Functional Assays in the Diagnosis of Heparin-Induced Thrombocytopenia: A Review

Heparin Induced Thrombocytopenia Testing

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

Flow cytometry vs conventional methods for the evaluation of anti-PF4/heparin antibodies: a single center study

Aim: Heparin-induced thrombocytopenia (HIT) is a rare, life-threatening, immune-mediated adverse effect of heparin administration. This study compares frequently used laboratory assays in terms of their effectiveness in HIT diagnosis.Materials & methods: Fifty patients with suspected HIT were tested by gel immunoassay and solid phase PF4/heparin antibody ELISA. On positive results, platelet activation markers P-selectin and Annexin V were assayed using flow cytometry.Results: Thirty/50 patients were negative for both immunoassays. Flow cytometry was performed in the 20 immunoassay positive patients. Platelet activation was observed in 7/20 in the presence of low heparin concentration (0.2 IU/ml).Conclusion: The results are in accordance with the currently available literature and flow cytometry seems a promising alternative in HIT laboratory investigation.

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

PURPOSE

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

SUMMARY

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

CONCLUSION

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

Association of medical comorbidities in obese subjects diagnosed with heparin-induced thrombocytopenia

Objectives

Heparin-induced thrombocytopenia can occur in obese subjects. The medical comorbidities associated with obesity may contribute to the pathogenesis of this disease. It is unknown, however, which specific medical comorbidities and if higher odds of thrombosis are present in obese heparin-induced thrombocytopenia patients. We sought to determine whether obese heparin-induced thrombocytopenia subjects had higher odds of both comorbidities and thrombosis, hypothesizing that this patient population would have higher odds of both these conditions.

Methods

This was a multi-center retrospective study utilizing TriNetX©, an electronic health record database, in subjects aged 18-99 years diagnosed with heparin-induced thrombocytopenia. The cohort was divided into two groups (1) non-obese (body mass index < 30 kg/m2) and (2) obese (body mass index ⩾ 30 kg/m2). We evaluated patient characteristics, diagnostic, laboratory, medication, and procedure codes.

Results

A total of 1583 subjects (696 (44.0%) non-obese and 887 (56.0%) obese) were included. Obese subjects had higher odds of diabetes with complications (OR = 1.73, 95% CI = 1.35-2.22, p < 0.001) and without complications (OR = 1.81, 95% CI = 1.47-2.22, p < 0.001). This association was still present after correcting for demographic and clinical factors. There were no increased odds of thrombosis observed in the obesity group.

Conclusions

Our study found that obese heparin-induced thrombocytopenia subjects had higher odds of having a diabetes mellitus comorbidity, but did not have higher odds of thrombosis. Given obesity is considered a hypercoagulable state, further study may be needed to understand why obese subjects diagnosed with heparin-induced thrombocytopenia do not have higher rates of thrombosis.

Heparin-induced thrombocytopenia after cardiac surgery. A single-center, retrospective cohort study

Background

Cardiac surgery is a high-risk setting for heparin-induced thrombocytopenia (HIT). However, large differences in its incidence, rate of thrombotic complications, and mortality have been reported in this context. Few studies address the pharmacologic management of HIT specifically in this setting.

Objectives

To describe the incidence, outcomes, and management of patients with HIT in our cohort and to compare them with patients presenting platelet factor 4/heparin antibodies but without platelet-activating capacity.

Methods

A retrospective observational study was conducted over a period of 10 years and 6 months on 13,178 cardiac operations in a single high-volume cardiac surgery center.

Results

HIT was diagnosed in 0.22% of patients. HIT with associated thromboembolic complications occurred in 0.04% of cases. Two deaths at 30 days were registered, both in patients with associated thrombosis. The 4T score showed a 99.9% negative predictive value. The immunoglobulin G-specific chemiluminescence test positivity rate was highly predictive of HIT. Warfarin was often started early after surgery, and although it was rarely stopped when the diagnosis of HIT was made, no new thromboembolic complications subsequently occurred. Thrombocytopenia appeared to be a poor prognostic sign, whatever the cause.

Conclusion

Although rare, HIT is characterized by high mortality in this setting, especially if thrombotic complications occur. Large multicentric studies or an international registry should be created to enhance the scientific evidence on HIT diagnosis and management in this context.

ChAdOx1 nCoV-19 Vaccine and Thrombosis with Thrombocytopenia Syndrome among Adults: A Systematic Review

Several vaccine-induced thrombotic thrombocytopenia syndrome (VITTS) cases have been reported after the ChAdOx1 nCov-19 vaccination. The current study systematically reviewed the reported post-ChAdOx1 nCoV-19 vaccination thrombotic thrombocytopenia cases. Their laboratory and clinical features, as well as the diagnostic and therapeutic measures, were investigated. Online databases were searched until 25 August 2021. Studies reporting post-ChAdOx1 nCov-19 vaccination thrombotic thrombocytopenia syndrome (TTS) were included. Overall, 167 cases (21-77 years old) from 53 publications were included showing a female dominance of 1.75 times. About 85% of the cases exhibited the primary symptoms within the first two weeks post-vaccination. Headache was the most common initial symptom (>44.2%), and hemorrhage/thrombotic problems (22.46%), as well as discoordination/weakness/numbness/ hemiparesis/cyanotic toes (19.6%), were the most prevalent uncommon initial symptoms. Prothrombin time (PT), D-dimers, and C-reactive protein were the most remarkable increased laboratory parameters in 50.6%, 99.1%, and 55.6% of cases, respectively. In comparison, platelet and fibrinogen were the most remarkable decreased laboratory parameters in 92.7% and 50.5% of cases, respectively. Most VITT cases presented with cerebral venous thrombosis/cerebral venous sinus thrombosis, supraventricular tachycardia, transverse sinus/cerebral thrombosis, pulmonary embolism, and cerebral hemorrhage. Anti-PF4 antibody measurement through immunoassays and functional assays were positive in 86.2% and 73% of cases, respectively. About 31% of the cases died. Early diagnosis and proper therapeutic measures are important in ChAdOx1 nCov-19 vaccine-induced VITTS patients. Therefore, experts are recommended to know the corresponding clinical and laboratory features, as well as diagnostic methods. Elucidation of the pathophysiologic mechanism of ChAdOx1 nCov-19 vaccine-induced TTS deserves further investigation.

