High sensitivity and specificity of an automated IgG-specific chemiluminescence immunoassay for diagnosis of HIT

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

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

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.

Anti-PF4 antibodies and their relationship with COVID infection

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

Optimization of laboratory diagnosis of heparin-induced thrombocytopenia using HemosIL-AcuStar-HIT-IgG assay

OBJECTIVE

The aim of this study was to determine an optimal cutoff value for the newly available HemosIL-AcuStar-HIT-IgG assay (AcuStar) for the diagnosis of heparin-induced thrombocytopenia (HIT).

METHOD

We evaluated the performance of AcuStar using serotonin release assay (SRA) as the gold standard and incorporated 4T score calculation in a cohort of suspected HIT cases. Statistical analysis was performed to determine optimal cutoff value for the diagnosis of HIT.

RESULT

A diagnosis of HIT can be excluded with a platelet factor 4 (PF4) value of <0.4 U/mL by AcuStar and 4T score in the low-risk category (≤3). All other cases will require confirmation with a functional test.

CONCLUSION

Our study resulted in the implementation of a diagnostic algorithm for laboratory diagnosis of HIT, which incorporates pretest calculation of 4T score and AcuStar as a screening test, with reflex confirmation by SRA. This new algorithm resulted in extended hours of test availability and a more rapid turnaround time in reporting PF4 results.

Anti-PF4 immunothrombosis without proximate heparin or adenovirus vector vaccine exposure

ABSTRACT

Platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies and anti-PF4 antibodies cause heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombocytopenia and thrombosis (VITT), respectively. Diagnostic and treatment considerations differ somewhat between HIT and VITT. We identified patients with thrombocytopenia and thrombosis without proximate heparin exposure or adenovirus-based vaccination who tested strongly positive by PF4/polyanion enzyme-immunoassays and negative/weakly positive by heparin-induced platelet activation (HIPA) test but strongly positive by PF4-induced platelet activation (PIPA) test (ie, VITT-like profile). We tested these patients by a standard chemiluminescence assay that detects anti-PF4/heparin antibodies found in HIT (HemosIL AcuStar HIT-IgG(PF4-H)) as well as a novel chemiluminescence assay for anti-PF4 antibodies found in VITT. Representative control sera included an exploratory anti-PF4 antibody-positive but HIPA-negative/weak cohort obtained before 2020 (n = 188). We identified 9 patients with a clinical-pathological profile of a VITT-like disorder in the absence of proximate heparin or vaccination, with a high frequency of stroke (arterial, n = 3; cerebral venous sinus thrombosis, n = 4), thrombocytopenia (median platelet count nadir, 49 × 109/L), and hypercoagulability (greatly elevated D-dimer levels). VITT-like serological features included strong reactivity by PIPA (aggregation <10 minutes in 9/9 sera) and positive testing in the novel anti-PF4 chemiluminescence assay (3/9 also tested positive in the anti-PF4/heparin chemiluminescence assay). Our exploratory cohort identified 13 additional patient sera obtained before 2020 with VITT-like anti-PF4 antibodies. Platelet-activating VITT-like anti-PF4 antibodies should be considered in patients with thrombocytopenia, thrombosis, and very high D-dimer levels, even without a proximate exposure to heparin or adenovirus vector vaccines.

Autoimmune Heparin-Induced Thrombocytopenia

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

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

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

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Platelets as a Gauge of Liver Disease Kinetics?

A multitude of laboratory and clinical interferences influence the utility of platelet-based diagnostic indices, including immature platelet fraction, in longitudinal monitoring and prognostication of patients with chronic liver disease (CLD). The complex yet highly regulated molecular basis of platelet production and clearance kinetics becomes dysregulated in liver pathogenesis. These underlying molecular mechanisms, including premature platelet clearance and bone marrow suppression in parallel with the progressive (e.g., treatment-naïve) or regressive (e.g., on-treatment and off-treatment) disease courses, involved in CLDs, may further confound the changes in platelet–liver correlations over time. Platelet count and function are commonly and secondarily altered in vivo in CLDs. However, the precise characterization of platelet functions during cirrhosis, including in vitro platelet aggregation, has proven challenging due to interferences such as thrombocytopenia. A flow cytometric approach may help monitor the unstably rebalanced hyper- and hypoaggregable states in patients with cirrhosis at risk of hyperaggregable, prothrombotic, or bleeding events. Studies have attempted to stratify patients with cirrhosis by substages and prognosis through the use of novel indices such as the ratio of in vitro endogenous platelet aggregation to platelet count. This review attempts to highlight clinical and laboratory precautions in the context of platelet-assisted CLD monitoring.

