The risk of major bleeding in patients with suspected heparin-induced thrombocytopenia

Management of Heparin-Induced Thrombocytopenia: A Contemporary Review

Heparin-induced thrombocytopenia (HIT) is a life- and limb-threatening immune-mediated emergency classically associated with heparin therapy. This review focuses on type II HIT, characterized by the development of antibodies against platelet-factor 4 (PF4) bound to heparin after exposure, causing life-threatening thrombocytopenia, arterial thrombosis, and/or venous thrombosis. The high morbidity and mortality rates emphasize the need for early recognition and urgent intervention with discontinuation of heparin and initiation of non-heparin anticoagulation. We discuss the management of HIT with an emphasis on recent developments: (i) incorporating the phases of HIT (i.e., suspected, acute, subacute A and B, and remote) into its management, categorized according to platelet count, immunoassay, and functional assay results and (ii) direct-acting oral anticoagulants (DOACs), which are increasingly used in appropriate cases of acute HIT (off-label). In comparison to parenteral options (e.g., bivalirudin and danaparoid), they are easier to administer, are more cost-effective, and obviate the need for transition to an oral anticoagulant after platelet recovery. We also identify the knowledge gaps and suggest areas for future research.

Electronic consultation to improve care outcomes in patients with suspected and confirmed heparin-induced thrombocytopenia

Background

Heparin-induced thrombocytopenia (HIT) is a complication of heparin exposure associated with high risk for morbidity and mortality. Diagnosis and management are complex due to limitations of laboratory testing and the need for nonheparin anticoagulation.

Objectives

To increase the delivery of evidence-based care of patients with suspected and confirmed HIT via electronic consultation (e-consult).

Methods

We describe the creation and implementation of an e-consult service for patients with concern for HIT at a large academic medical center. Hematology physicians with HIT expertise performed real-time chart review of all patients with a positive screening immunoassay result and provided written recommendations in their electronic health record.

Results

Comparison of outcomes for 1 year before and the year after the e-consult service implementation identified improvements in direct thrombin inhibitor stewardship, increased diagnostic accuracy, and decreased length of stay of patients with confirmed HIT.

Conclusion

The e-consult platform is a novel method for rapid, targeted consultative guidance, and this single-institution pilot demonstrates its feasibility and effectiveness to improve the care of patients with suspected and confirmed HIT.

Accuracy of Diagnosing Heparin-Induced Thrombocytopenia

Importance

Heparin-induced thrombocytopenia (HIT) is a life-threatening condition that requires urgent diagnostic clarification. However, knowledge of the diagnostic utility of the recommended diagnostic tests is limited in clinical practice.

Objective

To evaluate the current diagnostic practice for managing the suspicion of HIT.

Design, Setting, and Participants

This prospective diagnostic study was conducted from January 2018 to May 2021 among consecutive patients with suspected HIT from 11 study centers in Switzerland, Germany, and the United States. Detailed clinical data and laboratory information were recorded. Platelet factor 4/heparin antibodies were quantified using an automated chemiluminescent immunoassay (CLIA). A washed-platelet heparin-induced platelet activation (HIPA) test was used as a reference standard to define HIT.

Exposures

Suspicion of HIT.

Main Outcomes and Measures

The primary outcome was the diagnostic accuracy of the 4Ts score, the CLIA, and the recommended algorithm serially combining both tests.

Results

Of 1448 patients included between 2018 and 2021, 1318 were available for the current analysis (median [IQR] age, 67 [57-75] years; 849 [64.6%] male). HIPA was positive in 111 patients (prevalence, 8.4%). The most frequent setting was intensive care unit (487 [37.0%]) or cardiovascular surgery (434 [33.0%]). The 4Ts score was low risk in 625 patients (46.8%). By 2 × 2 table, the numbers of patients with false-negative results were 10 (9.0%; 4Ts score), 5 (4.5%; CLIA), and 15 (13.5%; recommended diagnostic algorithm). The numbers of patients with false-positive results were 592 (49.0%; 4Ts score), 73 (6.0%; CLIA), and 50 (4.1%; recommended diagnostic algorithm), respectively.

Conclusions and Relevance

In this diagnostic study of patients suspected of having HIT, when the recommended diagnostic algorithm was used in clinical practice, antibody testing was required in half the patients. A substantial number of patients were, however, still misclassified, which could lead to delayed diagnosis or overtreatment. Development of improved diagnostic algorithms for HIT diagnosis should be pursued.

A retrospective analysis of the frequency of heparin-induced thrombocytopenia in the intensive care unit at a tertiary care center in Riyadh, Saudi Arabia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an extremely serious and potentially fatal condition that can develop in patients taking heparin-based medications, such as unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). The incidence and risk factors for HIT in critically ill patients, however, are not well defined.