Incidence and clinical outcomes of heparin-induced thrombocytopenia: 11 year experience in a tertiary care university hospital

Heparin-induced thrombocytopenia (HIT) is a rare immuno-mediated adverse reaction with high thrombotic and mortality risk. To evaluate incidence and outcomes of HIT cases diagnosed at a tertiary care hospital from 2007 to 2018. A retrospective study was conducted. Patients with suspected HIT underwent 4Ts score assessment and anti-heparin PF4 IgG antibodies ELISA screening test. If the latter was positive, platelet aggregation test (PAT) was performed. If the latter was positive, any form of heparin was stopped, alternative anticoagulants were started and then overlapped with warfarin. HIT incidence was calculated by dividing HIT cases by the mean yearly number of admitted patients over 11 years. Follow-up was 90 days. Among 2125 screening tests, 96 (4.5%) were positive with confirmatory PAT in 82/90 (3.8% for missing data in 6). Median age was 75; 39 patients were surgical and 51 medical. The median 4Ts score was 5. Unfractionated heparin was employed in 34 (37%). HIT incidence was 0.16/1000/patient/years (95% CI: 0.12-0.23) in surgical and 0.15/1000/patient/years (95%: 0.12-0.20) in medical patients. HIT with thrombosis (HIT-T) was observed in 31 patients (0.05/1000/patient/years 95% CI: 0.04-0.1), with venous thromboses in 25 (80%). HIT without thrombosis was observed in 59 patients (0.1/1000 patient/years; 95% CI: 0.08-0.13, twofold vs HIT-T). All cause mortality was 25.5% (95% CI: 17.6-35.4), major bleeding 7.7% (95% CI:3.2-15.3), and thromboembolic complications 3.3% (95% CI:1.1-9.3). HIT is a rare event with high mortality, despite the use of non heparin anticoagulants.

A Functional Assay for the Determination of Heparin-Induced Thrombocytopenia via Flow Cytometry

Heparin-induced thrombocytopenia (HIT) is a life-threatening complication of heparin therapy (both unfractionated heparin and low-molecular-weight heparin). In our study, we examined a group of 122 patients with suspected HIT. The samples of all patients were analyzed in the first step using an immunoassay (ID-PaGIA Heparin/PF4, Hemos1L-Acustar HIT IgG, ZYMUTEST HIA Monostrip IgG) to detect the presence of antibodies against heparin-PF4 complexes (platelet factor 4). When the immunoassay was positive, the sample was subsequently analyzed for HIT with a functional flow cytometry assay, the HITAlert kit, the purpose of which was to demonstrate the ability of the antibodies present to activate platelets. A diagnosis of HIT can be made only after a positive functional test result. In this article, we present an overview of our practical experience with the use of the new functional method of analysis, HIT, with flow cytometry. In this work, we compared the mutual sensitivity of two functional tests, SRA and the flow cytometry HITAlert kit, in patients perceived as being at risk for HIT. This work aims to delineate the principle, procedure, advantages, pitfalls, and possibilities of the application of the functional test HITAlert using flow cytometry.

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).

Heparin-Independent and Heparin-Dependent Anti-CXCL4 Antibodies Have a Reciprocal Expression in a Systemic Sclerosis Patients' Cohort

Systemic sclerosis (SSc) is a chronic disease characterized by skin/internal organ fibrosis, vasculopathy and autoimmunity. Chemokine (C-X-C motif) ligand 4 (CXCL4) is an early SSc biomarker that predicts worse disease outcome. We previously reported that CXCL4 is an autoantigen in SSc, and anti-CXCL4 antibodies correlated with IFN-I and were more abundant in patients with lung fibrosis. However, it is unclear whether antibodies to CXCL4 in SSc are only directed to CXCL4 or recognize complexes formed by CXCL4 and heparin. Here, by analyzing an SSc cohort, we addressed the occurrence of circulating heparin-dependent VS heparin-independent anti-CXCL4 antibodies and their relationship with a few disease parameters. We found that heparin-dependent, like the heparin-independent antibodies, are higher in SSc as compared to healthy donors; they are detectable in 24% and 30% of the SSc patients, respectively, and appear inversely correlated and mutually exclusive. Like the heparin-independent antibodies, heparin-dependent antibodies correlated with digital ulcers. However, in contrast to heparin-independent antibodies, heparin-dependent antibodies did not correlate with IFN-I, but were largely expressed in patients with pulmonary arterial hypertension. This pilot study indicates that heparin-dependent antibodies are worth studying in larger SSc cohorts to address whether they discriminate SSc sub-groups with different pathological characteristics and outcomes.

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

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

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

Background

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

Objectives

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

Methods

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

Results

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

Conclusions

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

The potential impact of Covid-19 on the capacity of routine laboratory tests to detect heparin-induced thrombocytopenia

In Covid-19, anticoagulation with heparin is often administered to prevent or treat thromboembolic events. Heparin-induced thrombocytopenia (HIT) is a severe complication of heparin treatment, caused by heparin-dependent, platelet activating anti-platelet factor 4 (PF4)/heparin antibodies. Diagnosis of HIT is based on the combination of clinical parameters, allowing to determine the pretest probability, and laboratory testing for anti-PF4/heparin antibodies and confirmatory functional assays, such as the heparin-induced platelet activation (HIPA) test.

We report the case of a patient with severe Covid-19 pneumonia requiring ECMO treatment, who developed recurrent clotting of the ECMO filter and a drop in platelet count under heparin treatment. He was therefore suspected to have HIT and the anticoagulation was switched to argatroban. Despite high clinical probability and high titres of anti-PF4/heparin antibodies, the functional HIPA test was negative. Nevertheless, argatroban was continued rather than to reinstate anticoagulation with heparin. Reevaluation 7 days later then demonstrated a strongly positive functional HIPA test and confirmed the diagnosis of HIT. Under anticoagulation with argatroban the patient gradually improved and was finally weaned off the ECMO.

In conclusion, this case highlights the critical importance of clinical judgement, exploiting the 4 T score, given that Covid-19 patients may present a different pattern of routine laboratory test results in HIT diagnostics. The possibility of a false negative HIPA test has to be considered, particularly in early phases of presentation. In cases of a discrepancy with high clinical probability of HIT and/or high titre anti-PF4/heparin antibodies despite a negative HIPA test, a reevaluation within 3 to 5 days after the initial test should be considered in order to avoid precipitant reestablishment of unfractionated heparin, with potentially fatal consequences.

