How I treat heparin-induced thrombocytopenia

Abstract Heparin-induced thrombocytopenia is a prothrombotic adverse drug effect induced by platelet-activating antibodies against multimolecular complexes of platelet factor 4 and heparin. Diagnosis rests on a clinical assessment of disease probability and laboratory testing. Management involves immediate discontinuation of heparin and initiation of an alternative anticoagulant. Because of the frequency of thrombocytopenia among heparinized patients, the limited specificity of widely available immunoassays, the limited availability of more specific functional assays, and clinicians' fears of missing a case of true disease, overtesting, overdiagnosis, and overtreatment have become common. As a result, a substantial number of thrombocytopenic patients are unnecessarily exposed to costly alternative anticoagulants and their attendant risk of bleeding. In this review, we describe not only our approach to the evaluation and management of patients with heparin-induced thrombocytopenia, but also the measures we use to minimize misdiagnosis and unnecessary treatment of patients without the disease. In addition, we propose areas of investigation for improvement of the diagnosis and management of this potentially fatal disorder.

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Comparison of three different immunoassays in the diagnosis of heparin-induced thrombocytopenia

Clinical Chemistry and Laboratory Medicine (CCLM) ◽

10.1515/cclm-2014-0809 ◽

2015 ◽

Vol 53 (2) ◽

Cited By ~ 15

Author(s):

Fabrizio Vianello ◽

Luisa Sambado ◽

Pamela Scarparo ◽

Annamaria Lombardi ◽

Daniela Bernardi ◽

...

Keyword(s):

Laboratory Testing ◽

Screening Tests ◽

Antigen Binding ◽

Binding Assays ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Routine Testing ◽

Blood Samples ◽

Functional Assays ◽

Clinical Laboratories

AbstractHeparin-induced thrombocytopenia (HIT) is caused by platelet activating antibodies that recognize platelet factor 4/heparin (PF4/H) complexes. Laboratory testing plays a key role in the diagnosis of HIT. As functional assays are unfeasible for most clinical laboratories, antigen binding assays are commonly used in routine testing. However, their low specificity leads to overdiagnosis of HIT. Therefore, it is advisable to improve screening tests in this setting.Blood samples from 114 patients in whom HIT was suspected were investigated using a chemiluminescence test (HemosILTwenty-nine (25.4%) out of 114 subjects with suspected HIT had a positive HIPA test. None of patients with a 4Ts score <4 were positive at HIPA. HemosILThe HemosIL

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

Journal of Clinical Medicine ◽

10.3390/jcm9041226 ◽

2020 ◽

Vol 9 (4) ◽

pp. 1226 ◽

Cited By ~ 6

Author(s):

Brigitte Tardy ◽

Thomas Lecompte ◽

François Mullier ◽

Caroline Vayne ◽

Claire Pouplard

Keyword(s):

Platelet Activation ◽

Platelet Rich Plasma ◽

Serotonin Release ◽

Membrane Expression ◽

Platelet Factor ◽

Strict Adherence ◽

Heparin Induced Thrombocytopenia ◽

Functional Assays ◽

Healthy Donors ◽

Sensitivity Specificity

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.

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

Vascular Medicine ◽

10.1177/1358863x19898253 ◽

2020 ◽

Vol 25 (2) ◽

pp. 160-173 ◽

Cited By ~ 1

Author(s):

Marie Hogan ◽

Jeffrey S Berger

Keyword(s):

Laboratory Testing ◽

Clinical Suspicion ◽

High Index ◽

Heparin Induced Thrombocytopenia ◽

Diagnosis And Management ◽

Alternative Anticoagulation ◽

Clinical Scoring ◽

Rule Out ◽

Heparin Exposure

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

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Comparison of Antigen and Activation Assays with a Probability Scoring Model for the Diagnosis of Heparin Induced Thrombocytopenia.