METHODS

We retrospectively collected data on HIT test results, route of heparin administration, age, sex, heparin type (UFH or LMWH), and date of illness from patients admitted to the intensive care unit (ICU) and regular nursing floor (non-ICU) at our hospital between January 2011 and December 2014. We screened patients for HIT using the 4T score and confirmed the diagnosis through laboratory testing (direct enzyme immunoassay immunoglobulin G [IgG] or a platelet-activating antibody).

RESULTS

We screened a total of 946 patients, 56 (5.9%) of whom were positive for HIT. Among 776 patients receiving UFH and 180 receiving LMWH, 2.8 and 6.6% developed HIT, respectively (P = 0.051). We then classified our patients into two groups: ICU, and non-ICU. In the non-ICU group (n = 317), 4 (2.7%) patients receiving LMWH and 25 (5.1%) receiving UFH were positive for HIT (P = 0.221). In the ICU group (n = 639), 1 (3.1%) patient receiving LMWH and 26 (9.1%) receiving UFH were positive for HIT (P = 0.249). The ICU group, therefore, had a higher cumulative incidence rate of HIT than the non-ICU group (8.5 vs. 4.5%).

CONCLUSION

HIT was more common in ICU patients than non-ICU patients and in more patients receiving UFH than LMWH, although the differences were not statistically significant. Early diagnosis and appropriate treatment are essential to prevent adverse outcomes in patients with HIT.

Fibrinogenolysis and fibrinolysis in vaccine-induced immune thrombocytopenia and thrombosis

BACKGROUND

Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a rare syndrome associated with adenoviral vector vaccines for COVID-19. The syndrome is characterized by thrombosis, anti-platelet factor 4 (PF4) antibodies, thrombocytopenia, high D-dimer, and hypofibrinogenemia.

OBJECTIVES

To investigate abnormalities in fibrinolysis that contribute to the clinical features of VITT.

METHODS

Plasma samples from 18 suspected VITT cases were tested for anti-PF4 by ELISA and characterized as meeting criteria for VITT (11/18) or deemed unlikely (7/18; non-VITT). Antigen levels of PAI-1, factor XIII (FXIII), plasmin-α2antiplasmin (PAP), and inflammatory markers were quantified. Plasmin generation was quantified by chromogenic substrate. Western blotting was performed with antibodies to fibrinogen, FXIII-A, and plasminogen.

RESULTS

VITT patients 10/11 had scores indicative of overt disseminated intravascular coagulation, while 0/7 non-VITT patients met the criteria. VITT patients had significantly higher levels of inflammatory markers, IL-1β, IL-6, IL-8, TNFα, and C-reactive protein. In VITT patients, both fibrinogen and FXIII levels were significantly lower, while PAP and tPA-mediated plasmin generation were higher compared to non-VITT patients. Evidence of fibrinogenolysis was observed in 9/11 VITT patients but not in non-VITT patients or healthy controls. Fibrinogen degradation products were apparent, with obvious cleavage of the fibrinogen α-chain. PAP complex was evident in those VITT patients with fibrinogenolysis, but not in non-VITT patients or healthy donors.

CONCLUSION

VITT patients show evidence of overt disseminated intravascular coagulation and fibrinogenolysis, mediated by dysregulated plasmin generation, as evidenced by increased PAP and plasmin generation. These observations are consistent with the clinical presentation of both thrombosis and bleeding in VITT.

Quality improvement approaches to heparin-induced thrombocytopenia: a scoping review

Background

Heparin-induced thrombocytopenia (HIT) is a relatively uncommon condition characterized by 2 exceedingly common phenomena in hospitalized patients: thrombocytopenia and heparin exposure. Consequently, HIT is frequently overdiagnosed and inappropriately treated. These issues are the focus of many quality improvement (QI) initiatives.

Objectives

In this scoping review, we identified and characterized all published QI studies on improving the diagnosis and management of HIT.

Methods

We conducted a systematic literature search through April 2022 for studies reporting on QI interventions regarding the diagnosis, treatment, and/or prevention of HIT.

Results

Thirty studies were included in the final review. Studies were separated into 5 groups based on the focus of the interventions: increasing HIT recognition, reducing HIT incidence, reducing HIT overdiagnosis, promoting safer HIT management, and creating HIT task forces. Nine studies focused on the implementation of 4Ts score calculator into electronic medical record orders for HIT testing, while only 1 evaluated the impact of reducing unfractionated heparin use in favor of low-molecular-weight heparin. Six studies focused on the implementation of direct thrombin inhibitor management protocols, while none evaluated the use of alternative anticoagulants in HIT management.

Conclusion

The bulk of published HIT QI research focused on reducing overdiagnosis and promoting safer direct thrombin inhibitor therapy, while minimal attention has been devoted to HIT prevention and the use of evidence-based alternative HIT therapies.

