Serological investigation of patients with a previous history of heparin-induced thrombocytopenia who are reexposed to heparin

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).

How We Interpret Thrombosis with Thrombocytopenia Syndrome?

Platelets play an important role in hemostasis, and a low platelet count usually increases the risk of bleeding. Conditions in which thrombosis occurs despite low platelet counts are referred to as thrombosis with thrombocytopenia syndrome, including heparin-induced thrombocytopenia, vaccine-induced immune thrombotic thrombocytopenia, paroxysmal nocturnal hemoglobinuria, antiphospholipid syndrome, thrombotic microangiopathy (TMA), and disseminated intravascular coagulation. TMA includes thrombotic thrombocytopenic purpura, Shiga toxin-producing Escherichia coli-associated hemolytic uremic syndrome (HUS), and atypical HUS. Patients with these pathologies present with thrombosis and consumptive thrombocytopenia associated with the activation of platelets and the coagulation system. Treatment varies from disease to disease, and many diseases have direct impacts on mortality and organ prognosis if therapeutic interventions are not promptly implemented. Underlying diseases and the results of physical examinations and general laboratory tests as part of a thorough workup for patients should promptly lead to therapeutic intervention before definitive diagnosis. For some diseases, the diagnosis and initial treatment must proceed in parallel. Utilization of not only laboratory tests but also various scoring systems is important for validating therapeutic interventions based on clinical information.

Vaccine-Induced Immune Thrombotic Thrombocytopenia: Clinicopathologic Features and New Perspectives on Anti-PF4 Antibody-Mediated Disorders

INTRODUCTION

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare yet severe adverse complication first identified during the global vaccination effort against SARS-CoV-2 infection, predominantly observed following administration of the ChAdOx1-S (Oxford-AstraZeneca) and Ad26.CoV2.S (Johnson & Johnson/Janssen) adenoviral vector-based vaccines. Unlike other anti-platelet factor 4 (PF4) antibody-mediated disorders, such as heparin-induced thrombocytopenia (HIT), VITT arises with the development of platelet-activating anti-PF4 antibodies 4-42 days post-vaccination, typically featuring thrombocytopenia and thrombosis at unusual sites.

AIM

To explore the unique properties, pathogenic mechanisms, and long-term persistence of VITT antibodies in patients, in comparison with other anti-PF4 antibody-mediated disorders.

DISCUSSION

This review highlights the complexity of VITT as it differs in antibody behavior and clinical presentation from other anti-PF4-mediated disorders, including the high incidence rate of cerebral venous sinus thrombosis (CVST) and the persistence of anti-PF4 antibodies, necessitating a re-evaluation of long-term patient care strategies. The nature of VITT antibodies and the underlying mechanisms triggering their production remain largely unknown.

CONCLUSION

The rise in awareness and subsequent prompt recognition of VITT is paramount in reducing mortality. As vaccination campaigns continue, understanding the role of adenoviral vector-based vaccines in VITT antibody production is crucial, not only for its immediate clinical implications, but also for developing safer vaccines in the future.

Autoimmune Heparin-Induced Thrombocytopenia

Autoimmune thrombocytopenia (aHIT) is a severe subtype of heparin-induced thrombocytopenia (HIT) with atypical clinical features caused by highly pathological IgG antibodies ("aHIT antibodies") that activate platelets even in the absence of heparin. The clinical features of aHIT include: the onset or worsening of thrombocytopenia despite stopping heparin ("delayed-onset HIT"), thrombocytopenia persistence despite stopping heparin ("persisting" or "refractory HIT"), or triggered by small amounts of heparin (heparin "flush" HIT), most cases of fondaparinux-induced HIT, and patients with unusually severe HIT (e.g., multi-site or microvascular thrombosis, overt disseminated intravascular coagulation [DIC]). Special treatment approaches are required. For example, unlike classic HIT, heparin cessation does not result in de-escalation of antibody-induced hemostasis activation, and thus high-dose intravenous immunoglobulin (IVIG) may be indicated to interrupt aHIT-induced platelet activation; therapeutic plasma exchange may be required if high-dose IVIG is ineffective. Also, aHIT patients are at risk for treatment failure with (activated partial thromboplastin time [APTT]-adjusted) direct thrombin inhibitor (DTI) therapy (argatroban, bivalirudin), either because of APTT confounding (where aHIT-associated DIC and resulting APTT prolongation lead to systematic underdosing/interruption of DTI therapy) or because DTI inhibits thrombin-induced protein C activation. Most HIT laboratories do not test for aHIT antibodies, contributing to aHIT under-recognition.

How to Solve the Conundrum of Heparin-Induced Thrombocytopenia during Cardiopulmonary Bypass

Heparin-induced thrombocytopenia (HIT) is a major issue in cardiac surgery requiring cardiopulmonary bypass (CPB). HIT represents a severe adverse drug reaction after heparin administration. It consists of immune-mediated thrombocytopenia paradoxically leading to thrombotic events. Detection of antibodies against platelets factor 4/heparin (anti-PF4/H) and aggregation of platelets in the presence of heparin in functional in vitro tests confirm the diagnosis. Patients suffering from HIT and requiring cardiac surgery are at high risk of lethal complications and present specific challenges. Four distinct phases are described in the usual HIT timeline, and the anticoagulation strategy chosen for CPB depends on the phase in which the patient is categorized. In this sense, we developed an institutional protocol covering each phase. It consisted of the use of a non-heparin anticoagulant such as bivalirudin, or the association of unfractionated heparin (UFH) with a potent antiplatelet drug such as tirofiban or cangrelor. Temporary reduction of anti-PF4 with intravenous immunoglobulins (IvIg) has recently been described as a complementary strategy. In this article, we briefly described the pathophysiology of HIT and focused on the various strategies that can be applied to safely manage CPB in these patients.