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.

Fatal vaccine-induced immune thrombotic thrombocytopenia (VITT) post Ad26.COV2.S: first documented case outside US

Purpose

We reported the first described post Ad26.COV2.S (Janssen, Johnson & Johnson) vaccine-induced immune thrombocytopenia (VITT) case outside US. 

Case description

CA young woman without any medical history presented association of deep vein thrombosis and thrombocytopenia at day 10 after vaccine injection. The patient was treated with low-molecular weight heparin at a first medical institution. Twelve days post Ad26.COV2.S vaccination, the patient was admitted at our hospital for neurological deterioration and right hemiplegia. Medical imaging using MRI showed thrombosis of the major anterior part of the sagittal superior sinus with bilateral intraparenchymal hemorrhagic complications. Screening tests for antibodies against platelet factor 4 (PF4)–heparin by rapid lateral flow immunoassay and chemiluminescence techniques were negative. Platelet activation test using heparin-induced multiple electrode aggregometry confirmed the initial clinical hypothesis. Despite immediate treatment with intravenous immunoglobulin, dexamethasone, danaparoid and attempted neurosurgery the patient evolved toward brain death.

Conclusion

Even though it is an extremely rare complication of vaccination physicians should maintain a high index of suspicion of VITT in patients who received an adenovirus-vector-based SARS-CoV-2 vaccine within the last 30 days with persistent complains compatible with VITT or thromboembolic event associated with thrombocytopenia. The diagnosis should not be excluded if the rapid anti-PF4 immunological nor chemiluminescence techniques yield negative results. An adapted functional assay should be performed to confirm the diagnosis. Early treatment with intravenous immunoglobulin and non-heparin anticoagulants is essential as delayed diagnosis and administration of appropriate treatment is associated with poor prognosis.

HIT and VITT antibodies: fraternal-not identical-twins

Not applicable (no abstract in Invited Editorial Focus).

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.

PF4-Dependent Immunoassays in Patients with Vaccine-Induced Immune Thrombotic Thrombocytopenia: Results of an Interlaboratory Comparison

BACKGROUND

 Coronavirus disease 2019 vaccine ChAdOx1 nCov-19 may rarely lead to vaccine-induced thrombotic thrombocytopenia (VITT). Antibody-mediated, platelet factor 4 (PF4)-dependent platelet activation appears to resemble a key mechanism in VITT, partially comparable to heparin-induced thrombocytopenia. The use of PF4/heparin immunoassays has been proposed as part of a diagnostic approach, but their sensitivity has not been established.

METHODS

 Sera from 12 well-defined VITT patients were first studied by two different laboratories in functional assays. Sera where then used for an interlaboratory comparison, in which five different PF4/heparin immunoassays were used by four laboratories.

RESULTS

 Results for functional testing were highly concordant. VITT antibodies were also reliably detected by PF4/heparin enzyme-linked immunosorbent assays (ELISAs) (92-100%). In contrast, only 25% of VITT antibodies were reactive in a particle gel immunoassay (PaGIA), and 8% in a lateral flow assay (LFA). An automated chemiluminescence immunoassay (CLIA) was negative for all sera tested (0%).

CONCLUSION

 It seems feasible to establish functional antibody testing for the confirmation of VITT. For the initial screening of suspected VITT cases, PaGIA, LFA, and CLIA are useless when applied as single tests. Only ELISA-based PF4/heparin immunoassays are sensitive enough to be incorporated in the diagnostic workup. However, a combination of a positive ELISA and a negative CLIA may be useful to identify VITT antibodies in the absence of confirmatory functional assays.