Antibodies against platelet factor 4 and the risk of cerebral venous sinus thrombosis in patients with vaccine-induced immune thrombotic thrombocytopenia

BACKGROUND

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare complication of adenoviral vector-based vaccines against SARS-CoV-2. This syndrome is caused by antibodies against platelet factor 4 (PF4; CXCL4) that lead to platelet activation and is characterized by thrombocytopenia and thrombosis in unusual locations, including cerebral venous sinus thrombosis (CVST). VITT can be classified based on anti-PF4 antibodies properties in vitro: those that require PF4 to activate platelets (PF4-dependent) and those that can activate platelets without additional PF4 (PF4-independent) in the serotonin release assay.

OBJECTIVES

We aim to characterize the relationship of VITT platelet-activating profiles with CVST.

METHODS

We conducted a retrospective cohort study involving patients with confirmed VITT who were tested between March and June 2021. Data were collected with an anonymized form and cases were identified as VITT with high clinical suspicion according to platelet activation assays. Anti-PF4 antibody binding regions on PF4 were further characterized with alanine scanning mutagenesis.

RESULTS

Of the patients with confirmed VITT (n = 39), 17 (43.6%) had PF4-dependent antibodies and 22 (56.4%) had PF4-independent antibodies. CVST occurred almost exclusively in PF4-independent patients (11 of 22 vs 1 of 17; P < .05). Additionally, PF4-independent antibodies bound to 2 distinct epitopes on PF4, the heparin-binding region and a site typical for heparin-induced thrombocytopenia antibodies, whereas PF4-dependent antibodies bound to only the heparin-binding region.

CONCLUSION

These findings suggest that VITT antibodies that cause PF4-independent platelet activation represent a unique subset of patients more likely to be associated with CVST, possibly due to the 2 different types of anti-PF4 antibodies.

Different Types of Vasculitis Complicated by Heparin-Induced Thrombocytopenia-Analysis of Four Cases and Literature Review

Vasculitis and HIT have different etiologies, although both involve autoimmune mechanisms. Treatment of vasculitis often requires the use of an anticoagulant such as heparin, which can lead to the development of HIT and subsequent life-threatening complications. The analysis covered patients hospitalized in the Department of Internal Medicine, Nephrology and Dialysis in the period from September 2020 to March 2023. After analyzing the data, we selected four patients in whom vasculitis treatment was complicated by HIT. These included two patients with ANCA vasculitis and two patients with anti-GBM disease. We also described similar cases reported in the literature.

Heparin-induced thrombocytopenia associated with low-molecular-weight heparin: clinical feature analysis of cases and pharmacovigilance assessment of the FAERS database

Background: Unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) are commonly used anticoagulants for the management of arterial and venous thromboses. However, it is crucial to be aware that LMWH can, in rare cases, lead to a dangerous complication known as heparin-induced thrombocytopenia (HIT). The objective of this study was to evaluate the pharmacovigilance and clinical features of HIT associated with LMWH, as well as identify treatment strategies and risk factors to facilitate prompt management. Methods: We extracted adverse event report data from the FDA Adverse Event Reporting System (FAERS) database for pharmacovigilance assessment. Case reports on LMWH-induced thrombocytopenia dated up to 20 March 2023 were collected for retrospective analysis. Results: Significantly elevated reporting rates of HIT were shown in adverse event (AE) data of LMWHs in the FAERS database, while tinzaparin had a higher proportional reporting ratio (PRR) and reporting odds ratio (ROR) than other LMWHs, indicating a greater likelihood of HIT. Case report analysis indicated that a total of 43 patients showed evidence of LMWH-induced thrombocytopenia with a median onset time of 8 days. Almost half of the events were caused by enoxaparin. LMWHs were mainly prescribed for the treatment of embolism and thromboprophylaxis of joint operation. Patients with a history of diabetes or surgery appeared to be more susceptible to HIT. Clinical symptoms were mostly presented as thrombus, skin lesion, and dyspnea. Almost 90% of the patients experienced a platelet reduction of more than 50% and had a Warkentin 4T score of more than 6, indicating a high likelihood of HIT. In all patients, LMWHs that were determined to be the cause were promptly withdrawn. Following the discontinuation of LMWHs, almost all patients were given alternative anticoagulants and eventually achieved recovery. Conclusion: LMWH-induced thrombocytopenia is rare but serious, with increased risk in patients with diabetes or a surgical history. Prompt recognition and management are crucial for the safe use of LMWHs.

Clinical decision support to reduce unnecessary diagnostic testing for heparin-induced thrombocytopenia

Appropriate evaluation of heparin-induced thrombocytopenia (HIT) is imperative because of the potentially life-threatening complications. However, overtesting and overdiagnosis of HIT are common. Our goal was to evaluate the impact of clinical decision support (CDS) based on the HIT computerized-risk (HIT-CR) score, designed to reduce unnecessary diagnostic testing. This retrospective observational study evaluated CDS that presented a platelet count versus time graph and 4Ts score calculator to clinicians who initiated a HIT immunoassay order in patients with predicted low risk (HIT-CR score 0-2). The primary outcome was the proportion of immunoassay orders initiated but cancelled after firing of the CDS advisory. Chart reviews were conducted to assess anticoagulation usage, 4Ts scores and the proportion of patients who had HIT. In a 20-week period, 319 CDS advisories were presented to users who initiated potentially unnecessary HIT diagnostic testing. The diagnostic test order was discontinued in 80 (25%) patients. Heparin products were continued in 139 (44%) patients, and alternative anticoagulation was not given to 264 (83%). The negative predictive value of the advisory was 98.8% (95% CI: 97.2-99.5). HIT-CR score-based CDS can reduce unnecessary diagnostic testing for HIT in patients with a low pretest probability of HIT.