Breast cancer cell-based ELISA: a potential material for better detection of heparin-induced thrombocytopenia antibodies

Heparin-induced thrombocytopenia (HIT) is caused by newly formed platelet-activating antibodies against complexes formed between platelet factor 4 (PF4) and heparin (H). HIT can result in life-threatening complications; thus, early detection of HIT antibodies is crucial for the treatment of the disease. The enzyme-linked immune absorbance assay (ELISA) for the identification of HIT antibodies is widely used in many laboratories, but in general, this test provides only ∼50% accuracy while other methods show multiple limitations. Here, we developed a new cell-based ELISA to improve the detection of HIT antibodies. Instead of immobilizing PF4 or PF4/H complexes directly onto a plate as in the standard ELISA, we added the complexes on breast cancer cells, i.e., cell line MDA-MB-231, and applied the same protocol for antibody detection. Using confocal laser scanning microscopy and flow cytometry for the characterization of bound complexes, we identified two types of HIT-mimicked antibodies (KKO and 1E12), which were able to differentiate from the non-HIT antibody (RTO). PF4-treated MDA-MB-231 cells allowed binding of HIT-mimicked antibodies better than PF4/H complexes. With human sera, the cell-based ELISA allowed better differentiation of clinically relevant from non-clinically relevant HIT antibodies as compared with the standard ELISA. Our findings provide a potential approach that contributes to the development of better assays for the detection of HIT antibodies.

Prevalence and outcomes of patients developing heparin-induced thrombocytopenia during extracorporeal membrane oxygenation

Objectives

Unfractionated heparin (UFH) is the commonly used anticoagulant to prevent clotting of the ECMO circuit and thrombosis of the cannulated vessels. A side effect of UFH is heparin-induced thrombocytopenia (HIT). Little is known about HIT during ECMO and the impact of changing anticoagulation in ECMO patients with newly diagnosed HIT. The aim of the study was to determine the prevalence, complications, impact of switching anticoagulation to argatroban and outcomes of patients developing heparin-induced thrombocytopenia (HIT) during either veno-venous (VV) or veno-arterial (VA) ECMO.

Methods

Retrospective observational single centre study of prospectively collected data of consecutive patients receiving VV ECMO therapy for severe respiratory failure and VA ECMO for circulatory failure from January 2006 to December 2016 of the Medical intensive care unit (ICU) of the University Hospital of Regensburg. Treatment of HIT on ECMO was done with argatroban.

Results

507 patients requiring ECMO were included. Further HIT-diagnostic was conducted if HIT-4T-score was ≥4. The HIT-confirmed group had positive HIT-enzyme-linked-immunosorbent-assay (ELISA) and positive heparin-induced-platelet-activation (HIPA) test, the HIT-suspicion group a positive HIT-ELISA and missing HIPA but remained on alternative anticoagulation until discharge and the HIT-excluded group a negative or positive HIT-ELISA, however negative HIPA. These were compared to group ECMO-control without any HIT suspicion. The prevalence of HIT-confirmed was 3.2%, of HIT-suspicion 2.0% and HIT-excluded 10.8%. Confirmed HIT was trendwise more frequent in VV than in VA (3.9 vs. 1.7% p = 0.173). Compared to the ECMO control group, patients with confirmed HIT were longer on ECMO (median 13 vs. 8 days, p = 0.002). Different types of complications were higher in the HIT-confirmed than in the ECMO-control group, but in-hospital mortality was not different (31% vs. 41%, p = 0.804).

Conclusion

HIT is rare on ECMO, should be suspected, if platelets are decreasing, but seems not to increase mortality if treated promptly.

COVID-19 Vaccine-Associated Thrombosis With Thrombocytopenia Syndrome (TTS): A Systematic Review and Post Hoc Analysis

Background

A new clinical syndrome has been recognized following the COVID-19 vaccine, termed thrombosis with thrombocytopenia syndrome (TTS). The following systematic review focuses on extrapolating thrombotic risk factors, clinical manifestations, and outcomes of patients diagnosed with TTS following the COVID-19 vaccine.

Methods

We utilized the World Health Organization's criteria for a confirmed and probable case of TTS following COVID-19 vaccination and conducted a systematic review and posthoc analysis using the PRISMA 2020 statement. Data analysis was conducted using SPSS V25 for factors associated with mortality, including age, gender, anti-PF4/heparin antibodies, platelet nadir, D-dimer peak, time to event diagnosis, arterial or venous thrombi.

Results

Of the 175 studies identified, a total of 25 studies with 69 patients were included in this systematic review and post hoc analysis. Platelet nadir (P < .001), arterial or venous thrombi (χ2 = 41.911, P = .05), and chronic medical conditions (χ2 = 25.507, P = .041) were statistically associated with death. The ROC curve analysis yielded D-dimer (AUC = .646) and platelet nadir (AUC = .604) as excellent models for death prediction.

Conclusion

Adenoviral COVID-19 vaccines have been shown to trigger TTS, however, reports of patients having received mRNA COVID-19 vaccines are also present. Healthcare providers are recommended to maintain a high degree of suspicion among individuals who have received the COVID-19 vaccine within the last 4 weeks.

Heparin-Induced Thrombocytopenia in Iranian Cardiac Surgery Patients Using the 4Ts Clinical Scoring System and Laboratory Methods

Background: Heparin-induced thrombocytopenia (HIT) is a serious adverse drug reaction. HIT diagnosis needs an algorithmic approach including clinical evaluation and laboratory tests (screening and confirmatory). Few studies have been conducted on HIT in Iran, and most existing research has been general and based on clinical evaluations alone. The present study was conducted to determine the prevalence of HIT among cardiac surgery patients using an algorithmic approach.

Materials and Methods: A cross-sectional study was carried out over a period of 10 months, at Modares Hospital (Tehran, Iran) on 92 patients who were candidates for cardiac surgery. For the clinical evaluation, the 4Ts scoring system was used; in cases with 4Ts scores ≥4, a laboratory evaluation of anti-PF4/heparin antibody (Ab) was performed by enzyme-linked immunosorbent assay (ELISA) and a HIPA test too as a functional confirmatory method. The patients with 4Ts scores ≥4 who were ELISA positive (OD ≥0.2) and HIPA positive were taken as a definite case of HIT.

