Laboratory and demographic predictors of functional assay positive status in suspected heparin-induced thrombocytopenia: A multicenter retrospective cohort study.

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated immune response against platelet factor 4 (PF4) bound to heparin anticoagulants. A priori identification of patients at-risk for HIT remains elusive and a number of risk factors have been identified, but these associations and their effect sizes have limited validation in large cohorts of suspected HIT patients. The aim of this study was to investigate existing anti-PF4/heparin antibody thresholds and model the relationship of demographic variables and anti-PF4/heparin antibody levels with functional assay positivity across multiple institutions in the absence of detailed clinical data. In a large collection of suspected HIT patients (n = 8904), we tested for associations between laboratory and demographic variables and functional assay positive status as well as anti-PF4/heparin antibody levels. We also tested for correlation between IgG-specific and polyspecific (IgG/IgA/IgM) anti-PF4/heparin antibody values and their ability to predict functional assay positive status using area under the receiver operating characteristic (AUROC). Logistic regression identified increasing anti-PF4/heparin antibody OD levels (OR = 51.84 [37.27-74.34], p < 2.0 × 10-16) and female sex (OR = 1.47 [1.19-1.82], p = 3.5 × 10-4) as risk factors for positive functional assay in the largest cohort with consistent effect sizes in two other cohorts. In a subset of 1175 patients, polyspecific and IgG-specific anti-PF4/heparin antibody values were heterogeneous (mean coefficient of variation = 31.9 %), but strongly correlated (rho = 0.878; p < 2 × 10-16) with similar prediction of functional assay positivity (polyspecific AUROC = 0.976 and IgG-specific AUROC = 0.980). Thus, we recapitulate previously identified risk factors of functional assay positivity, providing precise effect sizes in a large observational population of suspected HIT patients. Our data reinforce the necessity of functional assay confirmation and suggest that, despite heterogeneity, polyspecific and IgG-specific anti-PF4/heparin antibody assays predict functional assay positive status similarly, even in the absence of 4Ts scores and detailed clinical data.

Multicenter evaluation of light transmission platelet aggregation reagents: communication from the ISTH SSC Subcommittee on Platelet Physiology.

BACKGROUND: Light transmission aggregation (LTA) is used widely by the clinical and research communities. Although it is a gold standard, there is a lack of interlaboratory harmonization. OBJECTIVES: The primary objective was to assess whether sources of activators (mainly adenosine diphosphate [ADP], collagen, arachidonic acid, epinephrine, and thrombin receptor activating peptide6) and ristocetin contribute to poor LTA reproducibility. The secondary objective was to evaluate interindividual variability of results to appreciate the distribution of normal values and consequently better interpret pathologic results. METHODS: An international multicenter study involving 28 laboratories in which we compared LTA results obtained with center-specific activators and a comparator that we supplied. RESULTS: We report variability in the potency (P) of activators in comparison with the comparator. Thrombin receptor activating peptide 6 (P, 1.32-2.68), arachidonic acid (P, 0.87-1.43), and epinephrine (P, 0.97-1.34) showed the greatest variability. ADP (P, 1.04-1.20) and ristocetin (P, 0.98-1.07) were the most consistent. The data highlighted clear interindividual variability, notably for ADP and epinephrine. Four profiles of responses were observed with ADP from high-responders, intermediate-responders, and low-responders. A fifth profile corresponding to nonresponders (5% of the individuals) was observed with epinephrine. CONCLUSION: Based on these data, the establishment and adoption of simple standardization principles should mitigate variability due to activator sources. The observation of huge interindividual variability for certain concentrations of activators should lead to a cautious interpretation before reporting a result as abnormal. Confidence can be taken from the fact that difference between sources is not exacerbated in patients treated with antiplatelet agents.

A French Real-World Evidence Study Evaluating the Efficacy, Safety, and Pharmacokinetic Parameters of rVIII-SingleChain in Patients with Hemophilia A Receiving Prophylaxis.