Treatment of thrombocytopenia and thrombosis in HIT in mice using deglycosylated KKO: a novel therapeutic?

Heparin-induced thrombocytopenia (HIT) is characterized by thrombocytopenia associated with a highly prothrombotic state due to the development of pathogenic antibodies that recognize human platelet factor 4 (hPF4) complexed with various polyanions. Although nonheparin anticoagulants are the mainstay of care in HIT, subsequent bleeding may develop, and the risk of developing new thromboembolic events remain. We previously described a mouse immunoglobulin G2bκ (IgG2bκ) antibody KKO that mimics the sentinel features of pathogenic HIT antibodies, including binding to the same neoepitope on hPF4-polyanion complexes. KKO, like HIT IgGs, activates platelets through FcγRIIA and induces complement activation. We then questioned whether Fc-modified KKO could be used as a novel therapeutic to prevent or treat HIT. Using the endoglycosidase EndoS, we created deglycosylated KKO (DGKKO). Although DGKKO retained binding to PF4-polyanion complexes, it inhibited FcγRIIA-dependent activation of PF4-treated platelets triggered by unmodified KKO, 5B9 (another HIT-like monoclonal antibody), and IgGs isolated from patients with HIT. DGKKO also decreased complement activation and deposition of C3c on platelets. Unlike the anticoagulant fondaparinux, injection of DGKKO into HIT mice lacking mouse PF4, but transgenic for hPF4 and FcγRIIA, prevented and reversed thrombocytopenia when injected before or after unmodified KKO, 5B9, or HIT IgG. DGKKO also reversed antibody-induced thrombus growth in HIT mice. In contrast, DGKKO was ineffective in preventing thrombosis induced by IgG from patients with the HIT-related anti-PF4 prothrombotic disorder, vaccine-induced immune thrombotic thrombocytopenia. Thus, DGKKO may represent a new class of therapeutics for targeted treatment of patients with HIT.

Novel Knowledge about Molecular Mechanisms of Heparin-Induced Thrombocytopenia Type II and Treatment Targets

Heparin-induced thrombocytopenia type II (HIT II), as stated in the literature, occurs in about 3% of all patients and in 0.1-5% of surgical patients. Thrombosis develops in 20-64% of patients with HIT. The mortality rate in HIT II has not decreased using non-heparin treatment with anticoagulants such as argatroban and lepirudin. An improved understanding of the pathophysiology of HIT may help identify targeted therapies to prevent thrombosis without subjecting patients to the risk of intense anticoagulation. The review will summarize the current knowledge about the pathogenesis of HIT II, potential new therapeutic targets related to it, and new treatments being developed. HIT II pathogenesis involves multi-step immune-mediated pathways dependent on the ratio of PF4/heparin and platelet, monocyte, neutrophil, and endothelium activation. For years, only platelets were known to take part in HIT II development. A few years ago, specific receptors and signal-induced pathways in monocytes, neutrophils and endothelium were revealed. It had been shown that the cells that had become active realised different newly formed compounds (platelet-released TF, TNFα, NAP2, CXCL-7, ENA-78, platelet-derived microparticles; monocytes-TF-MPs; neutrophils-NETs), leading to additional cell activation and consequently thrombin generation, resulting in thrombosis. Knowledge about FcγIIa receptors on platelets, monocytes, neutrophils and FcγIIIa on endothelium, chemokine (CXCR-2), and PSGL-1 receptors on neutrophils could allow for the development of a new non-anticoagulant treatment for HIT II. IgG degradation, Syk kinase and NETosis inhibition are in the field of developing new treatment possibilities too. Accordingly, IdeS and DNases-related pathways should be investigated for better understanding of HIT pathogenesis and the possibilities of being the HIT II treatment targets.

Modulation of ultralarge immune complexes in heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a serious thrombotic disorder caused by ultralarge immune complexes (ULICs) containing platelet factor 4 (PF4) and heparin that form the HIT antigen, together with a subset of anti-PF4 antibodies. ULICs initiate prothrombotic responses by engaging Fcγ receptors on platelets, neutrophils, and monocytes. Contemporary anti-thrombotic therapy for HIT is neither entirely safe nor entirely successful and acts downstream of ULIC formation and Fcγ receptor-initiated generation of thrombin.

OBJECTIVES

To determine whether HIT antigen and ULIC formation and stability could be modified favorably by inhibiting PF4-heparin interactions with fondaparinux, together with blocking formation of PF4 tetramers using a humanized monoclonal anti-PF4 antibody (hRTO).