Results: Of the 92 patients who had undergone cardiac surgery, 14 (15%) had 4Ts scores ≥4. Anti- PF4/heparin Ab was detected in eight patients using the ELISA and in six patients using the HIPA. Ultimately, definite HIT was confirmed in five of the patients.

Conclusion: The prevalence of HIT was 5.4% among the cardiac surgery patients assessed in the present study. To the researchers’ knowledge, this is the first time that HIT has been evaluated in Iran using a comprehensive algorithmic approach including clinical history-taking and both immunological and functional laboratory tests, and the findings showed a slightly higher HIT frequency in this single-center study in comparison with the other studies carried out in other countries.

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

BACKGROUND

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

MAIN BODY/TEXT

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

SHORT CONCLUSION

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

[Heparin-induced thrombocytopenia: Update]

Heparin-induced thrombocytopenia (HIT) is a serious complication of heparin therapy. It is due to the synthesis of antibodies most often directed against platelet factor 4 (FP4) modified by heparin (H). HIT is manifested by a platelet count fall, associated with a high risk of venous or arterial thrombosis. The diagnosis of HIT is based on the assessment of clinical probability (4Ts score or change in platelet count after cardiac surgery) and the demonstration of heparin-modified anti-FP4 antibodies (FP4/H). If the immunological tests are positive, functional tests should be performed. In case of suspicion of HIT, it is necessary to urgently stop heparin therapy, to perform a doppler ultrasound of the lower limbs, and to prescribe an alternative anticoagulation agent at a curative dose. Currently, danaparoid sodium and argatroban are authorized. The diagnosis and management of HIT remain complex and requires multidisciplinary collaboration.

Low ADAMTS-13 predicts adverse outcomes in hospitalized patients with suspected heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a life-threatening thrombotic complication after heparin exposure. However, the role of ADAMTS-13 and von Willebrand factor (VWF) in the disease process and outcomes of HIT is not known.

OBJECTIVE

To determine the potential role of ADAMTS-13 and VWF in hospitalized patients suspected with HIT.

METHODS

Associations of the HIT tests, ADAMTS-13 activity, and VWF antigen or activity with other clinical parameters and outcomes in the patients suspected with HIT were determined.

RESULTS

Of 261 patients, 87 (33.3%) were positive and 174 (66.7%) were negative for a HIT antibody determined by an enzyme immunoassay (EIA). Of these 87 EIA+ patients, 31 (35.6%) were also positive but 56 (64.4%) were negative for serotonin-releasing assay (SRA). There was no statistically significant difference among all three groups (i.e., EIA-, EIA+/SRA+, and EIA+/SRA-) as to their demographic features, reasons for admission to the hospital, type of procedures performed, and in-hospital mortality. Compared to those in the healthy controls, plasma ADAMTS-13 activity in patients suspected with HIT was significantly lower but plasma VWF antigen (VWFAg) and activity (VWFAc) in these patients were significantly higher. While there was no statistically significant difference among all three groups regarding plasma levels of ADAMTS-13 activity, VWFAg, and VWFAc, plasma levels of ADAMTS-13 activity <50% or the low ratios of ADAMTS-13 activity to VWFAg (or VWFAc) are highly predictive for a 90-day mortality rate, particularly in the EIA+SRA+ group.

CONCLUSIONS

These results demonstrate that relative deficiency of plasma ADAMTS-13 activity in hospitalized patients suspected with HIT is common, which may contribute at least in part to the adverse outcomes in this patient population, particularly in those with true HIT.

Minimizing cost associated with management of heparin-induced thrombocytopenia: A cost analysis of various laboratory testing models

INTRODUCTION

Management of patients with suspected heparin-induced thrombocytopenia (HIT) can lead to significant costs. Reported cost-saving initiatives have focused on minimizing inappropriate testing in low-risk patients and optimizing alternative anticoagulant selection. We sought to further investigate how utilizing various HIT laboratory testing models would impact total cost of testing and alternative anticoagulant use.

METHODS

Utilizing a retrospective cohort of adult patients tested for HIT over three years within our institution, we evaluated how utilization of four distinct laboratory models impacted total number of HIT test combinations completed, time to HIT testing finalization, percentage of patients discharged from the hospital prior to HIT testing finalization, total alternative anticoagulant days, and total anticipated major bleed events. Additionally, we calculated cost of laboratory testing and alternative anticoagulant associated with each model.

RESULTS

A total of 482 patients were included in our cohort. A laboratory testing model that utilized an in-house platelet factor 4 (PF4)-heparin enzyme-linked immunosorbent assay (ELISA) completed three days weekly, and reflex serotonin release assay (SRA) with a five-day turnaround resulted in the shortest mean time to HIT testing finalization, lowest percentage of patients discharged prior to HIT testing finalization, and lowest total alternative anticoagulant days.

CONCLUSIONS

Institutions should evaluate current HIT laboratory testing practices and assess for opportunities for optimization. Testing models utilizing a PF4-heparin antibody ELISA with a reflex SRA for positive results may improve testing metrics and lead to lower utilization of alternative anticoagulants.

US Case Reports of Cerebral Venous Sinus Thrombosis With Thrombocytopenia After Ad26.COV2.S Vaccination, March 2 to April 21, 2021

IMPORTANCE

Cerebral venous sinus thrombosis (CVST) with thrombocytopenia, a rare and serious condition, has been described in Europe following receipt of the ChAdOx1 nCoV-19 vaccine (Oxford/AstraZeneca), which uses a chimpanzee adenoviral vector. A mechanism similar to autoimmune heparin-induced thrombocytopenia (HIT) has been proposed. In the US, the Ad26.COV2.S COVID-19 vaccine (Janssen/Johnson & Johnson), which uses a human adenoviral vector, received Emergency Use Authorization (EUA) on February 27, 2021. By April 12, 2021, approximately 7 million Ad26.COV2.S vaccine doses had been given in the US, and 6 cases of CVST with thrombocytopenia had been identified among the recipients, resulting in a temporary national pause in vaccination with this product on April 13, 2021.

OBJECTIVE

To describe reports of CVST with thrombocytopenia following Ad26.COV2.S vaccine receipt.