Blood ◽

10.1182/blood.v110.11.3215.3215 ◽

2007 ◽

Vol 110 (11) ◽

pp. 3215-3215

Author(s):

Dimitrios Scarvelis ◽

Gail Rock

Keyword(s):

Laboratory Testing ◽

Laboratory Tests ◽

Serotonin Release ◽

Reference Standard ◽

Platelet Response ◽

Platelet Factor ◽

Platelet Recovery ◽

Heparin Induced Thrombocytopenia ◽

Activation Assay ◽

Heparin Anticoagulation

Background: Heparin induced thrombocytopenia (HIT) is a syndrome characterized by thrombocytopenia and an elevated risk of thrombosis. In the evaluation of patients with suspected HIT, laboratory testing is used to help differentiate between those patients who have HIT and those who do not and therefore do not require non-heparin anticoagulation. Laboratory tests include activation (serotonin release, heparin-induced platelet aggregation, ATP luminescence) and antigen (enzyme-linked immunoabsorbant assay (ELISA)) assays directed against the platelet factor 4-heparin or PF4/PVS complex. A pre-test scoring system (Warkentin 4T’s) has been derived to assess the probability of HIT which can be used in conjunction with laboratory testing in the evaluation for HIT. Objectives: To compare the activation assay used at The Ottawa Hospital (ATP luminescence) with the PF4 enhanced® ELISA and correlate results with clinical data. Methods: Patients undergoing HIT testing were identified. ELISA and activation assays were performed. Charts were reviewed in order to derive a 4T’s score and to assess response to therapeutic non-heparin anticoagulation (when administered). Correlation between ELISA, activation assay, 4T’s score and platelet response to alternative anticoagulation was determined. Sensitivity, specificity, PPV and NPV of laboratory tests or 4T’s scores were calculated depending on the measure chosen as the reference standard to diagnose HIT. Results: 111 patients undergoing HIT testing were evaluated. 41 and 43 were positive by ELISA (OD > 0.4) or activation assay respectively. 12 were positive only by ELISA (mean optical density (OD): 1.27) and 10 were positive by both activation assay and ELISA (mean OD: 2.28). Clinical information was available for 70 patients. 26 of these received therapeutic non-heparin anticoagulation as treatment for suspected HIT. Reference standard= activation assay: ELISA: Sens 95% (95% CI 73–99%), Spec 80% (66–89%), PPV 66% (46–81%), NPV 98% (86–100%). 4T’s (low vs. high score): Sens 85% (54–97%), Spec 95% (83–99%), PPV 85% (54–97%), NPV 95% (83–99%). ELISA OD > 1.5 + high 4T’s score: Sens 61% (36–82%), Spec 100% (56–100%), PPV 100% (68–100%), NPV 50% (24–76%). Reference standard= 4T’s score (high and low score only, excludes intermediate category): ELISA: Sens 85% (53–97%), Spec 77% (61–88%), PPV 52% (30–74%), NPV 94% (79–99%). Activation: Sens 85% (54–97%), Spec 95% (83–99%), PPV 85% (54–97%), NPV 95% (83–99%). Reference standard= “clear response to non-heparin anticoagulation” defined as significant platelet increase within 48 hours of start of therapy and no other explanation for platelet recovery: ELISA: Sens 100% (72–100%), Spec 54% (26–80%), PPV 68% (43–86%), NPV 100% (56–100%). Activation: Sens 85% (54–97%), Spec 69% (39–90%), PPV 73% (45–91%), NPV 82% (48–97%). 4T’s (low vs. high score): Sens 90% (54–99%), Spec 100% (60–100%), PPV 100% (63–100%), NPV 89% (51–99%). ELISA OD > 1.5 + 4T’s (low vs. high score): Sens 89% (51–99%), Spec 100% (56–100%), PPV 100% (60–100%), NPV 88% (47–99%). Conclusions: A high 4T’s score best predicts a clinical response to non-heparin therapeutic anticoagulation when HIT is suspected. ELISA OD > 1.5 does not add additional information in this respect. A negative ELISA and a low 4T’s score have comparable NPVs when an activation assay is the reference standard. Future trials should employ a clinical reference standard such as “clear response to non-heparin anticoagulation” when evaluating the operator characteristics of HIT assays.