METHODS

Results: The combination of fondaparinux and hRTO inhibited HIT antigen formation, promoted antigen dissociation, inhibited ULIC formation, and promoted ULIC disassembly at concentrations below the effective concentration of either alone and blocked Fcγ receptor-dependent induction of factor Xa activity by monocytic THP1 cells and activation of human platelets in whole blood. Combined with hRTO, fondaparinux inhibited HIT antigen and immune complex formation and activation through Fcγ receptors at concentrations at or below those used clinically to inhibit FXa coagulant activity.

CONCLUSIONS

HIT antigen and immune complexes are dynamic and amenable to modulation. Fondaparinux can be converted from an anticoagulant that acts at a downstream amplification step into a rationale, disease-specific intervention that blocks ULIC formation. Interventions that prevent ULIC formation and stability might increase the efficacy, permit use of lower doses, shorten the duration of antithrombotic therapy, and help prevent this serious thrombotic disorder.

A machine-learning model for reducing misdiagnosis in heparin-induced thrombocytopenia: A prospective, multicenter, observational study

BACKGROUND

Diagnosing heparin-induced thrombocytopenia (HIT) at the bedside remains challenging, exposing a significant number of patients at risk of delayed diagnosis or overtreatment. We hypothesized that machine-learning algorithms could be utilized to develop a more accurate and user-friendly diagnostic tool that integrates diverse clinical and laboratory information and accounts for complex interactions.

METHODS

We conducted a prospective cohort study including 1393 patients with suspected HIT between 2018 and 2021 from 10 study centers. Detailed clinical information and laboratory data were collected, and various immunoassays were conducted. The washed platelet heparin-induced platelet activation assay (HIPA) served as the reference standard.

FINDINGS

HIPA diagnosed HIT in 119 patients (prevalence 8.5%). The feature selection process in the training dataset (75% of patients) yielded the following predictor variables: (1) immunoassay test result, (2) platelet nadir, (3) unfractionated heparin use, (4) CRP, (5) timing of thrombocytopenia, and (6) other causes of thrombocytopenia. The best performing models were a support vector machine in case of the chemiluminescent immunoassay (CLIA) and the ELISA, as well as a gradient boosting machine in particle-gel immunoassay (PaGIA). In the validation dataset (25% of patients), the AUROC of all models was 0.99 (95% CI: 0.97, 1.00). Compared to the currently recommended diagnostic algorithm (4Ts score, immunoassay), the numbers of false-negative patients were reduced from 12 to 6 (-50.0%; ELISA), 9 to 3 (-66.7%, PaGIA) and 14 to 5 (-64.3%; CLIA). The numbers of false-positive individuals were reduced from 87 to 61 (-29.8%; ELISA), 200 to 63 (-68.5%; PaGIA) and increased from 50 to 63 (+29.0%) for the CLIA.

INTERPRETATION

Our user-friendly machine-learning algorithm for the diagnosis of HIT (https://toradi-hit.org) was substantially more accurate than the currently recommended diagnostic algorithm. It has the potential to reduce delayed diagnosis and overtreatment in clinical practice. Future studies shall validate this model in wider settings.

FUNDING

Swiss National Science Foundation (SNSF), and International Society on Thrombosis and Haemostasis (ISTH).

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.

Minimal role for the alternative pathway in complement activation by HIT immune complexes

BACKGROUND

Anti-platelet factor 4 (PF4)/heparin immune complexes that cause heparin-induced thrombocytopenia (HIT) activate complement via the classical pathway. Previous studies have shown that the alternative pathway of complement substantially amplifies the classical pathway of complement activation through the C3b feedback cycle.

OBJECTIVES

These studies sought to examine the contributions of the alternative pathway to complement activation by HIT antibodies.

METHODS

Using IgG monoclonal (KKO) and/or patient-derived HIT antibodies, we compared the effects of classical pathway (BBK32 and C1-esterase inhibitor [C1-INH]), alternative pathway (anti-factor B [fB] or factor D [fD] inhibitor) or combined classical and alternative pathway inhibition (soluble complement receptor 1 [sCR1]) in whole blood or plasma.

RESULTS

Classical pathway inhibitors BBK32 and C1-INH and the combined classical/alternative pathway inhibitor sCR1 prevented KKO/HIT immune complex-induced complement activation, including release of C3 and C5 activation products, binding of immune complexes to B cells, and neutrophil activation. The alternative pathway inhibitors fB and fD, however, did not affect complement activation by KKO/HIT immune complexes. Similarly, alternative pathway inhibition had no effect on complement activation by unrelated immune complexes consisting of anti-dinitrophenyl (DNP) antibody and the multivalent DNP--keyhole limpet hemocyanin antigen.

CONCLUSIONS

Collectively, these findings suggest the alternative pathway contributes little in support of complement activation by HIT immune complexes. Additional in vitro and in vivo studies are required to examine if this property is shared by most IgG-containing immune complexes or if predominance of the classic pathway is limited to immune complexes composed of multivalent antigens.

