Bayesian approach has been proposed using two rapid immunoassays in succession, which decreased analytic turnaround time to 60 minutes. Such an approach has the potential to be a much-needed clinical advance in improving accuracy and speed in the diagnosis of HIT.