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Clinical and laboratory tests for the diagnosis of heparin-induced thrombocytopenia

Thrombosis and Haemostasis ◽

10.1160/th16-03-0240 ◽

2016 ◽

Vol 116 (11) ◽

pp. 823-834 ◽

Cited By ~ 28

Author(s):

Michael Nagler ◽

Tamam Bakchoul

Keyword(s):

Clinical Assessment ◽

Laboratory Tests ◽

Clinical Decision Making ◽

Assessment Tool ◽

Diagnostic Errors ◽

Individual Characteristics ◽

Heparin Induced Thrombocytopenia ◽

Functional Assays ◽

Number Of Patients ◽

Activation Assay

SummaryA rapid diagnostic work-up is required in patients with suspected heparin-induced thrombocytopenia (HIT). However, diagnosis of HIT is challenging due to a number of practical issues and methodological limitations. Many laboratory tests and a few clinical scoring systems are available but the individual characteristics and the diagnostic accuracy of these are hard to appraise. The 4Ts score is a well evaluated clinical assessment tool with the potential to rule out HIT in many patients. Still, it requires experience and is subject to a relevant inter-observer variability. Immunoassays such as enzyme-linked immunosorbent assays or recently developed rapid assays are able to exclude HIT in a number of patients. But, accuracy of immunoassays differs depending on type of assay, threshold, antibody specificity and even manufacturer. Due to a comparatively low positive predictive value, HIT cannot be confirmed by immunoassays alone. In addition, only some of them are immediately accessible, particularly in small laboratories. While functional assays such as the serotonin release assay (SRA) and the heparin-induced platelet activation assay (HIPA) are considered as gold standard for diagnosis of HIT, they require a highly specialised laboratory. In addition, some of them are not adequately evaluated. In clinical practice, we recommend an integrated diagnostic approach combining not only clinical assessment (the 4Ts score) but immunoassays and functional assays as well. We propose a clear diagnostic algorithm supporting clinical decision-making. Furthermore, we provide an overview of all current laboratory techniques for HIT and discuss diagnostic pathways and strategies to reduce diagnostic errors, and future perspectives.

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Improving Recognition, Diagnosis, and Management Of Heparin Induced Thrombocytopenia By Implementing a Computer-Based Clinical Decision Support System

Blood ◽

10.1182/blood.v122.21.2966.2966 ◽

2013 ◽

Vol 122 (21) ◽

pp. 2966-2966 ◽

Cited By ~ 1

Author(s):

Jordan K Schaefer ◽

Rajiv K Pruthi ◽

Jennifer S Shin ◽

Mark W Dobie ◽

Pedro J Caraballo

Keyword(s):

At Risk ◽

Decision Support ◽

Clinical Decision Support ◽

Clinical Decision ◽

Control Group ◽

Antibody Assay ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Diagnosis And Management ◽