BACKGROUND: rVIII-SingleChain is a recombinant factor VIII (FVIII) with increased binding affinity to von Willebrand factor compared with other FVIII products. rVIII-SingleChain is indicated for the treatment and prevention of bleeding episodes in patients with hemophilia A. OBJECTIVES: To collect real-world evidence data from patients treated with rVIII-SingleChain to confirm the efficacy and safety established in the clinical trial program and carry out a population pharmacokinetic (PK) analysis. METHODS: This interim analysis includes data, collected between January 2018 - September 2021, from patients treated with rVIII-SingleChain prophylaxis at French Hemophilia Treatment centers. Data on annualized bleeding rates, dosing frequency, and consumption before and after switching to rVIII-SingleChain were recorded. A population PK analysis was also conducted to estimate PK parameters. RESULTS: Overall, 43 patients switched to prophylaxis with rVIII-SingleChain either from a previous prophylaxis regimen or from on-demand treatment. Following the switch to rVIII-SingleChain, patients maintained excellent bleed control. After switching to rVIII-SingleChain, most patients maintained or reduced their regimen. Interestingly, a majority of patients treated >2 ×/weekly with a standard half-life FVIII reduced both injection frequency and FVIII consumption with rVIII-SingleChain. A PK analysis revealed a lower clearance of rVIII-SingleChain (1.9 vs. 2.1 dL/h) and a longer half-life both in adolescents/adults (n = 28) and pediatric (n = 6) patients (15.5 and 11.9 hours, respectively vs. 14.5 and 10.3 hours) than previously reported. CONCLUSIONS: Patients who switched to rVIII-SingleChain prophylaxis demonstrated excellent bleed control and a reduction in infusion frequency. A population PK analysis revealed improved PK parameters compared with those reported in the clinical trial.

Variable serotonin release assay pattern and specificity of PF4-specific antibodies in HIT, and clinical relevance.

BACKGROUND: The diagnosis of heparin-induced thrombocytopenia (HIT) requires functional assays to demonstrate that platelet factor 4 (PF4)-specific antibodies activate platelets, typically when therapeutic heparin (H) concentrations are tested ("classical" pattern). Some HIT samples also activate platelets without heparin ("atypical" pattern), but with unclear clinical significance. OBJECTIVES: We aimed to assess whether platelet activation pattern and some characteristics of PF4-specific antibodies were associated with the severity of HIT. PATIENTS/METHODS: Serotonin release assay (SRA) pattern of 81 HIT patients were analyzed and compared with their clinical and biological data, including levels of anti-PF4/H immunoglobulin G (IgG) and anti-PF4 IgG in 47 of them. RESULTS: Higher anti-PF4/H IgG titers were measured in patients with an "atypical" SRA (optical density 2.52 vs. 1.94 in those with a "classical" pattern, p < .001). Patients of both groups had similar platelet count (PC) nadir and time to recovery, but those with an "atypical" SRA more frequently developed thrombotic events (69% vs. 34%, p = .037). Significant levels of anti-PF4 IgG were detected in both groups (38% and 61%, respectively). Whatever the SRA pattern, a lower PC nadir (35 vs. 53 G/L, p = .006) and a longer PC recovery time (6 vs. 3 days, p = .015) were evidenced in patients with anti-PF4 antibodies, compared with those with anti-PF4/H IgG only. CONCLUSIONS: An atypical SRA pattern with elevated anti-PF4/H IgG titers seems associated with an increased risk of thrombosis in HIT. IgG antibodies to native PF4 may contribute to more severe and persistent thrombocytopenia, and their detection could be useful in clinical practice.

Genome-wide association study of platelet factor 4/heparin antibodies in heparin-induced thrombocytopenia.

Heparin, a widely used anticoagulant, carries the risk of an antibody-mediated adverse drug reaction, heparin-induced thrombocytopenia (HIT). A subset of heparin-treated patients produces detectable levels of antibodies against complexes of heparin bound to circulating platelet factor 4 (PF4). Using a genome-wide association study (GWAS) approach, we aimed to identify genetic variants associated with anti-PF4/heparin antibodies that account for the variable antibody response seen in HIT. We performed a GWAS on anti-PF4/heparin antibody levels determined via polyclonal enzyme-linked immunosorbent assays. Our discovery cohort (n = 4237) and replication cohort (n = 807) constituted patients with European ancestry and clinical suspicion of HIT, with cases confirmed via functional assay. Genome-wide significance was considered at α = 5 × 10-8. No variants were significantly associated with anti-PF4/heparin antibody levels in the discovery cohort at a genome-wide significant level. Secondary GWAS analyses included the identification of variants with suggestive associations in the discovery cohort (α = 1 × 10-4). The top variant in both cohorts was rs1555175145 (discovery ß = -0.112 [0.018], P = 2.50 × 10-5; replication ß = -0.104 [0.051], P = .041). In gene set enrichment analysis, 3 gene sets reached false discovery rate-adjusted significance (q < 0.05) in both discovery and replication cohorts: "Leukocyte Transendothelial Migration," "Innate Immune Response," and "Lyase Activity." Our results indicate that genomic variation is not significantly associated with anti-PF4/heparin antibody levels. Given our power to identify variants with moderate frequencies and effect sizes, this evidence suggests genetic variation is not a primary driver of variable antibody response in heparin-treated patients with European ancestry.