Evaluation of Latex Immunoturbidimetric Assay Thresholds and HIT in Cardiothoracic Surgery

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a common differential diagnosis in cardiothoracic surgery. The latex immunoturbidimetric assay (LIA) is an enhanced immunoassay that has recently been introduced for the detection of total HIT immunoglobulin and retains a higher specificity of 95% compared to the enzyme-linked immunosorbent assay.

OBJECTIVES

To investigate if a semiquantitative relationship exists between increasing LIA levels beyond the current positivity threshold and its correlation to positive serotonin release assay results in cardiothoracic surgery.

METHODS

This was a multicenter, observational cohort of cardiothoracic surgery patients initiated on anticoagulation with heparin-based products. To conduct sensitivity and specificity analysis of LIA values, HIT positive was defined as a LIA value ≥1 unit/mL and HIT negative was defined as a LIA level <1 unit/mL. A receiver operating characteristic (ROC) analysis was utilized to evaluate the predictive performance of the LIA.

RESULTS

At manufactures' cutoffs of ≥1.0 unit/mL, LIA sensitivity and specificity was 93.8% and 22%, respectively, yielding a false positive rate of 78%. At a higher cutoff of 4.5 units/mL, LIA sensitivity and specificity was 75% and 71%, respectively, yielding a false positive rate of 29% and an area under the ROC curve of 0.75 (P = .01; 95% confidence interval: 0.621-0.889). Bivalirudin was initiated in 84.6% of false positive LIA results.

CONCLUSION

This study suggests that the diagnostic accuracy of the LIA can be optimized by increasing the LIA positivity threshold. Proposing a higher LIA cutoff, may mitigate unwarranted anticoagulation and bleeding outcomes.

Immunvermittelte Sinus- und Hirnvenenthrombosen: VITT und prä-VITT als Modellerkrankung

In diesem Übersichtsartikel beschreiben wir die klinischen und paraklinischen Charakteristika der Vakzin-induzierten immunthrombotischen Thrombozytopenie (VITT) und fassen den gegenwärtigen Kenntnisstand zur Pathogenese zusammen. Bei der VITT bilden sich 5–20 Tage nach einer Impfung mit einem Adenovirus-vektorbasiertem SARS-CoV-2-Vakzin (AstraZeneca oder Johnson & Johnson) lebensbedrohliche Thrombosen aus, vor allem in den zerebralen Sinus und Hirnvenen. Laborchemisch zeigt sich eine typische Thrombozytopenie mit erhöhten D-Dimeren. Der Pathogenese liegen immunologische Prozesse zugrunde, die Ähnlichkeiten mit der Heparin-induzierten Thrombozytopenie aufweisen: so geht die VITT mit hochtitrigem Immunoglobulin G gegen das thrombozytäre Protein Plättchenfaktor 4 (PF4) einher. Durch die Interaktion mit dem Impfstoff wird PF4 so verändert, dass es von Antikörper-produzierenden Zellen des Immunsystems erkannt wird. Die so produzierten Anti-PF4-Antikörper führen über thrombozytäre FcγIIa-Rezeptoren zu einer Plättchenaktivierung. Der Nachweis plättchenaktivierender Anti-PF4-Antikörper bestätigt die Diagnose einer VITT. Antikoagulanzien, die die Bildung von Thrombin oder Thrombin selbst blockieren und hochdosiertes i. v.-Immunglobulin G, das die Fcγ-Rezeptor-vermittelte Zellaktivierung inhibiert, stellen die wirksame und kausale Behandlung der VITT dar. Bei Patienten mit katastrophalem Verlauf kann ein Plasmaaustausch versucht werden. Bei einigen Patienten ist ein prä-VITT Syndrom als Prodromalstadium zu beoachten, das sich typischerweise mit Kopfschmerzen manifestieren kann und dessen frühe Behandlung hilft, thrombotische Komplikationen zu vermeiden. Die spezifische Dynamik der VITT-assozierten Immunreaktion entspricht einer transienten, sekundären Immunantwort. Aktuelle Studien gehen der Frage nach, wie PF4 an unterschiedliche adenovirale Proteine bindet und beleuchten die Rolle von anderen Impfstoff-Bestandteilen als potentielle Liganden für die PF4-Bindung. Einige dieser Faktoren sind auch an der Etablierung eines proinflammatorischen Milieus („danger signal“) beteiligt, das unmittelbar nach der Impfung die 1. Phase der VITT-Pathogenese triggert. Sobald in der 2. Phase der VITT-Pathogenese hohe Titer von Anti-PF4-Antikörper gebildet sind, aktivieren diese neben Thrombozyten auch Granulozyten. In einem als NETose (von „neutrophil extracellular traps“) bezeichneten Prozess setzen aktivierte Granulozyten dabei DNA frei, mit der PF4 weitere Komplexe bildet, an die Anti-PF4-Antikörper binden. Dies verstärkt die Fcγ-Rezeptor-vermittelte Zellaktivierung weiter mit der Folge einer ausgeprägten Thrombin-Bildung. Zum Ende des Artikels geben wir einen Ausblick, welchen Einfluss die bisherigen Erkenntnisse zur VITT auf weitere globale Impfkampagnen gegen SARS-CoV-2 haben und beleuchten, wie Anti-PF4-Antikörper jenseits von VITT und HIT auch eine Rolle bei seltenen Erkrankungen spielen, die mit rezidivierenden venösen und arteriellen Thrombosen einhergehen.

Temporal presentations of heparin-induced thrombocytopenia following cardiac surgery: A single-center, retrospective cohort study

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an important adverse drug reaction that can occur postcardiac surgery. Preoperative exposure to unfractionated heparin (UFH) is common, raising the issue of how frequently cardiac surgery-associated HIT occurs after immunizing preoperative exposure to heparin.

OBJECTIVE

To determine the frequency and clinical picture of HIT occurring within 4 days of cardiac surgery (early presentation) versus later presentations (typical, delayed).