Computer Based

Abstract Introduction Heparin induced thrombocytopenia (HIT) is often not considered as a potential etiology of thrombocytopenia. The gradual 5 to 14 day decline in platelet count (PC) may not be easily recognized by busy clinicians. The life-threatening complications of HIT are potentially preventable with prompt recognition and management. Implementation of computer-based clinical decision support systems (CDSS) may aid in addressing these issues. Such systems have shown promise in increasing provider recognition, appropriate testing, and management of a variety of conditions. Methods We developed, implemented and evaluated a CDSS designed to identify patients at risk for HIT. The CDSS was integrated into the electronic medical record (Centricity Enterprise, GE) at Mayo Clinic Rochester. The CDSS identified inpatients with recent or active use of heparin medications, who experienced a 50% PC drop. For such patients, the system generated an electronic message alerting their care team of the 50% PC drop and asked them to consider the possibility of HIT. A link to a website with recommendations for the diagnosis and management of HIT was included with the alert. Providers were encouraged to assess for HIT using the 4Ts Score and, if indicated, the anti-heparin/platelet factor 4 (PF4) antibody assay. We performed a retrospective chart review of consecutive patients identified by the system and equal number of randomly generated controls that did not trigger alerts. The accuracy of the algorithm was assessed and errors were analyzed for clinical significance. Patient risk of HIT was evaluated by using the 4Ts Score and the HIT Expert Probability (HEP) Score. Alert effect was classified as: Results Over the study period, 2,785 patients had a total of 37,060 PC (13.31 PC/patient) and the CDSS triggered a total of 124 alerts. About 10% of alerted clinicians reviewed the website with recommendations on the management of HIT. A total of 117 patients generating alerts were compared to 124 controls. Demographics between the two groups were similar. The CDSS had a 95% sensitivity for accurately detecting a 50% platelet drop for inpatients on heparin. The specificity was 74%. The Objective alert effect was found in 21% of the cases (16% of providers changed their management and 5% documented that HIT was felt to be unlikely). The most common actions taken after receiving the alert were obtaining a HIT antibody assay, withholding heparin, consulting a subspecialty service, and/or changing anticoagulants. The Probable alert effect was 14% of the cases. HIT had been considered before the alert was sent in 11% of patients. Alert effect was classified as None for the remaining 54% of patients. Anti-heparin/PF4 testing for HIT was ordered for 25% of the intervention patients compared to 3% of controls. For the intervention group, PF4 was negative for 86% of tested patients, equivocal for 10%, and positive in 3%. All PF4 tests were negative in the control group. 4Ts score was greater than or equal to 4 in 38% of patients and averaged 3.1 (range 7 to 0). HEP score averaged 4 (range 14 to -4). Risk calculations showed a trend to higher risk with therapeutic heparin compared to prophylactic heparin preparations. HIT risk scores were comparable by admitting service and diagnosis. Conclusions Our CDSS successfully identified upwards of 95% of inpatients at risk for HIT. However, approximately 26% of the alerts were false positives. Only a small proportion of clinicians accessed the website with recommendations for the diagnosis and management of HIT but patients with a suspected HIT were managed appropriately. Of the patients triggering a HIT alert, 38% had an intermediate or high clinical probability of HIT based on the 4Ts Score that would be an indication for further testing. This result highlights the success of the CDSS in identifying patients needing further evaluation but also the difficulties of a computer-based CDSS in representing all the important clinical criteria for the diagnosis of HIT. Disclosures: No relevant conflicts of interest to declare.

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PF4-dependent immunoassays in patients with vaccine-induced immune thrombotic thrombocytopenia (VITT): results of an inter-laboratory comparison

Thrombosis and Haemostasis ◽

10.1055/a-1535-9002 ◽

2021 ◽

Author(s):

Ulrich J. Sachs ◽

Nina Cooper ◽

Andreas Czwalinna ◽

Jens Müller ◽

Bernd Pötzsch ◽

...

Keyword(s):

Platelet Activation ◽

Functional Testing ◽

Initial Screening ◽

Antibody Testing ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Functional Assays ◽

Diagnostic Work ◽

Work Up ◽

Diagnostic Work Up

Background. COVID-19 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 (HIT). 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 inter-laboratory comparison, in which 5 different PF4/heparin immunoassays were used by 4 laboratories. Results. Results for functional testing were highly concordant. VITT antibodies were also reliably detected by PF4/heparin 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 assay (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 work-up. 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.

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Heparin-Induced Thrombocytopenia Testing: ELISA Based Anti-Platelet Factor 4 Polyspecific and IgG-Specific Antibody Detection Assays Compared to the Unfractionated Heparin Serotonin Release Assay

Blood ◽

10.1182/blood-2021-145373 ◽

2021 ◽

Vol 138 (Supplement 1) ◽

pp. 3209-3209

Author(s):

Edward C.C. Wong ◽

Laura A. Worfolk ◽

Caixia Bi ◽

Lina J. Noh ◽

Andrew Espinoza ◽

...

Keyword(s):

Unfractionated Heparin ◽

Specific Antibody ◽

Serotonin Release ◽

Test Results ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Specific Antibodies ◽