A practical approach to evaluating postoperative thrombocytopenia

Identifying the cause(s) of postoperative thrombocytopenia is challenging. The postoperative period includes numerous interventions, including fluid administration and transfusion of blood products, medication use (including heparin), and increased risk of organ dysfunction and infection. Understanding normal thrombopoietin physiology and the associated expected postoperative platelet count changes is the crucial first step in evaluation. Timing of thrombocytopenia is the most important feature when differentiating causes of postoperative thrombocytopenia. Thrombocytopenia within 4 days of surgery is commonly caused by hemodilution and increased perioperative platelet consumption prior to thrombopoietin-induced platelet count recovery and transient platelet count overshoot. A much broader list of possible conditions that can cause late-onset thrombocytopenia (postoperative day 5 [POD5] or later) is generally divided into consumptive and destructive causes. The former includes common (eg, infection-associated disseminated intravascular coagulation) and rare (eg, postoperative thrombotic thrombocytopenic purpura) conditions, whereas the latter includes such entities as drug-induced immune thrombocytopenia or posttransfusion purpura. Heparin-induced thrombocytopenia is a unique entity associated with thrombosis that is typically related to intraoperative/perioperative heparin exposure, although it can develop following knee replacement surgery even in the absence of heparin exposure. Very late onset (POD10 or later) of thrombocytopenia can indicate bacterial or fungal infection. Lastly, thrombocytopenia after mechanical device implantation requires unique considerations. Understanding the timing and severity of postoperative thrombocytopenia provides a practical approach to a common and challenging consultation.

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.

Heparin-induced thrombocytopenia and COVID-19

Heparin-induced thrombocytopenia (HIT) has not been included as a possible cause of thrombocytopenia in Coronavirus Disease 2019 (COVID-19) patients. We report a case of HIT in a patient with COVID-19 treated with heparin. A 78-yearold man was admitted to our hospital for acute respiratory failure and acute renal failure due to SARS-CoV-2 infection; in intensive care unit, one 5000IU heparin dose (day 0, platelet count 305000/μL). On day 2, haemoglobin started to decrease and heparin was stopped. On day 10, platelet count was 153000/μL and 5000IU calcium heparin subcutaneously twice daily was started. The platelet further decreased, reaching 49000/μL on day 17, and the patient was investigated for suspected HIT: an IgG specific chemiluminescence test for heparin- PF4 antibodies was positive and a femoral DVT was found at ultrasound. Argatroban was started, platelet count increased without any bleeding and thrombosis complication. Our experience shows that HIT may develop in heparin treated COVID-19 patients and should be included among the possible cause of thrombocytopenia in such patients.

Serotonin-release assay-positive but platelet factor 4-dependent enzyme-immunoassay negative: HIT or not HIT?

IgG-specific and polyspecific PF4-dependent enzyme-immunoassays (EIAs) have exceptionally high sensitivity (≥99%) for diagnosis of heparin-induced thrombocytopenia (HIT), a drug reaction caused by platelet-activating antibodies detectable by serotonin-release assay (SRA). The IgG-specific EIAs are recommended for screening, as their high sensitivity is accompanied by relatively high specificity vis-à-vis polyspecific EIAs. We investigated the frequency of SRA-positive/EIA-negative (SRA+/EIA-) HIT, prompted by referral to our reference HIT laboratory of serial blood samples from a patient ("index case") with false-negative IgG-specific EIAs. Despite initial clinical suspicion for HIT, repeat negative IgG-specific EIAs prompted heparin resumption, which triggered recurrent thrombocytopenia and near-fatal cardiac arrest, indicating likely post-heparin HIT-associated anaphylactoid reaction. Further investigations revealed a strong-positive SRA, whether performed with heparin alone, PF4 alone, or PF4/heparin, with inhibition by Fc receptor-blocking monoclonal antibody (indicating IgG-mediated platelet activation); however, five different IgG-specific immunoassays yielded primarily negative (or weak-positive) results. To investigate the frequency of SRA+/EIA- HIT, we reviewed the laboratory and clinical features of patients with this serological profile during a 6-year period in which our reference laboratory investigated for HIT using both SRA and IgG-specific EIA. Although ~0.2% of 8546 patients had an SRA+/EIA- profile, further review of 15 such cases indicated clerical/laboratory misclassification or false-positive SRA in all, with no SRA+/EIA- HIT case identified. We conclude that while SRA+/EIA- HIT is possible-as shown by our index case-this clinical picture is exceptionally uncommon. Moreover, the requirement for a positive EIA is a useful quality control maneuver that reduces risk of reporting a false-positive SRA result.