Acute lower limb ischemia caused by vaccine-induced immune thrombotic thrombocytopenia: focus on perioperative considerations for 2 cases

BACKGROUND

ChAdOx1 nCoV-19 (AstraZeneca) and Ad26COV2.S (Johnson & Johnson/Janssen) adenoviral vector vaccines have been associated with vaccine-induced immune thrombotic thrombocytopenia (VITT). Arterial thrombosis and acute limb ischemia have been described in a minority of patients with VITT. These patients usually need a revascularization, but they potentially are at a higher risk of complications. Optimal perioperative care of patients undergoing vascular surgery in acute VITT is unknown and important considerations in such context need to be described.

CASES PRESENTATIONS

We report 2 cases of VITT presenting with acute limb ischemia who needed vascular surgery and we describe the multidisciplinary team decisions for specific treatment surrounding the interventions. Both patients' platelet counts initially increased after either intravenous immune globulin (IVIG) or therapeutic plasma exchange (TPE). None received platelet transfusion. They both received argatroban as an alternative to heparin for their surgery. Despite persistent positivity of anti-platelet factor 4 (PF4) antibodies and serotonin-release assay with added PF4 (PF4-SRA) in both patients, only one received a repeated dose of IVIG before the intervention. Per- and post-operative courses were both unremarkable.

CONCLUSION

In spite of persistent anti-PF4 and PF4-SRA positivity in the setting of VITT, after platelet count improvement using either IVIG or TPE, vascular interventions using argatroban can show favorable courses. Use of repeated IVIG or TPE before such interventions still needs to be defined.

Management of Postoperative Myocardial Injury After Non-cardiac Surgery in Patients Aged ≥ 80 Years: Our 10 Years' Experience

Background

Postoperative myocardial injury (PMI) is associated with short- and long-term mortality. The incidence of PMI in very old patients is currently unknown. There is currently neither known effective prophylaxis nor a uniform strategy for the elderly with PMI.

Objective

To share our 10 years of experience in the comprehensive management of PMI after non-cardiac surgery in patients aged ≥ 80 years.

Methods

In this case series, we retrospectively collected and assessed the 2,984 cases aged ≥ 80 years who accepted non-cardiac surgery from 2011 to 2021 at the second Medical Center, Chinese PLA General Hospital. The incidence, risk factors, management strategy, and prognosis of surgical patients with PMI were analyzed.

Results

A total of 2,984 patients met our inclusion criteria. The overall incidence of PMI was 14%. In multivariable analysis, coronary artery disease, chronic heart failure, and hypotension were independently associated with the development of PMI. The patients with PMI were at a higher risk of death (OR, 2.69; 95% CI, 1.78–3.65). They were more likely to have received low molecular heparin, anti-plantlet therapy, beta-blocker, early coronary angiography, and statin than patients without PMI. The 30-day (0.96% vs. 0.35%; OR 3.46; 95% CI, 1.49–7.98; P &lt; 0.001) and 1-year mortality (5.37% vs. 2.60%; OR 2.35; 95% CI, 1.12–6.53; P &lt; 0.001) was significantly higher in patients with PMI compared with those without PMI.

Conclusions

The incidence of PMI in very old patients was high. The PMI is associated with an increased risk of 30 days and 1-year mortality. These patients can benefit from intensification of assessment and individualized care of multi-morbidities during the perioperative period. Especially cardiovascular medical treatments, such as antiplatelet, anticoagulation, β-blockers, and statins are very important for patients with PMI.

Measurement of procoagulant platelets provides mechanistic insight and diagnostic potential in heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a prothrombotic, immune-mediated adverse drug reaction associated with high rates of thrombosis-related morbidity and mortality caused by FcγRIIa-activating pathogenic antibodies to PF4-heparin. Procoagulant platelets are a platelet subset that promote thrombin generation, are clinically relevant in prothrombotic diseases, and are formed when platelet G-protein-coupled receptor (GPCR) and ITAM-linked receptors are co-stimulated.

OBJECTIVES

We examined the procoagulant platelet response of healthy donors to platelet agonists in the presence of HIT plasma and determined the contribution of FcγRIIa.

PATIENTS/METHODS

Our previously established flow cytometry-based procoagulant platelet assay was modified to incorporate plasma samples, performed using FcγRIIa-responsive donor platelets. Plasma samples were serotonin-release assay-confirmed HIT (HIT+), or negative on HIT screening.

RESULTS

In response to GPCR stimulation, only HIT+ plasma produced a heparin-dependent sensitization that required active FcγRIIa. As a potential diagnostic tool, the procoagulant platelet assay achieved 98% accuracy in identifying clinically verified HIT when performed blinded to the diagnoses of a validation cohort. Samples inducing a higher procoagulant platelet response were more likely from patients with thrombotic complications. Thrombin stimulation markedly increased the procoagulant platelet response with HIT+ plasma that was heparin independent and only partially reversed by FcγRIIa blockade, possibly reflecting ongoing thrombotic risk after heparin cessation.