Functional Assays ◽

Antibody Assays ◽

Release Assay

Abstract Introduction: Antibodies that cause heparin-induced thrombocytopenia (HIT) can be detected with either antigenic or functional assays. Previously, it has been shown that antigenic (ELISA based) assays that detect anti-platelet factor 4 (anti-PF4) IgG, IgM, or IgA (polyspecific) antibodies are more sensitive but less specific than functional assays such as the unfractionated serotonin release assay (UFH SRA), and that the use of anti-PF4 assays that detect IgG antibodies only, would increase the specificity but decrease the sensitivity of these assays for the detection of HIT antibodies that are prothrombotic (associated with positive functional assay). To date large epidemiologic studies have not confirm these findings. To evaluate the relative performance of anti-PF4 polyspecific and IgG-specific antibodies in their ability to detect prothrombotic HIT antibodies, we evaluated results of non-reflexive HIT panels that contained either anti-PF4 polyspecific or IgG-specific assays and unfractionated heparin serotonin release assays over an eight-year period at a U.S. reference laboratory. Methods: Test results for 2 HIT detection panels were compared: 1 panel had UFH SRA plus the polyspecific PF4 ENHANCED® assay (GTI Diagnostics, Waukesha, WI) and 1 panel had UFH SRA plus the IgG-specific Zymutest HIA IgG assay (Hyphen Biomed, France). Test results were from the last 4 years of use for each panel (2009 to 2012 for the polyspecific panel; 2017 to 2020 for the IgG-specific panel). UFH SRA was performed as described by Sheridan et al, (1986) with positivity defined as ≥20% serotonin release by low dose UFH and >50% suppression of release at high dose (100 U/mL) UFH. For each year and assay, test results were stratified by optical density (OD) results, and the percent of results positive by UFH SRA was determined for each OD range. Median yearly UFH SRA positivity rates for each OD interval were compared for anti-PF4 polyspecific vs IgG-specific antibody assays using non-parametric statistical testing, Mann-Whitney U test, two-tailed, with significance defined as <0.05. Results: HIT panels with either ELISA based assays detecting either anti-PF4 polyspecific or IgG specific antibodies demonstrated increasing UFH SRA positivity rates as OD increased. Approximately 50% UFH SRA positivity occurred when OD was in the 2.000 to range. No significant differences in SRA positivity were seen at any positive OD interval when comparing anti-PF4 polyspecific vs IgG-specific assays. A small but significant difference was seen when OD results were considered This observation may have been due to a in the review process (2017-2020): when a UFH SRA result was positive with a negative OD result, repeat UFH SRA testing was performed. Conclusions: Our study demonstrates that the correlations of UFH SRA positivity and OD measurements are similar for anti-PF4 IgG-specific and polyspecific antibody assays. These results suggest the assay types may perform similarly for the detection of HIT and importantly provide important predictive information as to when an optical density value will lead to a positive UFH SRA result. Figure 1 Figure 1. Disclosures Wong: Quest Diagnostics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Worfolk: Quest Diagnostics: Current Employment. Bi: Quest Diagnostics: Current Employment. Noh: Quest Diagnostics: Current Employment. Espinoza: Quest Diagnostics: Current Employment. Wu: Quest Diagnostics: Current Employment. Sahud: Quest Diagnostics: Current Employment. Racke: Quest Diagnostics: Current Employment. Dlott: Quest Diagnostics: Current Employment.

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Pharmacist involvement in clinical assessment and laboratory testing for heparin-induced thrombocytopenia

Journal of Thrombosis and Thrombolysis ◽

10.1007/s11239-019-02011-8 ◽

2019 ◽

Vol 50 (1) ◽

pp. 195-200

Author(s):

Amanda J. Condon ◽

Adam J. Hood ◽

Katie L. Willenborg ◽

Kraig Kumfer ◽

Anne E. Rose

Keyword(s):

Clinical Assessment ◽

Laboratory Testing ◽

Heparin Induced Thrombocytopenia

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Novel Diagnostic Assays for Heparin-Induced Thrombocytopenia

Blood ◽

10.1182/blood.v120.21.267.267 ◽

2012 ◽

Vol 120 (21) ◽

pp. 267-267

Author(s):

Adam Cuker ◽

Ann H Rux ◽

Jillian L Hinds ◽

May Dela Cruz ◽

Serge Yarovoi ◽

...