ABO O blood group as a risk factor for platelet reactivity in heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is an unpredictable, potentially catastrophic adverse effect resulting from an immune response to platelet factor 4 (PF4)/heparin complexes. We performed a genome-wide association study (GWAS) with positive functional assay as the outcome in a large discovery cohort of patients divided into 3 groups: (1) functional assay-positive cases (n = 1269), (2) antibody-positive (functional assay-negative) controls (n = 1131), and (3) antibody-negative controls (n = 1766). Significant associations (α = 5 × 10-8) were investigated in a replication cohort (α = 0.05) of functional assay-confirmed HIT cases (n = 177), antibody-positive (function assay-negative) controls (n = 258), and antibody-negative controls (n = 351). We observed a strong association for positive functional assay with increasing PF4/heparin immunoglobulin-G (IgG) level (odds ratio [OR], 16.53; 95% confidence interval [CI], 13.83-19.74; P = 1.51 × 10-209) and female sex (OR, 1.15; 95% CI, 1.01-1.32; P = .034). The rs8176719 C insertion variant in ABO was significantly associated with positive functional assay status in the discovery cohort (frequency = 0.41; OR, 0.751; 95% CI, 0.682-0.828; P = 7.80 × 10-9) and in the replication cohort (OR, 0.467; 95% CI, 0.228-0.954; P = .0367). The rs8176719 C insertion, which encodes all non-O blood group alleles, had a protective effect, indicating that the rs8176719 C deletion and the O blood group were risk factors for HIT (O blood group OR, 1.42; 95% CI, 1.26-1.61; P = 3.09 × 10-8). Meta-analyses indicated that the ABO association was independent of PF4/heparin IgG levels and was stronger when functional assay-positive cases were compared with antibody-positive (functional assay-negative) controls than with antibody-negative controls. Sequencing and fine-mapping of ABO demonstrated that rs8176719 was the causal single nucleotide polymorphism (SNP). Our results clarify the biology underlying HIT pathogenesis with ramifications for prediction and may have important implications for related conditions, such as vaccine-induced thrombotic thrombocytopenia.

The deglycosylated form of 1E12 inhibits platelet activation and prothrombotic effects induced by VITT antibodies.

In order to improve the safety of COVID-19 vaccines, there is an urgent need to unravel the pathogenesis of vaccineinduced immune thrombotic thrombocytopenia (VITT), a severe complication of recombinant adenoviral vector vaccines used to prevent COVID-19, and likely due to anti-platelet factor 4 (PF4) IgG antibodies. In this study, we demonstrated that 1E12, a chimeric anti-PF4 antibody with a human Fc fragment, fully mimics the effects of human VITT antibodies, as it activates platelets to a similar level in the presence of platelet factor 4 (PF4). Incubated with neutrophils, platelets and PF4, 1E12 also strongly induces NETosis, and in a microfluidic model of whole blood thrombosis, it triggers the formation of large platelet/leukocyte thrombi containing fibrin(ogen). In addition, a deglycosylated form of 1E12 (DG-1E12), which still binds PF4 but no longer interacts with Fcγ receptors, inhibits platelet, granulocyte and clotting activation induced by human anti-PF4 VITT antibodies. This strongly supports that 1E12 and VITT antibodies recognize overlapping epitopes on PF4. In conclusion, 1E12 is a potentially important tool to study the pathophysiology of VITT, and for establishing mouse models. On the other hand, DG-1E12 may help the development of a new drug that specifically neutralizes the pathogenic effect of autoimmune anti-PF4 antibodies, such as those associated with VITT.

What is the impact of blood pressure on neurological symptoms and the risk of ESKD in primary and secondary thrombotic microangiopathies based on clinical presentation: a retrospective study.