METHODS

We identified patients with laboratory-confirmed HIT following cardiac surgery over 30 years in a single cardiac surgery center. Three different clinical presentations of HIT were identified: typical (HIT-related platelet count fall beginning between postoperative days [PODs] 5-10), delayed (patients with falls after POD10 or who presented following hospital discharge), and early (established before POD5, including during cardiac surgery [acute intraoperative HIT]).

RESULTS

Of 129 patients identified with HIT complicating cardiac surgery, 100 had typical and 16 had delayed presentation of HIT; only 13 patients (10.1%) presented with early HIT, all of whom had received exposure to UFH during the 10 days before cardiac surgery. No patient was identified in whom remote preoperative UFH exposure was implicated in explaining early HIT. Notably, five patients appeared to have had acute intraoperative HIT, without immediate adverse consequences.

CONCLUSIONS

Approximately 90% of patients with HIT after cardiac surgery appear to develop this complication due to immunization triggered by cardiac surgery; however, in approximately 10% of patients, early presentation during the first four PODs (or intraoperatively) can be explained by recent immunizing exposure to heparin.

Association of trauma severity with antibody seroconversion in heparin-induced thrombocytopenia: A multicenter, prospective, observational study

BACKGROUND

Heparin administration can induce the production of anti-platelet factor 4 (PF4)/heparin antibodies with platelet-activating properties, causing heparin-induced thrombocytopenia (HIT). Previous studies have suggested that trauma severity influences HIT immune responses, but their relationship has not been fully explained. This study aimed to clarify this association by multicenter prospective observational study.

METHODS

Trauma patients who met the criteria of age 18 years or older and Injury Severity Scores (ISSs) of ≥9 from March 2018 to February 2019 were included. Patients who did not receive any heparin and those who received it as flushes or for treatment were also included. Patients were divided into three groups based on trauma severity (to mild [ISS 9-15], moderate [ISS 16-24], and severe injury groups [ISS ≥25]) and were compared by the seroconversion time and rate, as well as the disappearance rate of antibodies on day 30.

RESULTS

A total of 184 patients were included: 55, 62, and 67 patients were classified into the mild, moderate, and severe injury groups, respectively. Overall, the seroconversion rates of anti-PF4/heparin immunoglobulin G (IgG) and HIT antibodies by washed platelet activation assay were 26.6% and 16.3%, respectively. There was a significant difference in the seroconversion rates of anti-PF4/heparin IgG ( p = 0.016) and HIT antibodies ( p = 0.046) among the groups. Seroconversion rates in both assays increased with increasing trauma severity. The time required to achieve seroconversion was similar (between 5 and 10 days of trauma onset) regardless of heparin administration. Anti-PF4/heparin IgG and HIT antibodies were no longer detected on day 30 in 28.6% and 60.9% of seroconverted patients, respectively.

CONCLUSION

Development of HIT antibodies was observed commonly in severely injured trauma patients. Heparin-induced thrombocytopenia antibody development may be related to trauma severity, with a high disappearance frequency on day 30.

LEVEL OF EVIDENCE

Therapeutic/Care Management; Level III.

Longitudinal Aspects of VITT

In hundreds of patients worldwide, vaccination against COVID-19 with adenovirus vector vaccines (ChAdOx1 nCoV-19; Ad26.COV2.S) triggered platelet-activating anti-platelet factor 4 (PF4) antibodies inducing vaccine-induced immune thrombotic thrombocytopenia (VITT). In most VITT patients, platelet-activating anti-PF4-antibodies are transient and the disorder is discrete and non-recurring. However, in some patients platelet-activating antibodies persist, associated with recurrent thrombocytopenia and sometimes with relapse of thrombosis despite therapeutic-dose anticoagulation. Anti-PF4 IgG antibodies measured by enzyme-immunoassay (EIA) are usually detectable for longer than platelet-activating antibodies in functional assays, but duration of detectability is highly assay-dependent. As more than 1 vaccination dose against COVID-19 is required to achieve sufficient protection, at least 69 VITT patients have undergone subsequent vaccination with an mRNA vaccine, with no relevant subsequent increase in anti-PF4 antibody titers, thrombocytopenia, or thrombotic complications. Also, re-exposure to adenoviral vector-based vaccines in 5 VITT patients was not associated with adverse reactions. Although data are limited, vaccination against influenza also appears to be safe. SARS-CoV-2 infection reported in 1 patient with preceding VITT did not influence anti-PF4 antibody levels. We discuss how these temporal characteristics of VITT provide insights into pathogenesis.

Pathogenesis of vaccine-induced immune thrombotic thrombocytopenia (VITT)

Vaccine-induced immune thrombotic thrombocytopenia (VITT; synonym, thrombosis with thrombocytopenia syndrome, is associated with high-titer immunoglobulin G antibodies directed against platelet factor 4 (PF4). These antibodies activate platelets via platelet FcγIIa receptors, with platelet activation greatly enhanced by PF4. Here we summarize the current concepts in the pathogenesis of VITT. We first address parallels between heparin-induced thrombocytopenia and VITT, and provide recent findings on binding of PF4 to adenovirus particles and non-assembled adenovirus proteins in the 2 adenovirus vector-based COVID-19 vaccines, ChAdOx1 nCoV-19 and Ad26.COV2.S. Further, we discuss the potential role of vaccine constituents such as glycosaminoglycans, EDTA, polysorbate 80, human cell-line proteins and nucleotides as potential binding partners of PF4. The immune response towards PF4 in VITT is likely triggered by a proinflammatory milieu. Human cell-line proteins, non-assembled virus proteins, and potentially EDTA may contribute to the proinflammatory state. The transient nature of the immune response towards PF4 in VITT makes it likely that-as in heparin-induced thrombocytopenia -marginal zone B cells are key for antibody production. Once high-titer anti-PF4 antibodies have been formed 5 to 20 days after vaccination, they activate platelets and granulocytes. Activated granulocytes undergo NETosis and the released DNA also forms complexes with PF4, which fuels the Fcγ receptor-dependent cell activation process, ultimately leading to massive thrombin generation. Finally, we summarize our initial observations indicating that VITT-like antibodies might also be present in rare patients with recurrent venous and arterial thrombotic complications, independent of vaccination.