DESIGN, SETTING, AND PARTICIPANTS

Case series of 12 US patients with CVST and thrombocytopenia following use of Ad26.COV2.S vaccine under EUA reported to the Vaccine Adverse Event Reporting System (VAERS) from March 2 to April 21, 2021 (with follow-up reported through April 21, 2021).

EXPOSURES

Receipt of Ad26.COV2.S vaccine.

MAIN OUTCOMES AND MEASURES

Clinical course, imaging, laboratory tests, and outcomes after CVST diagnosis obtained from VAERS reports, medical record review, and discussion with clinicians.

RESULTS

Patients' ages ranged from 18 to younger than 60 years; all were White women, reported from 11 states. Seven patients had at least 1 CVST risk factor, including obesity (n = 6), hypothyroidism (n = 1), and oral contraceptive use (n = 1); none had documented prior heparin exposure. Time from Ad26.COV2.S vaccination to symptom onset ranged from 6 to 15 days. Eleven patients initially presented with headache; 1 patient initially presented with back pain and later developed headache. Of the 12 patients with CVST, 7 also had intracerebral hemorrhage; 8 had non-CVST thromboses. After diagnosis of CVST, 6 patients initially received heparin treatment. Platelet nadir ranged from 9 ×103/µL to 127 ×103/µL. All 11 patients tested for the heparin-platelet factor 4 HIT antibody by enzyme-linked immunosorbent assay (ELISA) screening had positive results. All patients were hospitalized (10 in an intensive care unit [ICU]). As of April 21, 2021, outcomes were death (n = 3), continued ICU care (n = 3), continued non-ICU hospitalization (n = 2), and discharged home (n = 4).

CONCLUSIONS AND RELEVANCE

The initial 12 US cases of CVST with thrombocytopenia after Ad26.COV2.S vaccination represent serious events. This case series may inform clinical guidance as Ad26.COV2.S vaccination resumes in the US as well as investigations into the potential relationship between Ad26.COV2.S vaccine and CVST with thrombocytopenia.

Comparison of vaccine-induced thrombotic events between ChAdOx1 nCoV-19 and Ad26.COV.2.S vaccines

Cerebral venous thrombosis (CVT) events have been reported after vaccination with adenoviral COVID-19 vector vaccines. This study aimed to compare the clinical presentations and courses of vaccine-induced thrombotic thrombocytopenia (VITT) between the two adenoviral vector vaccines, Ad26.COV.2.S (Janssen/Johnson & Johnson) and ChAdOx1 nCoV-19 (Astra-Zeneca). We found that CVT after Ad26.COV.2.S vaccination presents later with similar symptoms compared to CVT after administration of ChAdOx1 nCoV-19, albeit with more thrombosis and intracerebral hemorrhage, lower D-dimer and aPTT levels but similar mortality. These findings could help guide clinical assessment and management of CVT after COVID-19 vaccination.

Accuracy of the functional, flow cytometer-based Emo-Test HIT Confirm® for the diagnosis of heparin-induced thrombocytopenia

INTRODUCTION

Rapid functional assays have been proposed to overcome the limitations of washed platelet assays in the work-up of patients with suspected heparin-induced thrombocytopenia (HIT). Data on the diagnostic accuracy are, however, scarce and conflicting. We aimed to study the diagnostic accuracy of a rapid, flow cytometer-based assay and to explore sources of variability.

MATERIAL AND METHODS

Frozen serum samples of 103 consecutive patients, evaluated for suspected HIT at our institution in 2017, and characterized with 4Ts score, IgG-PF4/heparin ELISA (GTI), HemosIL®Acustar (IgG), as well as heparin-induced platelet activation test (HIPA), were further tested using HIT Confirm, determining P-selectin release of donor platelets after incubation with patient's serum. The diagnosis of HIT was defined as a positive HIPA result.

RESULTS

HIT was confirmed in 15 out of 103 patients corresponding to a prevalence of 14.6%. HIT Confirm was positive in 11 patients (10.7%), negative in 88 patients (85.4%), and inconclusive in 4 patients (3.9%). According to the intention-to-diagnose principle, the number of true positives was 9, the number of true negatives 83, the number of false negatives was 6, the number of false positives 5. This corresponds to a sensitivity of 60.0%, and a specificity of 94.3%. Modifications of the test did not improve sensitivity.

CONCLUSIONS

The rapid, flow cytometer-based assay HIT Confirm is able to verify HIT in positive patient samples but cannot rule-out HIT in clinical practice. Other rapid functional assays shall be studies in appropriately designed diagnostic accuracy studies.

Clinical validation of immunoassay HemosIL® AcuStar HIT-IgG (PF4-H) in the diagnosis of Heparin-induced thrombocytopenia

Heparin induced thrombocytopenia (HIT) is a life and limb-threatening complication of heparin exposure. The misdiagnosis of this disease can have major consequences on the patients. The objective of this study was to evaluate a diagnostic strategy that combines the 4Ts score with the result of HemosIL® AcuStar HIT-IgG (PF4-H) to confirm the diagnosis of HIT. Citrated plasmas from 1300 patients with suspicion of HIT were analyzed with a fully automated quantitative chemiluminescent immunoassay (HemosIL® AcuStar HIT-IgG (PF4/H)). If the IgG anti-PF4/H antibodies were positive (cut-off, 1 U/mL), HIT diagnosis was confirmed using functional tests. In total, 1300 samples of consecutive patients were enrolled, 94 (7.2%) of which gave positive results in HemosIL® AcuStar-IgG. HIT was diagnosed in 65 out of these patients, corresponding to a prevalence of 5%. Using ROC curve analysis, patients were divided into three groups according to their titer of antibodies. Higher values of the IgG (PF4-H) were associated with increased probability of HIT, and the diagnostic specificity was greatly increased using the combination of a 4Ts score > 3 and a positive titer ≥ 3.25 U/mL. Importantly, the diagnostic specificity is 100% when the titer is > 12.40 U/mL. We demonstrated that higher values of Anti PF4/H Antibodies were associated with a high probability of having HIT. A titer of HemosIL® IgG (PF4-H) > 12.40 U/mL has a specificity of 100% which should no require a functional test to confirm the diagnosis of HIT.