CONCLUSIONS

We demonstrate that HIT plasma together with platelet agonists increased the procoagulant platelet proportions, which may contribute to thrombotic risk in HIT. Targeting procoagulant platelet activation may represent a novel treatment strategy. This assay may be a rapid, clinically relevant functional assay for accurately detecting pathological HIT antibodies.

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.

Vaccine-induced thrombosis and thrombocytopenia (VITT) in Ireland: A review of cases and current practices

Since the beginning of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS CoV-2) virus pandemic, several highly effective and safe vaccines have been produced at remarkable speed. Following global implementation of vaccination programmes, cases of thrombosis with thrombocytopenia following administration of adenoviral vector-based vaccines started being reported. In this review we discuss the known pathogenesis and epidemiology of so-called vaccine induced thrombocytopenia and thrombosis (VITT). We consider the available guidelines, diagnostic laboratory tests and management options for these patients. Finally, we discuss important unanswered questions and areas for future research in this novel pathoclinical entity.

Postoperative Natural Killer Cell Dysfunction: The Prime Suspect in the Case of Metastasis Following Curative Cancer Surgery

Surgical resection is the foundation for the curative treatment of solid tumors. However, metastatic recurrence due to the difficulty in eradicating micrometastases remain a feared outcome. Paradoxically, despite the beneficial effects of surgical removal of the primary tumor, the physiological stress resulting from surgical trauma serves to promote cancer recurrence and metastasis. The postoperative environment suppresses critical anti-tumor immune effector cells, including Natural Killer (NK) cells. The literature suggests that NK cells are critical mediators in the formation of metastases immediately following surgery. The following review will highlight the mechanisms that promote the formation of micrometastases by directly or indirectly inducing NK cell suppression following surgery. These include tissue hypoxia, neuroendocrine activation, hypercoagulation, the pro-inflammatory phase, and the anti-inflammatory phase. Perioperative therapeutic strategies designed to prevent or reverse NK cell dysfunction will also be examined for their potential to improve cancer outcomes by preventing surgery-induced metastases.

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.

Vaccine-induced immune thrombotic thrombocytopenia: what we know and do not know

The development of vaccines to fight COVID-19 has been a remarkable medical achievement. However, this global immunization effort has been complicated by a rare vaccine-related outcome characterized by thrombocytopenia and thrombosis in association with platelet-activating anti-platelet factor 4 antibodies. In this Spotlight, we will discuss the recently described complication of vaccine-induced immune thrombotic thrombocytopenia (VITT) occurring in response to certain COVID-19 vaccines. Although information about this clinical condition is rapidly evolving, we will summarize our current understanding of VITT.

Complement mediates binding and procoagulant effects of ultra-large HIT immune complexes

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder mediated by ultra-large immune complexes (ULICs) containing IgG antibodies to a multivalent antigen composed of platelet factor 4 (PF4) and heparin. The limitations of current anti-thrombotic therapy in HIT supports the need to identify additional pathways that may be targets for therapy. Activation of FcgRIIA by HIT ULICs initiates diverse procoagulant cellular effector functions. HIT ULICs are also known to activate complement, but the contribution of this pathway to the pathogenesis of HIT has not been studied in detail. We observed that HIT ULICs physically interact with C1q in buffer and plasma, activate complement via the classical pathway, promote co-deposition of IgG and activated C3 complement fragments (C3c) on neutrophil and monocyte cell surfaces. Complement activation by ULICs, in turn, facilitates Fcg receptor(R)-independent monocyte tissue factor expression, enhances IgG binding to the cell surface FcgRs and promotes platelet adhesion to injured endothelium. Inhibition of the proximal, but not terminal, steps in the complement pathway, abrogates monocyte tissue factor expression by HIT ULICs. Together, these studies suggest a major role for complement activation in regulating Fc-dependent effector functions of HIT ULICs, identify potential non-anticoagulant targets for therapy, and provide insights into the broader roles of complement in immune complex-mediated thrombotic disorders.

Soluble glycoprotein VI is a predictor of major bleeding in patients with suspected heparin-induced thrombocytopenia