BACKGROUND: The impact of blood pressure on neurological symptoms and risk of end-stage kidney disease (ESKD) is unknown in primary and secondary thrombotic microangiopathies (TMAs). METHODS: We measured baseline systolic (SBP) and diastolic (DBP) BP in consecutive 563 patients with adjudicated primary and secondary TMAs, and assessed its association with the risk of ESKD. RESULTS: Normal BP, grade 1, 2 and 3 hypertension were present in 243 (43.1%), 132 (23.4%), 101 (17.9%) and 88 (15.6%), respectively. Significant BP differences were noted in relation to the cause of TMA: highest BP values were found in patients with atypical hemolytic-uremic syndrome (aHUS), pregnancy, transplantation and auto-immune-related TMAs. Normal BP or grade 1 hypertension was found in 17/18 (94.4%) patients with thrombotic thrombocytopenic patients (only 1/18 (5.6%) had a SBP value>150 mmHg). In contrast, BP values could not differentiate isolated "essential" malignant hypertension (MH) from MH associated with aHUS (isolated MH (n=15): BP (median (IQR)): 220 (182-249)/132 (101-150) mmHg; MH with aHUS (n=5): BP: 223 (196-245)/131 (111-144) mmHg). The risk of vigilance disturbances (6.9%, 15.0%, 25.0%, respectively), epileptic seizures (1.5%, 4.0%, 12.5%, respectively) and posterior reversible encephalopathy syndrome (0.76%, 2.97%, 6.82%, respectively) increased with increasing baseline BP values from grade 1 to grade 3 hypertension. ESKD occurred in 35/563 (6.2%) patients (1.23%, 2.27%, 11.9% and 19.3% of patients with normal BP, grade 1, 2 and 3 hypertension, respectively). As compared to patients with normal BP (<120/139 mmHg), grade 1, grade 2 and grade 3 hypertension were associated with a greater risk of ESKD in univariate (OR: 1.91 [0.83-4.40], 13.2 [3.56-48.9] and 34.8 [9.31-130], respectively) and multivariate (OR: 0.89 [0.30-2.69], 7.00 [1.57-31.3] and 19.7 [4.53-85.2], respectively) analyses. The association between BP and the risk of ESRD was unchanged after adjustment on eculizumab use (OR: 3.46 [1.41-8.49], 17.7 [4.44-70.0] and 70.6 [8.61-579], respectively). Patients with MH, regardless of its cause, had a greater risk of ESKD (OR: 26.4 [10.0-69.8] vs other patients). CONCLUSIONS: Baseline BP differs in primary and secondary TMAs. High BP reduces the neurological tolerance of TMAs and is a powerful independent risk factor of ESKD, even after adjustment on TMA's cause.

Multicentre evaluation of 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, as an internal quality control in HIT functional assays: Communication from the ISTH SSC Subcommittee on Platelet Immunology.

BACKGROUND: Functional tests for the diagnosis of heparin-induced thrombocytopenia (HIT) exhibit variable performance. OBJECTIVES: We evaluated in a multicenter study whether 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, could be used as an internal quality control. METHODS: 5B9 was sent to 11 laboratories in seven countries, and six initial concentrations ranging from 10 to 400 µg/mL were tested by heparin-induced platelet activation assay (HIPA), serotonin release assay (SRA), platelet aggregation test (PAT), flow cytometry (FC), or heparin-induced multiple-electrode aggregometry (HIMEA). Each method was evaluated in three different laboratories using experimental procedures identical to those usually applied for the diagnosis of HIT by testing platelets from 10 different healthy donors. RESULTS: The procedures used varied among the laboratories, particularly when platelet-rich plasma and whole blood were used. Nevertheless, positive results were obtained with at least 100 µg/ml of 5B9 for most donors tested by all centers (except one) performing HIPA, SRA, or HIMEA. FC and PAT results were more heterogeneous. FC results from one center that used washed platelets preincubated with PF4 were positive with all donors at 50 µg/ml 5B9, but at least 200 µg/ml of 5B9 were required to activate cells with most donors tested using PAT. CONCLUSION: This study confirms that HIT functional tests are not well standardized and exhibit variable sensitivity for the detection of platelet-activating antibodies. However, 5B9 is a potentially useful tool to standardize functional tests, to select responding platelet donors, and consequently to improve the performance of these assays and comparability between laboratories.

Heparin-induced Thrombocytopenia Diagnosis: A Retrospective Study Comparing Heparin-induced Platelet Activation Test to 14 C-serotonin Release Assay.