Heparin-induced thrombocytopenia in patients readmitted after open cardiac surgical procedures: A case series

Objectives

Heparin-induced thrombocytopenia (HIT) is an immune-mediated complication that occurs in a small percentage of patients exposed to heparin. Concerns of HIT are particularly high in patients undergoing cardiac procedures requiring cardiopulmonary bypass, as they are exposed to high doses of heparin intraoperatively. Our aim was to identify and assess the hospital courses of patients who were diagnosed with HIT during readmission following cardiac surgery.

Methods

A retrospective review of patients who underwent open cardiac surgical procedures from June 2017 through October 2019 was performed. Of these, we identified patients who were newly diagnosed with HIT upon readmission. HIT positivity was defined as a positive anti-PF4 antibody screening test, plus a positive serotonin release assay.

Results

Of the 2496 patients identified, 13 patients were HIT positive on index admission and were excluded. Of the remaining 2483 patients, 351 were readmitted within 30 days. Six were newly diagnosed with HIT during readmission, 5 of whom presented with thrombotic complications. One patient was readmitted with thrombocytopenia and was started on argatroban; the remaining 5 did not have a significantly lower platelet count on readmission. Of the 12 patients readmitted for venous thromboembolism, 4 tested positive for HIT.

Conclusions

HIT can have a delayed appearance following open heart surgery. Venous thromboembolism appears to be a significant indicator for HIT during readmission, even in the absence of thrombocytopenia. This may support the use of non-heparin anticoagulation for cardiac surgery patients readmitted with thromboembolism until HIT status is determined.

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: ELISA optical density value and 4T score in correlation with panel donor platelets activation in functional flow cytometric assay

BACKGROUND

Serological assays for the diagnosis of heparin-induced thrombocytopenia (HIT) detect both platelet-activating and platelet non-activating anti-heparin/platelet factor 4 (PF4) antibodies and have therefore a limited positive predictive value. Functional assays confirm the presence of platelet-activating antibodies but require platelets from healthy donors, whose response to patient serum can differ. Our aim was to investigate the correlation between the level of anti-heparin/PF4 antibodies, 4T score, and the extent of panel donor platelet activation in the functional assay.

MATERIALS AND METHODS

In total, 38 sera from enzyme immunoassays (ELISA) positive patients were tested against panel platelets obtained from 10 healthy, randomly selected donors, using our routine flow cytometry functional test for CD62P expression. Levels of anti-heparin/PF4 antibodies from medical and surgical patients and 4T pretest probability scores (where available) were correlated with the number of activated panel platelets.

RESULTS

Sera with low ELISA optical density (OD) values (0.4-1) activated on average 5.6, sera with intermediate ELISA OD values (>1-2.5) activated on average 7.3, and sera with high ELISA OD values (>2.5) activated on average 8.6 out of 10 panel platelets. One serum with low 4T score did not activate donor platelets, 12 sera with intermediate 4T score activated on average 6.3 donors, 8 sera with high 4T score activated on average 8.5 panel platelets.

DISCUSSION

Sera with higher ELISA OD values activated platelets from a higher number of platelet donors, independently of patient type (medical or surgical). The average number of activated panel platelets increased with rising 4T score. Results indicate that both donor platelet reactivity and quantity of anti-heparin/PF4 antibodies affect the result of the functional assay, meaning special attention is needed in platelet donor selection when testing sera with low levels of antibodies.

Potential and Limitations of the New P2Y12 Inhibitor, Cangrelor, in Preventing Heparin-Induced Platelet Aggregation During Cardiac Surgery: An In Vitro Study

BACKGROUND

Heparin-induced thrombocytopenia (HIT) can put cardiac surgery patients at a high risk of lethal complications. If anti-PF4/heparin antibodies (anti-PF4/Hep Abs) are present, 2 strategies exist to prevent intraoperative aggregation during bypass surgery: first, using an alternative anticoagulant, and second, using heparin combined with an antiaggregant. The new P2Y12 inhibitor, cangrelor, could be an attractive candidate for the latter strategy; several authors have reported its successful use. The present in vitro study evaluated cangrelor's ability to inhibit heparin-induced platelet aggregation in the presence of anti-PF4/Hep Abs.

METHODS

Platelet-poor plasma (PPP) from 30 patients with functional anti-PF4/Hep Abs was mixed with platelet-rich plasma (PRP) from 5 healthy donors.Light transmission aggregometry was used to measure platelet aggregation after adding 0.5 IU·mL of heparin (HIT) to the plasma, and this was compared with samples spiked with normal saline (control) and samples spiked with cangrelor 500 ng·mL and heparin 0.5 IU·mL (treatment). Friedman test with post hoc Dunn-Bonferroni test was used for between-group comparisons.

RESULTS

Heparin 0.5 IU·mL triggered aggregation in 22 of 44 PPP-PRP mixtures, with a median aggregation of 86% (interquartile range [IQR], 69-91). The median aggregation of these 22 positive samples' respective control tests was 22% (IQR, 16-30) (P < .001). Median aggregation in the cangrelor-treated samples was 29% (IQR, 19-54) and significantly lower than the HIT samples (P < .001). Cangrelor inhibited heparin-induced aggregation by a median of 91% (IQR, 52-100). Cangrelor only reduced heparin-induced aggregation by >95% in 10 of the 22 positive samples (45%). Cangrelor inhibited heparin-induced aggregation by <50% in 5 of the 22 positive samples (22%) and by <10% in 3 samples (14%).

CONCLUSIONS

This in vitro study found that cangrelor was an unreliable inhibitor of heparin-induced aggregation in the presence of anti-PF4/Hep Abs. We conclude that cangrelor should not be used as a standard antiaggregant for cardiac patients affected by HIT during surgery. Unless cangrelor's efficacy in a particular patient has been confirmed in a presurgery aggregation test, other strategies should be chosen.

Current Perspectives on Diagnostic Assays and Anti-PF4 Antibodies for the Diagnosis of Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a recognized clinical entity in patients receiving unfractionated heparin and low–molecular weight heparin. Currently, diagnosing HIT includes the combination of a physician’s clinical suspicion based on a clinical scoring system and a series of laboratory tests. In the present article, we discuss challenges in suspecting and diagnosing HIT in consideration of the turnaround time of available tests and recent advances in techniques and methodologies of newer immunoassays and functional assays.