Elucidation of Cellular Contributions to Heparin-Induced Thrombocytopenia Using Omic Approaches

Heparin-induced thrombocytopenia (HIT) is an unpredictable, complex, immune-mediated adverse drug reaction associated with a high mortality. Despite decades of research into HIT, fundamental knowledge gaps persist regarding HIT likely due to the complex and unusual nature of the HIT immune response. Such knowledge gaps include the identity of a HIT immunogen, the intrinsic roles of various cell types and their interactions, and the molecular basis that distinguishes pathogenic and non-pathogenic PF4/heparin antibodies. While a key feature of HIT, thrombocytopenia, implicates platelets as a seminal cell fragment in HIT pathogenesis, strong evidence exists for critical roles of multiple cell types. The rise in omic technologies over the last decade has resulted in a number of agnostic, whole system approaches for biological research that may be especially informative for complex phenotypes. Applying multi-omics techniques to HIT has the potential to bring new insights into HIT pathophysiology and identify biomarkers with clinical utility. In this review, we review the clinical, immunological, and molecular features of HIT with emphasis on key cell types and their roles. We then address the applicability of several omic techniques underutilized in HIT, which have the potential to fill knowledge gaps related to HIT biology.

Heparin-induced thrombocytopenia and cardiovascular surgery

Clinicians generally counsel patients with a history of heparin-induced thrombocytopenia (HIT) to avoid heparin products lifelong. Although there are now many alternative (nonheparin) anticoagulants available, heparin avoidance remains challenging for cardiac surgery. Heparin is often preferred in the cardiac surgery setting based on the vast experience with the agent, ease of monitoring, and reversibility. To "clear" a patient with a history of HIT for cardiac surgery, hematologists must first confirm the diagnosis of HIT, which can be challenging due to the ubiquity of heparin exposure and frequency of thrombocytopenia in patients in the cardiac intensive care unit. Next, the "phase of HIT" (acute HIT, subacute HIT A/B, or remote HIT) should be established based on platelet count, immunoassay for antibodies to platelet factor 4/heparin complexes, and a functional assay (eg, serotonin release assay). As long as the HIT functional assay remains positive (acute HIT or subacute HIT A), cardiac surgery should be delayed if possible. If surgery cannot be delayed, an alternative anticoagulant (preferably bivalirudin) may be used. Alternatively, heparin may be used with either preoperative/intraoperative plasma exchange or together with a potent antiplatelet agent. The optimal strategy among these options is not known, and the choice depends on institutional experience and availability of alternative anticoagulants. In the later phases of HIT (subacute HIT B or remote HIT), brief intraoperative exposure to heparin followed by an alternative anticoagulant as needed in the postoperative setting is recommended.

Management of heparin-induced thrombocytopenia: systematic reviews and meta-analyses

Heparin-induced thrombocytopenia (HIT) is a prothrombotic adverse drug reaction occurring in <0.1% to 7% of patients receiving heparin products depending on the patient population and type of heparin. Management of HIT is highly dependent on a sequence of tests for which clinicians may or may not have the results when care decisions need to be made. We conducted systematic reviews of the effects of management strategies in persons with acute HIT, subacute HIT A or B, and remote HIT. We searched Medline, EMBASE, and the Cochrane Database through July 2019 for previously published systematic reviews and primary studies. Two investigators independently screened and extracted data and assessed the certainty of the evidence using the Grading of Recommendations Assessment, Development and Evaluation approach. We found primarily noncomparative studies and case series assessing effects of treatments, which led to low to very low certainty evidence. There may be little to no difference in the effects between nonheparin parenteral anticoagulants and direct oral anticoagulants in acute HIT. The benefits of therapeutic-intensity may be greater than prophylactic-intensity anticoagulation. Using inferior vena cava filters or platelet transfusion may result in greater harm than not using these approaches. Evidence for management in special situations, such as for patients undergoing cardiovascular interventions or renal replacement therapy, was also low to very low certainty. Additional research to evaluate nonheparin anticoagulants is urgently needed, and the development of novel treatments that reduce thrombosis without increasing hemorrhage should be a priority.

Heparin-Induced Thrombocytopenia: A Review of New Concepts in Pathogenesis, Diagnosis, and Management

Knowledge on heparin-induced thrombocytopenia keeps increasing. Recent progress on diagnosis and management as well as several discoveries concerning its pathogenesis have been made. However, many aspects of heparin-induced thrombocytopenia remain partly unknown, and exact application of these new insights still need to be addressed. This article reviews the main new concepts in pathogenesis, diagnosis, and management of heparin-induced thrombocytopenia.

How would we treat our own heparin-induced thrombocytopenia during cardiac surgery?

The aim of this article is to provide a comprehensive review of the current state of knowledge on heparin-induced thrombocytopenia (HIT) in cardiac surgery. The management of HIT patients undergoing cardiac surgery with cardiopulmonary bypass is complex and requires an interdisciplinary and patient-tailored approach because available evidence is limited and current anticoagulation strategies have potential risks. An index case is used to discuss both the established and new perioperative therapeutic options in HIT patients undergoing urgent cardiac surgery with cardiopulmonary bypass.

Heparin Induced Thrombocytopenia for the Perioperative and Critical Care Clinician

Purpose of Review

This review will illustrate the importance of heparin-induced thrombocytopenia in the intraoperative and critical care settings.

Recent Findings

Heparin-induced thrombocytopenia (HIT) occurs more frequently in surgical patients compared with medical patients due to the inflammatory release of platelet factor 4 and perioperative heparin exposure. Recognition of this disease requires a high index of suspicion. Diagnostic tools and therapeutic strategies have been expanded and refined in recent years.

Summary

HIT is a condition where antibodies against the heparin/platelet factor 4 complex interact with platelet receptors to promote platelet activation, aggregation, and thrombus formation. Our review will focus on intraoperative and postoperative considerations related to HIT to help the clinician better manage this rare but often devastating hypercoagulable disease process.