We have shown that patients with suspected heparin-induced thrombocytopenia (HIT) have a high incidence of major bleeding. Recent studies have implicated elevated soluble glycoprotein VI (sGPVI) levels as a potential risk factor for bleeding. We sought to determine if elevated sGPVI plasma levels are associated with major bleeding events in patients with suspected HIT. We used a cohort of 310 hospitalized adult patients with suspected HIT who had a blood sample collected at the time HIT was suspected. Plasma sGPVI levels were measured by using enzyme-linked immunosorbent assay. Patients were excluded who had received a platelet transfusion within 1 day of sample collection because of the high levels of sGPVI in platelet concentrates. We assessed the association of sGPVI (high vs low) with International Society on Thrombosis and Haemostasis major bleeding events by multivariable logistic regression, adjusting for other known risk factors for bleeding. Fifty-four patients were excluded due to recent platelet transfusion, leaving 256 patients for analysis. Eighty-nine (34.8%) patients had a major bleeding event. Median sGPVI levels were significantly elevated in patients with major bleeding events compared with those without major bleeding events (49.09 vs 31.93 ng/mL; P < .001). An sGPVI level >43 ng/mL was independently associated with major bleeding after adjustment for critical illness, sepsis, cardiopulmonary bypass surgery, and degree of thrombocytopenia (adjusted odds ratio, 2.81; 95% confidence interval, 1.51-5.23). Our findings suggest that sGPVI is associated with major bleeding in hospitalized patients with suspected HIT. sGPVI may be a novel biomarker to predict bleeding risk in patients with suspected HIT.

A prospective, blinded study of a PF4-dependent assay for HIT diagnosis

Heparin-induced thrombocytopenia (HIT) is a life-threatening, prothrombotic, antibody-mediated disorder. To maximize the likelihood of recovery, early and accurate diagnosis is critical. Widely available HIT assays, such as the platelet factor 4 (PF4) heparin enzyme-linked immunosorbent assay (ELISA) lack specificity, and the gold-standard carbon 14-labeled serotonin release assay (SRA) is of limited value for early patient management because it is available only through reference laboratories. Recent studies have demonstrated that pathogenic HIT antibodies selectively activate PF4-treated platelets and that a technically simpler assay, the PF4-dependent P-selectin expression assay (PEA), may provide an option for rapid and conclusive results. Based upon predefined criteria that combined 4Ts scores and HIT ELISA results, 409 consecutive adults suspected of having HIT were classified as disease positive, negative, or indeterminate. Patients deemed HIT indeterminate were considered disease negative in the primary analysis and disease positive in a sensitivity analysis. The ability of PEA and SRA to identify patients judged to have HIT was compared using receiver operating characteristic curve statistics. Using these predefined criteria, the diagnostic accuracy of PEA was high (area under the curve [AUC], 0.94; 95% confidence interval [CI], 0.87-1.0) and similar to that of SRA (AUC, 0.91; 95% CI, 0.82-1.0). In sensitivity analysis, the AUCs of PEA and SRA were also similar at 0.88 (95% CI, 0.78-0.98) and 0.86 (95% CI, 0.77-0.96), respectively. The PEA, a technically simple nonradioactive assay that uses ∼20-fold fewer platelets compared with the SRA, had high accuracy for diagnosing HIT. Widespread use of the PEA may facilitate timely and more effective management of patients with suspected HIT.

Heparin-Induced Thrombocytopenia: A Focus on Thrombosis

Heparin-induced thrombocytopenia is an immune-mediated disorder caused by antibodies that recognize complexes of platelet factor 4 and heparin. Thrombosis is a central and unpredictable feature of this syndrome. Despite optimal management, disease morbidity and mortality from thrombosis remain high. The hypercoagulable state in heparin-induced thrombocytopenia is biologically distinct from other thrombophilic disorders in that clinical complications are directly attributable to circulating ultra-large immune complexes. In some individuals, ultra-large immune complexes elicit unchecked cellular procoagulant responses that culminate in thrombosis. To date, the clinical and biologic risk factors associated with thrombotic risk in heparin-induced thrombocytopenia remain elusive. This review will summarize our current understanding of thrombosis in heparin-induced thrombocytopenia with attention to its clinical features, cellular mechanisms, and its management.

Pathogenesis of heparin-induced thrombocytopenia

There are currently no effective substitutes for high intensity therapy with unfractionated heparin (UFH) for cardiovascular procedures based on its rapid onset of action, ease of monitoring and reversibility. The continued use of UFH in these and other settings requires vigilance for its most serious nonhemorrhagic complication, heparin induced thrombocytopenia (HIT). HIT is an immune prothrombotic disorder caused by antibodies that recognize complexes between platelet factor 4 (PF4) and polyanions such as heparin (H).The pathogenicity of anti-PF4/H antibodies is likely due to the formation of immune complexes that initiate intense procoagulant responses by vascular and hematopoietic cells that lead to the generation of platelet microparticles, monocyte and endothelial cell procoagulant activity, and neutrophil extracellular traps, among other outcomes. The development of anti-PF4/H antibodies after exposure to UFH greatly exceeds the incidence of clinical disease, but the biochemical features that distinguish pathogenic from nonpathogenic antibodies have not been identified. Diagnosis relies on pretest clinical probability, screening for anti-PF4/H antibodies and documentation of their platelet activating capacity. However, both clinical algorithms and test modalities have limited predictive values making diagnosis and management challenging. Given the unacceptable rates of recurrent thromboembolism and bleeding associated with current therapies, there is an unmet need for novel rational nonanticoagulant therapeutics based on the pathogenesis of HIT. We will review recent developments in our understanding of the pathogenesis of HIT and its implications for future approaches to diagnosis and management.

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.