Pathophysiology and Diagnosis of Drug-Induced Immune Thrombocytopenia

Drug-induced immune thrombocytopenia (DITP) is a life-threatening clinical syndrome that is under-recognized and difficult to diagnose. Many drugs can cause immune-mediated thrombocytopenia, but the most commonly implicated are abciximab, carbamazepine, ceftriaxone, eptifibatide, heparin, ibuprofen, mirtazapine, oxaliplatin, penicillin, quinine, quinidine, rifampicin, suramin, tirofiban, trimethoprim-sulfamethoxazole, and vancomycin. Several different mechanisms have been identified in typical DITP, which is most commonly characterized by severe thrombocytopenia due to clearance and/or destruction of platelets sensitized by a drug-dependent antibody. Patients with typical DITP usually bleed when symptomatic, and biological confirmation of the diagnosis is often difficult because detection of drug-dependent antibodies (DDabs) in the patient's serum or plasma is frequently not possible. This is in contrast to heparin-induced thrombocytopenia (HIT), which is a particular DITP caused in most cases by heparin-dependent antibodies specific for platelet factor 4, which can strongly activate platelets in vitro and in vivo, explaining why affected patients usually have thrombotic complications but do not bleed. In addition, laboratory tests are readily available to diagnose HIT, unlike the methods used to detect DDabs associated with other DITP that are mostly reserved for laboratories specialized in platelet immunology.

Detection of Platelet-Activating Antibodies Associated with Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic immune drug reaction caused by platelet-activating antibodies that in most instances recognize platelet factor 4 (PF4)/polyanion complexes. Platelet activation assays (i.e., functional assays) are more specific than immunoassays, since they are able to discern clinically relevant heparin-induced antibodies. All functional assays used for HIT diagnosis share the same principle, as they assess the ability of serum/plasma from suspected HIT patients to activate fresh platelets from healthy donors in the presence of several concentrations of heparin. Depending on the assay, donors’ platelets are stimulated either in whole blood (WB), platelet-rich plasma (PRP), or in a buffer medium (washed platelets, WP). In addition, the activation endpoint studied varies from one assay to another: platelet aggregation, membrane expression of markers of platelet activation, release of platelet granules. Tests with WP are more sensitive and serotonin release assay (SRA) is considered to be the current gold standard, but functional assays suffer from certain limitations regarding their sensitivity, specificity, complexity, and/or accessibility. However, the strict adherence to adequate preanalytical conditions, the use of selected platelet donors and the inclusion of positive and negative controls in each run are key points that ensure their performances.

Evaluation of functional assays for the diagnosis of heparin induced thrombocytopenia using 5B9, a monoclonal IgG that mimics human antibodies

BACKGROUND

Serotonin release assay (SRA) is considered as the "gold standard" for detecting pathogenic heparin-induced thrombocytopenia (HIT) antibodies. However, this method is time consuming, expensive, and uses radioelements. Heparin-induced multiple electrode aggregometry (HIMEA), light transmission aggregometry (LTA) with platelet rich plasma (PRP) or washed platelets (WP), adenosine triphosphate (ATP) release, and flow cytometry (FC) are available alternatives.

OBJECTIVES

To evaluate the performance of these assays, comparatively with SRA, for detecting HIT antibodies, using 5B9, a monoclonal IgG fully mimicking human HIT antibodies.

PATIENTS/METHODS

Heparin-dependent platelet activation induced by 5B9 (50/20/10 µg/mL) was evaluated by all assays performed on the same day using platelets from 20 healthy donors. The three methods exhibiting the highest sensitivity to 5B9 were then assessed by testing samples from patients with either likely (n = 10), or indeterminate/unlikely HIT (n = 10).

RESULTS

All methods exhibited good sensitivity for detecting 5B9 50 µg/mL, but only SRA and HIMEA were positive with 100% of donors using 5B9 20 µg/mL, followed by FC (83%). SRA detected 5B9 10 μg/mL with 90% of donors, while HIMEA and FC were positive in 45% and 44% of cases, respectively. Whereas SRA was positive with 9/10 samples from likely HIT, HIMEA and FC were positive with 6 and 7 of them, respectively. Neither SRA nor HIMEA was positive with indeterminate/unlikely HIT samples, while FC was positive or doubtful in three cases.

CONCLUSIONS

Serotonin release assay likely remains the most sensitive and specific assay for detecting platelet activating HIT antibodies, but HIMEA or FC are potential alternatives, despite being less performant.

Heparin-induced Thrombocytopenia: Pathophysiology, Diagnosis and Management

Heparin-induced thrombocytopenia (HIT), even rare, is a life-threatening, immune-mediated complication of heparin exposure. It is considered the most severe non-bleeding adverse reaction of heparin treatment and one of the most important adverse drug reactions. The pathophysiological basis of HIT results from the formation of an immunocomplex consisting of an auto-antibody against platelet factor 4 (PF4) - heparin complex, which binds to the surface of platelets and monocytes, provoking their activation by cross-linking FcgIIA receptors. Platelets and monocyte activation, leads to the generation of catastrophic arterial and venous thrombosis, with a mortality rate of 20%, without early recognition. The definitive diagnosis of HIT i.e., clinical and laboratory evidence, can not be done at the onset of symptoms because laboratory results may not be available for several days. Thus, the initial approach is to predict the likelihood of HIT, because in highly suspected patients immediate heparin cessation and initiation of alternative anticoagulation treatment are crucial for the prevention of the devastating thrombotic sequelae. Herein, we describe the pathophysiology, the clinical manifestations, the diagnostic approach, and the management of patients with HIT.

Establishment of a megakaryoblastic cell line for conventional assessment of platelet calcium signaling

Platelet function tests utilizing agonists or patient serum are generally performed to assess platelet activation ex vivo. However, inter-individual differences in platelet reactivity and donor requirements make it difficult to standardize these tests. Here, we established a megakaryoblastic cell line for the conventional assessment of platelet activation. We first compared intracellular signaling pathways using CD32 crosslinking in several megakaryoblastic cell lines, including CMK, UT-7/TPO, and MEG-01 cells. We confirmed that CD32 was abundantly expressed on the cell surface, and that intracellular calcium mobilization and tyrosine phosphorylation occurred after CD32 crosslinking. We next employed GCaMP6s, a highly sensitive calcium indicator, to facilitate the detection of calcium mobilization by transducing CMK and MEG-01 cells with a plasmid harboring GCaMP6s under the control of the human elongation factor-1α promoter. Cells that stably expressed GCaMP6s emitted enhanced green fluorescent protein fluorescence in response to intracellular calcium mobilization following agonist stimulation in the absence of pretreatment. In summary, we have established megakaryoblastic cell lines that mimic platelets by mobilizing intracellular calcium in response to several agonists. These cell lines can potentially be utilized in high-throughput screening assays for the discovery of new antiplatelet drugs or diagnosis of disorders caused by platelet-activating substances.