Intravenous Immune Globulin (IVIG) for Treatment of Autoimmune Heparin-Induced Thrombocytopenia: A Systematic Review

OBJECTIVE

To evaluate intravenous immune globulin (IVIG) for autoimmune heparin-induced thrombocytopenia (aHIT), including platelet recovery, IVIG dose, dosing weight, IVIG product used, and complications reported.

DATA SOURCES

PubMed and EMBASE were searched from inception through June 21, 2020. Search terms included heparin-induced thrombocytopenia, HIT, intravenous immune globulin, IVIG, autoimmune HIT, aHIT, and immune globulin.

STUDY SELECTION AND DATA EXTRACTION

Patients administered IVIG for HIT and diagnosed by immunoassay (optical density ≥2) or positive activation assay were included.

DATA SYNTHESIS

Twenty-four cases were reviewed; 92% had persistent aHIT. Time to IVIG administration post-nonheparin anticoagulant initiation was 9 days (median). Most common IVIG cumulative dose was 2 g/kg (dosed as 1 g/kg/d for 2 consecutive days); 75% had a favorable platelet increase (≥50 × 10⁹/L) within 5 days of initial IVIG dosing.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

aHIT is characterized by critically low platelets, thrombosis, and a persistent delay in platelet recovery despite treatment with a nonheparin anticoagulant. An immunoassay and subsequent confirmatory activation assay (at low, high, and 0 IU/mL unfractionated heparin levels) is recommended to confirm diagnosis. Patients nonresponsive to nonheparin anticoagulants within 5 days of initiation should be evaluated for IVIG treatment (2 g/kg cumulative dose). More data are needed to clarify appropriate IVIG dosing weight, although based on current published literature, it is recommended to use actual body weight.

CONCLUSIONS

Data reported support use of IVIG as adjunctive therapy for patients with aHIT. Judicious IVIG use based on key clinical and laboratory findings is critical.

Heparin-induced thrombocytopenia (HIT): Review of incidence, diagnosis, and management

Heparin-induced thrombocytopenia (HIT) is a life and limb-threatening complication of heparin exposure. Here, we review the pathogenesis, incidence, diagnosis, and management of HIT. The first step in thwarting devastating complications from this entity is to maintain a high index of clinical suspicion, followed by an accurate clinical scoring assessment using the 4Ts. Next, appropriate stepwise laboratory testing must be undertaken in order to rule out HIT or establish the diagnosis. In the interim, all heparin must be stopped immediately, and the patient administered alternative anticoagulation. Here we review alternative anticoagulation choice, therapy alternatives in the difficult-to-manage patient with HIT, and the problem of overdiagnosis.

Cardiac Surgery Successfully Managed With Cangrelor in a Patient With Persistent Anti-PF4/Heparin Antibodies 8 Years After Heparin-Induced Thrombocytopenia

A 66-YEAR-OLD female requiring cardiac surgery had persisting anti-platelet factor 4 (PF4)/heparin antibodies (HIT-abs) 8 years after heparin-induced thrombocytopenia (HIT). In 2010, she developed thrombotic thrombocytopenic purpura (TTP) (ADAMTS-13 <5%, inhibitor at 1.0 BU/mL), which was treated successfully with corticotherapy, plasmapheresis, and intravenous heparin. While taking heparin, she developed HIT, as evidenced by a positive functional test. Her platelet count fully resolved without thrombotic complications with danaparoid treatment. In 2018, the preoperative titer of HIT-abs was still 0.38 U/mL by chemoluminescent immunoassay (CLIA), and positive by particle-gel agglutination immunoassay (PaGIA) with a titer of 2 and was strongly positive on an enzyme-linked immunosorbent assay (ELISA). The authors of the case report chose to use cangrelor combined with heparin during cardiopulmonary bypass (CPB). Cangrelor was used without increased postoperative bleeding or thrombotic complications. Postoperatively she exhibited a huge rise in HIT-abs (14.22 U/mL on postoperative day 11) with a positive functional assay. There was no recurrence of HIT, however. This case illustrates the importance of excluding the presence of persisting HIT-abs before CPB and ensuring close medical follow-up after even a single exposure to heparin.

Autoimmune heparin-induced thrombocytopenia and venous limb gangrene after aortic dissection repair: in vitro and in vivo effects of intravenous immunoglobulin

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder characterized by heparin-dependent antibodies that activate platelets (PLTs) via PLT FcγIIa receptors. "Autoimmune" HIT (aHIT) indicates a HIT subset where thrombocytopenia progresses or persists despite stopping heparin; aHIT sera activate PLTs strongly even in the absence of heparin (heparin-independent PLT-activating properties). Affected patients are at risk of severe complications, including dual macro- and microvascular thrombosis leading to venous limb gangrene. High-dose intravenous immunoglobulin (IVIG) offers an approach to interrupt heparin-independent PLT-activating effects of aHIT antibodies.

CASE REPORT

A 78-year-old male who underwent cardiopulmonary bypass for aortic dissection developed aHIT, disseminated intravascular coagulation, and deep vein thrombosis; progression to venous limb gangrene occurred during partial thromboplastin time (PTT)-adjusted bivalirudin infusion (underdosing from "PTT confounding"). Thrombocytopenia recovered with high-dose IVIG, although the PLT count increase began only after the third dose of a 5-day IVIG regimen (0.4 g/kg/day × 5 days). We reviewed case reports and case series of IVIG for treating HIT, focusing on various IVIG dosing regimens used.

RESULTS

Patient serum-induced PLT activation was inhibited in vitro by IVIG in a dose-dependent fashion; inhibition of PLT activation by IVIG was much more marked in the absence of heparin versus the presence of heparin (0.2 U/mL). Our literature review indicated 1 g/kg × 2 IVIG dosing as most common for treating HIT, usually associated with rapid PLT count recovery.