Laboratory confirmation of heparin-induced thrombocytopenia (HIT) is of crucial importance and remains challenging and relies on platelet functional assays highlighting the presence of heparin-dependent platelet-activating antibodies in patient serum or plasma. Platelet functional assays using washed platelets include the 14 C-serotonin release assay (SRA), usually described as the gold standard, and the heparin-induced platelet activation assay (HIPA). Since its first comparison with SRA there has been no additional published study regarding HIPA diagnostic performances compared with SRA. Aim of our retrospective study was to compare the concordance between HIPA and SRA in HIT suspected-patients with positive anti-PF4/heparin antibodies between October 2010 and October 2015. Fifty-five HIT-suspected patients who beneficiated from both HIPA and SRA were included. Positive and negative percent agreements were 83.8% (95% CI 68.0-93.8%) and 66.7% (95% CI 41.0-86.7%), respectively. Overall percent agreement was 78.2% (95% CI 65.0-92.2%). Agreement was higher in patients who underwent cardiopulmonary bypass with extracorporeal circulation circuit for cardiac surgery. We also confirm that the use of a minimum of 2 platelet donors to establish positive HIT diagnosis and 4 platelet donors to exclude HIT diagnosis allows obtaining a good agreement with SRA. Although HIPA and SRA were performed with different platelet donors and in different laboratories, HIPA had a good positive agreement with SRA for HIT diagnosis, showing that HIPA is a useful functional assay that does not require radioactivity and could be developed worldwide to improve HIT diagnosis.

Infection in Patients with Suspected Thrombotic Microangiopathy Based on Clinical Presentation.

BACKGROUND AND OBJECTIVES: In contrast to shigatoxin-associated Escherichia coli (STEC) causing hemolytic uremic syndrome, STEC-unrelated infections associated with thrombotic microangiopathy are less characterized. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Our retrospective study in a four-hospital institution of 530 consecutive patients with adjudicated thrombotic microangiopathies during the 2009-2016 period studied STEC-unrelated infections' epidemiology and major outcomes (death, acute dialysis, and major cardiovascular events). RESULTS: STEC-unrelated infection was present in 145 of 530 (27%) patients, thrombotic microangiopathies without infection were present in 350 of 530 (66%) patients, and STEC causing hemolytic and uremic syndrome was present in 35 of 530 (7%) patients. They (versus thrombotic microangiopathy without infection) were associated with age >60 years (36% versus 18%), men (53% versus 27%), altered consciousness (32% versus 11%), mean BP <65 mm Hg (21% versus 4%), lower hemoglobin and platelet count, and AKI (72% versus 49%). They were associated with more than one pathogen in 36 of 145 (25%) patients (either isolated [14%] or combined [86%] to other causes of thrombotic microangiopathy); however, no significant clinical or biologic differences were noted between the two groups. They were more frequently due to bacteria (enterobacteria [41%], Staphylococcus aureus [11%], and Streptococcus pneumonia [3%]) than viruses (Epstein-Barr [20%], cytomegalovirus [18%], influenza [3%], hepatitis C [1%], HIV [1%], and rotavirus [1%]). STEC-unrelated infections were independent risk factors for in-hospital death (odds ratio, 2.22; 95% confidence interval, 1.18 to 4.29), major cardiovascular event (odds ratio, 3.43; 95% confidence interval, 1.82 to 6.69), and acute dialysis (odds ratio, 3.48; 95% confidence interval, 1.78 to 7.03). Bacteria (versus other pathogens), and among bacteria, enterobacteria, presence of more than one bacteria, and E. coli without shigatoxin were risk factors for acute dialysis. CONCLUSIONS: Infections are frequent thrombotic microangiopathy triggers or causes, and they are mostly unrelated to STEC. Infections convey a higher risk of death and major complications. The most frequent pathogens were enterobacteria, S. aureus, Epstein-Barr virus, and cytomegalovirus. PODCAST: This article contains a podcast at https://www.asn-online.org/media/podcast/CJASN/2021_09_07_CJN17511120.mp3.

International multicenter, multiplatform study to validate Taipan snake venom time as a lupus anticoagulant screening test with ecarin time as the confirmatory test: Communication from the ISTH SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies.