Anticoagulation management strategies in heart transplantation

Anticoagulation before, during, and after heart transplantation (HT) presents unique challenges to clinicians. Bleeding and thrombotic morbidity continues to affect this patient population throughout all phases of the HT journey. Reversal is commonly required since patients are commonly bridged to HT with left ventricular assist devices, which require chronic anti platelet and anticoagulation. Caution must be exercised in patients requiring cardiopulmonary bypass during surgery who are at risk of complications from heparin induced thrombocytopenia. The reported incidence of venous thromboembolism following HT is high, particularly during the first post-HT year, most likely due to surgery, biopsies, specific immunosuppression (mTOR inhibitors) and immobilization. It is crucial to maintain long-term oral anticoagulation after the first venous thromboembolism event, especially when risk factors exist. A major issue, and one for which there remains considerable debate, is the optimal treatment of such complications, particularly upper extremity venous thrombosis. For both warfarin and the thrombin inhibitors or Factor Xa inhibitors, the clinician must determine potential drug interactions based on the HT drug regimen, and then develop a patient-specific management strategy.

Heparin-induced thrombocytopenia: Diagnostic challenges in intensive care patients especially with extracorporeal circulation

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a serious drug induced reaction that may be associated with life threatening complications. Platelet-activating antibodies directed against platelet factor 4 (PF4)/heparin complexes cause the disease. The diagnosis of HIT is challenging, as thrombocytopenia is a frequent finding in intensive care (ICU) patient population, especially during extracorporeal membrane oxygenation.

OBJECTIVE

To investigate the performance of a diagnostic algorithm for HIT in ICU patients.

METHODS

ICU patients who developed thrombocytopenia or thrombosis under heparin treatment were included in this study. The pretest probability for HIT was estimated using the 4Ts-score and patient's sera were tested using two rapid immunoassays (RA) LFI-HIT and PaGIA (from Milenia Biotec and DiaMed), and within 72 h using the IgG enzyme immunoassay (EIA) from Hyphen and the heparin induced platelet activation assay (HIPA).

RESULTS

392 consecutive ICU patients with suspected HIT were enrolled in this study, of whom 83/392 (21.2%) patients had extracorporeal circulation. Sera from 120/392 (30.6%) and 98/392 (25.0%) patients revealed positive results in RA and IgG EIA, respectively. The HIPA test revealed heparin-dependent platelet activation in a total of 15/392 (3.8%) ICU patients (3 medical and 12 surgical patients). In addition, sera from 7 patients revealed indeterminate HIPA results, of whom 2 patients had a clinical course compatible with HIT.

CONCLUSIONS

Data from our study confirm the high frequency of IgG PF4/heparin antibodies in ICU patients under unfractionated heparin and shows that the combination of 4Ts-score and RA does not reduce the laboratory overinvestigation for HIT in these patients.

Pathogenic heparin-induced thrombocytopenia and thrombosis (HIT) antibodies determined by rapid functional flow cytometry

OBJECTIVES

Reliable diagnosis of heparin-induced thrombocytopenia and thrombosis (HIT) is mandatory for patient management, yet prompt determination of pathogenic antibodies remains an unmet clinical challenge. Common immunoassays carry inherent limitations and functional assays which detect antibody-mediated platelet activation are not usually readily available to routine laboratories, especially the serotonin release assay (SRA), being technically demanding, time consuming, and requires high level expertise. To overcome some of these limitations, we have developed a practical functional flow cytometric assay (FCA) for routine clinical use.

METHODS

A simple FCA is described which avoids platelet manipulation, is highly specific and sensitive compared with SRA, and provides rapid results.

RESULTS

Of the 650 consecutive samples, from HIT-suspected patients, 99 (15.3%) were positive by the PaGIA Heparin/PF4 immunoassay and 31 (4.8%) by FCA. Average platelet activation was 11-fold higher in PaGIA+/FCA+ vs PaGIA-/FCA- samples. Of 21 SRA-positive samples, 19 were FCA-positive (relative sensitivity 90.5%), and of 42 SRA-negative samples, 40 were FCA-negative (relative specificity 95.2%). The FCA showed significantly higher correlation with the clinical presentation of HIT (4Ts score) performed on 182 patients, compared with PaGIA Heparin/PF4 (ROC-plot analysis, AUC 0.93 vs 0.63, P < 0.001). At a 92% sensitivity, the assay specificity was 96%.

CONCLUSIONS

The present FCA is practical for routine testing, providing prompt reliable results for initial diagnosis and confirmation, to effectively assist in HIT patient management.

Heparinized Polyurethane Surface Via a One-Step Photografting Method

Traditional methods using coupling chemistry for surface grafting of heparin onto polyurethane (PU) are disadvantageous due to their generally low efficiency. In order to overcome this problem, a quick one-step photografting method is proposed here. Three heparin derivatives incorporating 0.21, 0.58, and 0.88 wt% pendant aryl azide groups were immobilized onto PU surfaces, leading to similar grafting densities of 1.07, 1.17, and 1.13 μg/cm², respectively, yet with increasing densities of anchoring points. The most negatively charged surface and the maximum binding ability towards antithrombin III were found for the heparinized PU with the lowest amount of aryl azide/anchor sites. Furthermore, decreasing the density of anchoring points was found to inhibit platelet adhesion to a larger extent and to prolong plasma recalcification time, prothrombin time, thrombin time, and activated partial thromboplastin time to a larger extent. This was also found to enhance the bioactivity of immobilized heparin from 22.9% for raw heparin to 36.9%. This could be explained by the enhanced molecular mobility of immobilized heparin when it is more loosely anchored to the PU surface, as well as a higher surface charge.