Keyword(s):

Laboratory Testing ◽

Research Funding ◽

Cell Activation ◽

B Lymphocyte ◽

Roc Curves ◽

Serotonin Release ◽

Luciferase Reporter ◽

Advisory Committees ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia

Abstract Abstract 267 Introduction Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder mediated by platelet, monocyte, and endothelial cell-activating antibodies (Abs) against ultralarge complexes of platelet factor 4 (PF4) and heparin. Laboratory testing plays a key role in the diagnosis of HIT, but is associated with important shortcomings. Immunoassays such as the PF4/heparin ELISA frequently yield false-positive results due to their inability to discriminate cellular activating-Abs from their non-pathogenic counterparts. Functional assays such as the 14C-serotonin release assay (SRA) are more specific, but are unfeasible for most clinical laboratories due to the requirement for radioisotope and fresh platelets from reactive donors. KKO is a monoclonal Ab that causes a HIT-like thrombocytopenic disorder in a mouse model. Binding of KKO to immobilized PF4/heparin is inhibited by human HIT plasma, but not by plasma from patients with non-pathogenic anti-PF4/heparin Abs.1 We exploited this property of KKO to develop a KKO-inhibition (KKO-I) ELISA to detect platelet-activating Abs. We recently described a system to measure cell activation by HIT Abs: DT40 (chicken B lymphocyte) cells transfected with human FcgRIIa coupled to a luciferase reporter.2 We hypothesized that this system (DT40-luc) could be used to identify cell-activating anti-PF4/heparin Abs without need for donor platelets or radioactivity. Here we describe the KKO-I and DT40-luc assays and compare their performance to two commercially available immunoassays and the SRA in samples from 58 patients with suspected HIT and circulating anti-PF4/heparin Abs. Methods Patient samples consecutively referred to a clinical coagulation laboratory for HIT laboratory testing that tested positive by polyspecific anti-PF4/heparin ELISA were included. In addition to the polyspecific ELISA, citrated plasma samples from all patients were tested by an IgG-specific PF4/heparin ELISA, an in-house SRA, and the investigational KKO-I and DT40-luc assays. A 4Ts score to estimate the clinical likelihood of HIT was determined for each subject. The investigator performing 4Ts scoring was blinded to the results of HIT laboratory assays. Investigators performing the KKO-I and DT40-luc assays were blinded to the 4Ts score and the results of the SRA and anti-PF4/heparin ELISA. The KKO-I and DT40-luc assays were performed as previously described.1,2 HIT was defined as the combination of an intermediate or high probability 4Ts score (≥4) and a positive SRA. The performance of each assay with respect to this reference standard was evaluated by receiver-operating characteristic (ROC) analysis. Areas under the ROC curves (AUCs) were calculated and compared using the Delong method for correlated samples. Results Fifty-eight subjects were enrolled, 21 of whom met prespecified criteria for HIT. There were no significant differences in demographic characteristics between the 21 HIT-positive and 37 HIT-negative subjects. The ability of the polyspecific ELISA, IgG-specific ELISA, KKO-I, and DT40-luc assay to discriminate HIT-positive from HIT-negative subjects is shown in Figure 1. HIT-positive plasma showed significantly greater mean inhibition of KKO binding than HIT-negative plasma (70.1%, 95% CI 64.8–75.4 vs. 40.4%, 33.5–47.4, p<0.0001) (Figure 1C). Plasma from HIT-positive subjects also induced significantly greater luciferase activity (3.14, 2.25–4.03 vs. 0.96, 0.85–1.07, p<0.0001) in the DT40-luc assay (Figure 1D). ROC curves for each assay are shown in Figure 2. The AUC for KKO-I (0.92, 0.85–1.00) was significantly greater than the AUC for the polyspecific (0.82, 0.70–0.95) and IgG-specific (0.76, 0.62–0.90) ELISAs (p<0.05 for both comparisons). The AUC for DT40-luc (0.89, 0.79–0.99) was significantly greater than the AUC for the IgG-specific (p=0.046), but not the polyspecific ELISA (p=0.28). Conclusion KKO-I and DT40-luc showed better discrimination than commercially available ELISAs in a small cohort of patients with suspected HIT and anti-PF4/heparin Abs. These assays are simple to perform, do not require donor platelets or radioactivity, and hold promise for improving the specificity and feasibility of HIT laboratory testing. Further evaluation in a larger cohort of patients is required. Disclosures: Cuker: Baxter: Consultancy, Research Funding; Bayer: Consultancy; Canyon: Consultancy; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees; Stago: Research Funding. Arepally:Teva Pharmaceuticals: Research Funding. Cines:Amgen: Consultancy; GSK: Consultancy; Eisai: Consultancy; T2 Biosystems: Research Funding.

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