CONCLUSION

Our clinical and laboratory observations support dose-dependent efficacy of IVIG for decreasing PLT activation and thus correcting thrombocytopenia in aHIT. Our case experience and literature review suggests dosing of 1 g/kg IVIG × 2 for patients with severe aHIT.

American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction mediated by platelet-activating antibodies that target complexes of platelet factor 4 and heparin. Patients are at markedly increased risk of thromboembolism.

OBJECTIVE

These evidence-based guidelines of the American Society of Hematology (ASH) are intended to support patients, clinicians, and other health care professionals in their decisions about diagnosis and management of HIT.

METHODS

ASH formed a multidisciplinary guideline panel balanced to minimize potential bias from conflicts of interest. The McMaster University GRADE Centre supported the guideline development process, including updating or performing systematic evidence reviews. The panel prioritized clinical questions and outcomes according to their importance for clinicians and patients. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was used to assess evidence and make recommendations, which were subject to public comment.

RESULTS

The panel agreed on 33 recommendations. The recommendations address screening of asymptomatic patients for HIT, diagnosis and initial management of patients with suspected HIT, treatment of acute HIT, and special situations in patients with acute HIT or a history of HIT, including cardiovascular surgery, percutaneous cardiovascular intervention, renal replacement therapy, and venous thromboembolism prophylaxis.

CONCLUSIONS

Strong recommendations include use of the 4Ts score rather than a gestalt approach for estimating the pretest probability of HIT and avoidance of HIT laboratory testing and empiric treatment of HIT in patients with a low-probability 4Ts score. Conditional recommendations include the choice among non-heparin anticoagulants (argatroban, bivalirudin, danaparoid, fondaparinux, direct oral anticoagulants) for treatment of acute HIT.

Cellular immune responses to platelet factor 4 and heparin complexes in patients with heparin-induced thrombocytopenia

Essentials The immunogenesis of Heparin-induced thrombocytopenia (HIT) is not well understood. Immunization to platelet factor 4 (PF4)-heparin occurs early in life, before any heparin exposure. PF4 and PF4-heparin complexes induce the proliferation of CD14+ cells. Reduced levels of regulatory cytokines contribute to immune dysregulation in HIT.

SUMMARY

Background Heparin-induced thrombocytopenia (HIT) is an adverse reaction to heparin characterized by thrombocytopenia and thrombotic complications. HIT is caused by pathogenic antibodies that bind to complexes of platelet factor 4 (PF4) and heparin, leading to platelet activation and inducing a hypercoagulable state. Previous studies have shown immunity to PF4-heparin complexes occurs early in life, even before heparin exposure; however, the immunogenesis of HIT is not well characterized. Objectives To investigate cellular proliferation in response to PF4-heparin complexes in patients with HIT. Patients/Methods Peripheral blood mononuclear cells (PBMCs) from healthy controls (n = 30), postoperative cardiac surgery patients who had undergone cardiopulmonary bypass (CPB) (n = 17) and patients with confirmed HIT (n = 41) were cultured with PF4 and PF4-heparin complexes. Cellular proliferation was assessed by [3 H]thymidine uptake and 5-ethynyl-2'-deoxyuridine detection. Results and Conclusions PBMCs proliferated in the presence of PF4, and this was enhanced by the addition of heparin in all study groups. CPB and HIT patients showed significantly greater proliferative responses than healthy controls. PBMC proliferation was antigen-specific, depended on the presence of platelets, and only CD14+ cells were identified as proliferating cells. Culture supernatants were tested for the levels of regulatory cytokines, and both CPB and HIT patients produced significantly lower levels of interleukin-10 and transforming growth factor-β1 than healthy controls. These findings further demonstrate cellular immune sensitization to PF4-heparin complexes occurs before heparin exposure, and suggests immune dysregulation can contribute to HIT.

Intravenous immunoglobulin as an adjunct therapy in persisting heparin-induced thrombocytopenia

Heparin induced thrombocytopenia (HIT) is a serious adverse drug reaction caused by transient antibodies against platelet factor 4 (PF4)/heparin complexes, resulting in platelet activation and potentially fatal arterial and/or venous thrombosis. Most cases of HIT respond to cessation of heparin and administration of an alternative non-heparin anticoagulant, but there are cases of persisting HIT, defined as thrombocytopenia due to platelet activation/consumption for greater than seven days despite standard therapy. These patients remain at high risk for thrombotic events, which may result in limb-loss and mortality. Intravenous immunoglobulin (IVIg) has been proposed as an adjunct therapy for these refractory cases based on its ability to saturate FcγRIIa receptors on platelets, thus preventing HIT antibody binding and platelet activation. We describe 2 cases of persisting HIT (strongly positive antigen and functional assays, and persisting thrombocytopenia >7 days) with rapid clinical response to IVIg. We performed in-vitro experiments to support IVIg response. Healthy donor platelets (1 × 10e6) were treated with PF4 (3.75 μg/mL) for 20 min followed by 1-hour incubation with patients' sera. Platelet activation with and without addition of IVIg (levels equivalent to those reached in a patient after treatment with 2 gm/Kg) was evaluated in the PF4-dependent P-selectin expression assay (PEA). A significantly decreased platelet activation was demonstrated after the addition of IVIg to both patient samples, which correlated well with the rapid clinical response that each patient experienced. Thus, our study supports the use of IVIg as an adjunct therapy for persisting HIT.

A Patient With Remote Heparin-Induced Thrombocytopenia and Antiphospholipid Syndrome Requiring Cardiopulmonary Bypass: Do Current Guidelines Apply?