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

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

Heparin induced thrombocytopenia: position paper from the Italian Society on Thrombosis and Haemostasis (SISET)

Heparin induced thrombocytopenia (HIT) is a rare immune mediated adverse drug reaction occurring after exposure to heparin. It is a serious and potentially fatal condition, which may be associated with the development of arterial or venous thrombotic events. Although known for many years, HIT is still often misdiagnosed. Pre- test clinical probability, screening for anti-PF4/heparin antibodies and documentation of their platelet activating capacity are the cornerstones of diagnosis. However, both clinical algorithms and test modalities have limited predictive values and limited diffusion so that the diagnosis and management is challenging in the clinical practice. For this reason, there is an unmet need for novel rational non-anticoagulant therapies based on the pathogenesis of HIT.The present paper reports the position of the Italian Society on Haemostasis and Thrombosis (SISET) in order to increase awareness of HIT among clinicians and other health care professionals and to provide information on the most appropriate management.

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.

Prospective evaluation of two specific IgG immunoassays (HemosIL® AcuStar HIT-IgG and HAT45G® ) for the diagnosis of heparin-induced thrombocytopenia: A Bayesian approach

INTRODUCTION

The accurate diagnosis of heparin-induced thrombocytopenia (HIT) is essential to ensure adequate treatment and prevent complications. First step diagnosis test are immunoassays including enzyme-linked immunosorbent assays (ELISAs) and rapid immunoassays.

METHODS

Using a Bayesian approach, we prospectively evaluated the performance of the IgG PF4/polyvinylsulfonate ELISA and a chemiluminescent immunoassay (CLIA), which are specific for IgG and use the same antigenic target to detect HIT antibodies.

RESULTS

One hundred and eighty-four 184 consecutive patients with an intermediate (n = 159) or high (n = 25) clinical pretest probability of HIT based on the 4Ts score or platelet pattern were included. Both immunoassays (IAs) were performed on all 184 samples, and definite HIT was confirmed with a positive serotonin release assay in 29 patients (12.7%). The sensitivity (Ss) and negative predictive value (NPV) of ELISA were excellent (100%) allowing HIT to be excluded with good confidence when the test was negative. In addition, the Ss and NPV of the CLIA equalled 93.1% and 98.6%, respectively, as it was negative in two definite HIT. When the CLIA was negative, the post-test probability of HIT was 0.7% in case of intermediate risk. Although there was excellent agreement between CLIA and ELISA results, the quantitative values provided by the two IAs were not correlated.

CONCLUSION

AcuStar HIT^® detects more than 90% of HIT, as do all rapid IAs, and appears to be a good tool for excluding HIT when the pretest probability is intermediate. A chemiluminescent signal higher than 10 IU/mL is highly predictive of definite HIT with a PPV of 100%.

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

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

Challenges in Detecting Clinically Relevant Heparin-Induced Thrombocytopenia Antibodies

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated hypercoagulable state featuring high thrombosis risk and distinct pathogenesis involving immunoglobulin G-mediated platelet activation. The target of the immune response is a cationic “self” protein, platelet factor 4 (PF4), rendered antigenic by heparin. A key problem is that only a minority of anti-PF4/polyanion antibodies induced by heparin are pathogenic, i.e., capable of causing platelet activation and thereby clinical HIT. Since thrombocytopenia occurs frequently in hospitalized, heparin-treated patients, testing for “HIT antibodies” is common; thus, the problem of distinguishing between pathogenic and nonpathogenic antibodies is important. The central concept is that those antibodies that have platelet-activating properties demonstrable in vitro correlate well with pathogenicity, as shown by platelet activation tests such as the serotonin-release assay (SRA) and heparin-induced platelet activation assay. However, in most circumstances, immunoassays are used for first-line testing, and so it is important for clinicians to appreciate which immunoassay result profiles—in the appropriate clinical context—predict the presence of platelet-activating antibodies (Bayesian analysis). Clinicians with access to rapid, on-demand HIT immunoassays (e.g., particle gel immunoassay, latex immunoturbidimetric assay, chemiluminescent immunoassay) can look beyond simple dichotomous result interpretation (“negative”/“positive”) and incorporate semiquantitative interpretation, where, for example, a strong-positive immunoassay result (or even combination of two immunoassays) points to a greater probability of detecting platelet-activating antibodies, and hence supporting a diagnosis of HIT. Recent recognition of “SRA-negative HIT” has increased the importance of semiquantitative interpretation of immunoassays, given that strong immunoassay reactivity is a potential clue indicating possible HIT despite a (false) negative platelet activation assay.

A multicentre assessment of contemporary laboratory assays for heparin induced thrombocytopenia

Heparin induced thrombocytopenia (HIT) is a rare but potentially fatal complication of heparin therapy. In some patients, HIT causes platelet activation and thrombosis (sometimes abbreviated HITT), which leads to adverse clinical sequalae ('pathological HIT'). The likelihood of HIT is initially assessed clinically, typically using a scoring system, of which the 4T score is that most utilised. Subsequent laboratory testing to confirm or exclude HIT facilitates exclusion or diagnosis and management. The current investigation comprises a multicentre (n=9) assessment of contemporary laboratory testing for HIT, as performed over the past 1-3 years in each site and comprising testing of over 1200 samples. The primary laboratory test used by study participants (n=8) comprised a chemiluminescence procedure (HIT-IgG_(PF4-H)) performed on an AcuStar instrument. Additional immunological testing performed by study sites included lateral flow (STiC, Stago), enzyme linked immunosorbent assay (ELISA), Asserachrom (HPIA IgG), PaGIA (BioRad), plus functional assays, primarily serotonin release assay (SRA) or platelet aggregation methods. The chemiluminescence procedure yielded a highly sensitive screening method for identifying functional HIT, given high area under the curve (AUC, generally ≥0.9) in a receiver operator characteristic (ROC) analysis against SRA as gold standard. ELISA testing resulted in lower ROC AUC scores (<0.8) and higher levels of false positives. Although there is clear association with the likelihood of HIT, the 4T score had less utility than literature suggests, and was comparable to a previous study reported by some of the authors.

High-dose IVIG plus cangrelor platelet "anesthesia" during urgent heparin-CPB in a patient with recent SRA-negative HIT-thrombosis with persisting platelet-activating antibodies

In a high-risk patient with subacute heparin-induced thrombocytopenia (HIT) type A (platelet count recovery following acute HIT but with persisting platelet-activating antibodies), in whom urgent cardiac surgery was required, a key clinical question arose: could intraoperative heparin be given safely with "platelet anesthesia" provided with high-dose intravenous immunoglobulin (IVIG) plus cangrelor (ultra-short-acting antiplatelet agent)? This approach proved successful, without unexpected postoperative thrombocytopenia or thromboembolism. In vitro studies confirmed that both IVIG and cangrelor contributed to perioperative inhibition of HIT antibody-induced platelet activation. Interestingly, despite the patient testing strongly positive in 4 HIT immunoassays (latex immunoturbidimetric assay and 3 enzyme-immunoassays), the serotonin-release assay (SRA) was consistently negative. Nevertheless, platelet-activating HIT antibodies were detectable using modified (platelet factor 4-enhanced) SRA. Our protocol of heparin rechallenge following IVIG/cangrelor provides both intraoperative and early postoperative inhibition of HIT antibody-induced platelet activation and is applicable to patients with circulating functional HIT antibodies requiring urgent heart surgery, including those with "SRA-negative HIT."