Anticoagulation for cardiopulmonary bypass (CPB) is required to prevent acute disseminated intravascular coagulation and clot formation within the bypass circuit. Unfractionated heparin is the standard anticoagulant for CPB due to its many advantages and long history of successful use. However, heparin has the unique drawback of triggering Heparin-PF4 (PF4) antibodies potentially leading to heparin-induced thrombocytopenia (HIT). We have limited data regarding reformation of antibodies if a patient has had a prior (remote) antibody production or full HIT. Patients with antiphospholipid antibodies undergoing CPB with unfractionated heparin have a high complication rate, even in the absence of HIT. Antiphospholipid antibodies have a multifaceted, cumulatively inhibitory effect on the normal anticoagulation armamentarium in vivo. Even more concerning is the possibility that antiphospholipid syndrome and HIT may be synergistic. We report a patient with risk factors for both thromboembolic (remote history of HIT and antiphospholipid syndrome) and hemorrhagic complications who underwent an aortic valve replacement and coronary artery bypass grafting on CPB using bivalirudin. We discuss the complex decision making regarding anticoagulant for CPB, particularly with regard to American College of Chest Physicians guidelines.

Management of heparin-induced thrombocytopenia

PURPOSE OF REVIEW

The purpose of this review is to summarize recent findings on heparin-induced thrombocytopenia (HIT), a prothrombotic disorder caused by platelet-activating IgG targeting platelet factor 4 (PF4)/polyanion complexes.

RECENT FINDINGS

HIT can explain unusual clinical events, including adrenal hemorrhages, arterial/intracardiac thrombosis, skin necrosis, anaphylactoid reactions, and disseminated intravascular coagulation. Sometimes, HIT begins/worsens after stopping heparin ('delayed-onset' HIT). Various HIT-mimicking disorders are recognized (e.g., acute disseminated intravascular coagulation/'shock liver' with limb ischemia). HIT has features of both B-cell and T-cell immune responses; uptake of PF4/heparin complexes into macrophages ('macropinocytosis') facilitates the anti-PF4/heparin immune response. Antibody-induced activation of monocytes and platelets via their FcγIIA receptors triggers an intense procoagulant response. Sometimes, HIT antibodies recognize PF4 bound to (platelet-associated) chondroitin sulfate, explaining how HIT might occur without concurrent or recent heparin (delayed-onset HIT, 'spontaneous HIT syndrome'). The molecular structure of HIT antigen(s) has been characterized, providing a rationale for future drug design to avoid HIT and improve its treatment. The poor correlation between partial thromboplastin time and plasma argatroban levels (risking subtherapeutic anticoagulation) and need for intravenous administration of argatroban have led to increasing 'off-label' treatment with fondaparinux or one of the direct oral anticoagulants.

SUMMARY

Understanding the molecular mechanisms and unusual clinical features of HIT will improve its management.

Treatment of refractory delayed onset heparin-induced thrombocytopenia after thoracic endovascular aortic repair with intravenous immunoglobulin (IVIG)

Delayed onset heparin induced thrombocytopenia (HIT), is characterized by a late nadir due to persistent platelet-activating IgG antibodies. It typically begins or worsens 5 or more days after heparin is discontinued with complications such as thrombosis up to 3 weeks after exposure to heparin.1-3 In 50% of cases, the platelet count can decrease to very low numbers (<20 000/μL), which is not usual for typical HIT. Here we report 2 cases of post-operative delayed onset HIT manifesting as severe thrombocytopenia that persisted despite cessation of heparin and initiation of argatroban. Key Clinical Question: Is intravenous immunoglulin beneficial in severe refractory delayed-onset HIT?

IVIg for Treatment of Severe Refractory Heparin-Induced Thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) complicated by severe thrombocytopenia and thrombosis can pose significant treatment challenges. Use of alternative anticoagulants in this setting may increase bleeding risks, especially in patients who have a protracted disease course. Additional therapies are lacking in this severely affected patient population.

METHODS

We describe three patients with HIT who had severe thromboembolism and prolonged thrombocytopenia refractory to standard treatment but who achieved an immediate and sustained response to IVIg therapy. The mechanism of action of IVIg was evaluated in these patients and in five additional patients with severe HIT. The impact of a common polymorphism (H/R 131) in the platelet IgG receptor FcγRIIa on IVIg-mediated inhibition of platelet activation was also examined.

RESULTS

At levels attained in vivo, IVIg inhibits HIT antibody-mediated platelet activation. The constant domain of IgG (Fc) but not the antigen-binding portion (Fab) is required for this effect. Consistent with this finding, IVIg had no effect on HIT antibody binding in a solid-phase HIT immunoassay (platelet factor 4 enzyme-linked immunoassay). The H/R131 polymorphism in FcγRIIa influences the susceptibility of platelets to IVIg treatment, with the HH131 genotype being most susceptible to IVIg-mediated inhibition of antibody-induced activation. However, at high doses of IVIg, activation of platelets of all FcγRIIa genotypes was significantly inhibited. All three patients did well on long-term anticoagulation therapy with direct oral anticoagulants.

CONCLUSIONS

These studies suggest that IVIg treatment should be considered in patients with HIT who have severe disease that is refractory to standard therapies.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune complication of heparin therapy caused by antibodies to complexes of platelet factor 4 (PF4) and heparin. Pathogenic antibodies to PF4/heparin bind and activate cellular FcγRIIA on platelets and monocytes to propagate a hypercoagulable state culminating in life-threatening thrombosis. It is now recognized that anti-PF4/heparin antibodies develop commonly after heparin exposure, but only a subset of sensitized patients progress to life-threatening complications of thrombocytopenia and thrombosis. Recent scientific developments have clarified mechanisms underlying PF4/heparin immunogenicity, disease susceptibility, and clinical manifestations of disease. Insights from clinical and laboratory findings have also been recently harnessed for disease prevention. This review will summarize our current understanding of HIT by reviewing pathogenesis, essential clinical and laboratory features, and management.