Combination of two complementary automated rapid assays for diagnosis of heparin-induced thrombocytopenia (HIT)

BACKGROUND

HIT diagnosis typically uses complementary diagnostic assays (eg, a PF4-dependent enzyme-immunoassay [EIA] and a platelet activation assay such as the serotonin-release assay [SRA]).

OBJECTIVES

To determine whether the combination of two automated assays-a latex immunoturbidimetric assay (LIA) that evaluates competitive inhibition of a HIT-like monoclonal antibody and a chemiluminescence immunoassay (CLIA) for detecting anti-PF4/heparin IgG-optimizes diagnostic sensitivity while also yielding good specificity, particularly at high assay reactivities.

PATIENTS/METHODS

We determined operating characteristics using combined LIA/CLIA results from a HIT observational trial (n = 430; derivation cohort) and 147 consecutive patients with HIT (n = 147; supplementary derivation cohort). We also evaluated 678 consecutive samples referred for HIT testing (replication cohort). LIA/CLIA reactivities were scored individually as "negative" (<1.00 U/mL, 0 points), "weak" (1.00-4.99 U/mL, 1 point), "moderate" (5.00-15.99 U/mL, 2 points) and "strong" (≥16.00 U/mL, 3 points), thus contributing up to 6 points (maximum) when LIA/CLIA results were combined. We also examined whether higher LIA/CLIA scores predicted presence of platelet-activating antibodies by conventional and modified (PF4- or PF4/heparin-enhanced) SRA.

RESULTS

Combined LIA/CLIA testing yielded high diagnostic sensitivity (~99%) similar to EIA. Interpretation of LIA/CLIA results using the 6-point scale indicated progressively greater likelihood for the presence of platelet-activating antibodies with increasing scores (semi-quantitative reactivity). A LIA/CLIA score ≥ 4 points predicted the presence of platelet-activating antibodies by SRA or PF4-enhanced SRA with high probability (~98%).

CONCLUSION

Combined LIA/CLIA testing optimizes diagnostic sensitivity, with progressively greater probability of detecting platelet-activating antibodies with higher assay reactivity that reaches 98% when both automated assays yield moderate or strong results.

Laboratory diagnosis of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a clinical-pathological disorder; thus, laboratory testing for the pathogenic platelet-activating antiplatelet factor 4 (PF4)/heparin antibodies is central for diagnosis. The "iceberg" model summarizes the inter-relationship between platelet activation assays and PF4-dependent immunoassays, with platelet-activating antibodies comprising a subset of anti-PF4/heparin antibodies. The platelet serotonin-release assay (SRA), performed by reference laboratories, has high sensitivity and specificity for HIT (~95% each), and is especially suited for detecting highly pathogenic HIT sera containing both heparin-dependent and heparin-independent platelet-activating antibodies; this latter subgroup of antibodies explains "autoimmune HIT" disorders (delayed-onset, persisting, spontaneous, heparin "flush," fondaparinux-associated). Recently, SRA-negative HIT has become recognized, in which serum from some HIT patients contains subthreshold levels of platelet-activating antibodies (by SRA) that become detectable using a PF4-enhanced platelet activation assay. Unusual immunologic features of HIT include early antibody detectability (at onset of platelet count fall) and antibody transience (seroreversion). Widely available PF4-dependent enzyme immunoassays (EIAs) have high sensitivity but poor specificity for HIT, although specificity is enhanced with IgG-specific EIAs and strong positive results; unfortunately, EIA results are usually not available in real time. Automated rapid immunoassays, such as the chemiluminescence immunoassay (CLIA) and latex immunoturbidimetric assay (LIA), facilitate real-time laboratory diagnosis. Recently available likelihood ratio (LR) data for positive (LR+) and negative (LR-) test results allow clinicians to adjust their pretest probabilities for HIT, using Bayesian analysis, into real-time posttest probabilities that are dramatically increased (test positive) or decreased (test negative). Moreover, (semi-)quantitative CLIA- and LIA-positive results (weak, moderate, strong positive) can further refine the posttest probability of HIT.