Clinical picture of heparin-induced thrombocytopenia (HIT) and its differentiation from non-HIT thrombocytopenia

HIT is an acquired antibody-mediated disorder strongly associated with thrombosis, including microthrombosis secondary to disseminated intravascular dissemination (DIC). The clinical features of HIT are reviewed from the perspective of the 4Ts scoring system for HIT, which emphasises its characteristic timing of onset of thrombocytopenia. HIT antibodies recognize multimolecular complexes of platelet factor 4 (PF4)/heparin. However, a subset of HIT sera recognise PF4 bound to platelet chondroitin sulfate; these antibodies activate platelets in vitro and in vivo even in the absence of heparin, thus explaining: delayed-onset HIT (where HIT begins or worsens after stopping heparin); persisting HIT (where HIT takes several weeks to recover); spontaneous HIT syndrome (a disorder clinically and serologically resembling HIT but without proximate heparin exposure); and fondaparinux-associated HIT (four distinct syndromes featuring thrombocytopenia that begins or worsens during treatment with fondaparinux), with a new patient case presented with ongoing thrombocytopenia (and fatal haemorrhage) during treatment of HIT with fondaparinux, with fondaparinux-dependent platelet activation induced by patient serum ("fondaparinux cross-reactivity"). Ironically, despite existence of fondaparinux-associated HIT, this pentasaccharide anticoagulant is a frequent treatment for HIT (including one used by the author). HIT can be confused with other disorders, including those with a) timing similar to HIT (e. g. abciximab-associated thrombocytopenia of delayed-onset); b) combined thrombocytopenia/thrombosis (e. g. symmetrical peripheral gangrene secondary to acute DIC and shock liver); and c) both timing of onset and thrombosis (e. g. warfarin-associated venous limb gangrene complicating cancer-associated DIC). By understanding clinical and pathophysiological similarities and differences between HIT and non-HIT mimicking disorders, the clinician is better able to make the correct diagnosis.

Protamine (heparin)-induced thrombocytopenia: a review of the serological and clinical features associated with anti-protamine/heparin antibodies

Protamine is widely used in medicine as a rapidly-acting antidote to heparin, particularly for reversing heparin anticoagulation after cardiac surgery. Protamine is also used as a stabilizing additive to certain preparations of insulin. Recent reports demonstrate that protamine and heparin form multimolecular complexes that result in high rates of immunization in post-cardiac surgery patients, particularly of immunoglobulin G (IgG) class antibodies; a subset of these anti-protamine/heparin IgG antibodies activates platelets through their FcγIIA (IgG) receptors. Although the clinical consequences of anti-protamine/heparin antibodies that are newly generated after cardiac surgery are unknown, there is evidence that platelet-activating anti-protamine/heparin antibodies already present at the time of cardiac surgery might occasionally explain more severe thrombocytopenia with delayed platelet count recovery, as well as thromboembolic complications, in the post-cardiac surgery setting. Triggers for such antibodies remain poorly-defined, but could include preoperative administration of heparin to diabetic patients receiving protamine-insulin as well as recent previous cardiac surgery. Anti-protamine/heparin antibodies have several features in common with anti-platelet factor 4 (PF4) PF4/heparin antibodies implicated in heparin-induced thrombocytopenia (HIT), including immunization by heparin-containing multimolecular complexes, predominant IgG class, pathological platelet-activating properties, relatively rapid IgG formation without IgM precedence, and antibody transience. Despite these similarities, the risk of anti-protamine/heparin antibody-mediated complications seems to affect the early post-cardiac surgery period, whereas HIT usually occurs at least 5 days following cardiac surgery. Clinicians need to become aware of this recently recognized immunohematological disorder, and research is needed to identify triggers of immunization, improve detection of pathological antibodies and identify patients at risk of this complication.

Heparin-induced thrombocytopenia

PURPOSE OF REVIEW

Thrombocytopenia and heparin exposure are common in critically ill patients, yet immune heparin-induced thrombocytopenia (HIT), a prothrombotic adverse effect of heparin, rarely accounts for thrombocytopenia in this patient population. The review discusses the clinical and laboratory features that distinguish HIT from non-HIT thrombocytopenia.

RECENT FINDINGS

The frequency of HIT in heparin-exposed critically ill patients is approximately 0.3-0.5% versus at least a 30-50% background frequency of non-HIT thrombocytopenia. Most patients who form anti-PF4/heparin antibodies do not develop HIT, contributing to HIT overdiagnosis. Disseminated intravascular coagulation (DIC), particularly in the setting of cardiogenic or septic shock associated with 'shock liver', can cause ischemic limb gangrene with pulses, mimicking a clinical picture of HIT. However, whereas non-HIT-related DIC with microthrombosis can be treated with heparin, HIT usually requires nonheparin anticoagulation. HIT-associated DIC can result in an elevated INR, which could reflect factor VII depletion because of extrinsic (tissue factor) pathway-mediated activation of coagulation.

SUMMARY

Greater understanding of the various clinical and laboratory features that distinguish HIT from non-HIT thrombocytopenia could help improve outcomes in patients who develop thrombocytopenia and coagulopathies in the ICU.

Immune pathogenesis of heparin-induced thrombocytopenia

The immune response to heparin is one of the most common drug-induced allergies, and yet, atypical for a drug hypersensitivity reaction. Whereas most drug-induced allergies are rare, idiosyncratic and life-long, the allergic response to heparin is common, predictable in certain clinical settings and transient. Advances in the last decade with regards to structural characterisation of the PF4/heparin antigenic complex, contributions of innate immunity and development of animal models have provided insights into the distinctive features of the HIT immune response. Recent descriptions of the crystal structure of the PF4/heparin complex, alongside other biophysical studies, have clarified the structural requirements for immunogenicity and heparin-dependency of antibody formation. Studies of interactions of PF4 with bacterial cell walls as well as epidemiologic associations of anti-PF4/heparin antibody formation and infection suggest a role for immune priming and explain the rapid evolution of an isotype-switched immune response in sensitised patients. Murine models have greatly facilitated investigations of cellular basis of the HIT response and identified a major role for T-cells and marginal zone B-cells, but key findings have yet to be validated in human disease. This chapter will summarise recent investigations of the HIT immune response in the context of major pathways of immune activation and identify areas of uncertainty.