BACKGROUND: Lupus anticoagulant (LA) assays are compromised in anticoagulated patients, and existing strategies to overcome the interferences have limitations. The prothrombin-activating Taipan snake venom time (TSVT) screening test and ecarin time (ET) confirmatory test are innately insensitive to vitamin K antagonists (VKA) and direct factor Xa inhibitors (DFXaI). OBJECTIVES: Validate standardized TSVT/ET reagents for LA detection, in a multicenter, multiplatform study. PATIENTS/METHODS: Six centers from four countries analyzed samples with TSVT/ET from 81 nonanticoagulated patients with LA, patients with established antiphospholipid syndrome (APS), and proven persistent LA who were either not anticoagulated (n = 120) or were anticoagulated with VKAs (n = 180) or DFXaIs (n = 71). Additionally, 339 nonanticoagulated LA-negative patients, and 575 anticoagulated non-APS patients (172 VKA, 403 DFXaI) were tested. Anticoagulant spiking experiments were performed and 112 samples containing potential interferences (i.e., direct thrombin inhibitors) were tested. Results were evaluated against locally derived cutoffs. Imprecision was evaluated. RESULTS: Cutoffs were remarkably similar despite use of different analyzers and donor populations. Cutoffs for TSVT ratio, ET ratio, percent correction, and normalized TSVT ratio/ET ratio ranged between 1.08 and 1.10, 1.09 and 1.12, 9.3% and 14.8%, and 1.10 and 1.15, respectively. Coefficients of variation for TSVT and ET ratios were ≤5.0%. TSVT/ET exhibited sensitivity, specificity, and negative and positive predictive values of 78.2%/95.0%/86.3%/91.5%, respectively, with established APS as the LA-positive population, and 86.9%/95.0%/76.8%/97.4%, respectively, with triple-positive APS. Interference was seen with direct thrombin inhibitors, unfractionated heparin, and low molecular weight heparins, but not VKAs or DFXaIs. CONCLUSIONS: TSVT/ET are validated for LA detection in nonanticoagulated patients and those on VKAs or DFXaIs.

Heparin-induced thrombocytopenia: Construction of a pretest diagnostic score derived from the analysis of a prospective multinational database, with internal validation.

BACKGROUND: Diagnosis of heparin-induced thrombocytopenia (HIT) requires pretest probability assessment and dedicated laboratory assays. OBJECTIVE: To develop a pretest score for HIT. DESIGN: Observational; analysis of prospectively collected data of hospitalized patients suspected with HIT (ClinicalTrials.gov NCT00748839). SETTING: Thirty-one tertiary hospitals in France, Switzerland, and Belgium. PATIENTS: Patients tested for HIT antibodies (2280 evaluable), randomly allocated to derivation and validation cohorts. MEASUREMENTS: Independent adjudicators diagnosed HIT based on the prospectively collected data and serotonin release assay results. RESULTS: Heparin-induced thrombocytopenia was diagnosed in 234 (14.7%) and 99 (14.5%) patients in the two cohorts. Eight features were associated with HIT (in brackets, points assigned for score calculation of the score): unfractionated heparin (1); therapeutic-dose heparin (1); cardiopulmonary bypass (cardiac surgery) (2); major trauma (3); 5- to 21-day interval from anticoagulation initiation to suspicion of HIT (4); ≥40% decrease in platelet count over ≤6 days (3); thrombotic event, arterial (3) or venous (3). The C-statistic was 0.79 (95% CI, 0.76-0.82). In the validation cohort, the area under the receiver operating characteristic curve was 0.77 (95% CI, 0.74-0.80). Three groups of scores were defined; HIT prevalence reached almost 30% in the high-probability group. LIMITATION: The performance of the score may depend on settings and practices. CONCLUSION: The objective, easy-to-collect, clinical features of HIT we evidenced were incorporated into a pretest score, which may guide clinical decisions regarding diagnostic testing and anticoagulation.

Acquired platelet defect associated with gabapentin treatment: a case-report.

Gabapentin, a structural analog of gamma-aminobutyric acid, is used to treat peripheral neuropathic pain. Here we report the first case of platelet function disorder associated with gabapentin treatment in a 44-year-old woman without a history of bleeding. She presented with mucocutaneous bleeding approximately 1 month after initiation of gabapentin and platelet function tests showed no aggregation to arachidonic acid and epinephrine, a defective response to ADP and a slightly decreased response to collagen. Gabapentin's imputability was supported by the fact that all platelet functions were normalized 6 days after drug discontinuation, with the simultaneous disappearance of bleedings.

Characterization of New Monoclonal PF4-Specific Antibodies as Useful Tools for Studies on Typical and Autoimmune Heparin-Induced Thrombocytopenia.

BACKGROUND: Heparin-induced thrombocytopenia (HIT) is typically caused by platelet-activating immunoglobulin G (IgG) antibodies (Abs) against platelet factor 4 (PF4) complexed with heparin (H). Much less frequent "autoimmune" HIT is distinguished from typical HIT by platelet activation without heparin and the presence of both anti-PF4/H and anti-PF4 IgG. We developed three murine monoclonal anti-PF4 Abs with a human Fc-part, 1E12, 1C12, and 2E1, resembling autoimmune HIT Abs. OBJECTIVES: To characterize 1E12, 1C12, and 2E1 in comparison to the heparin-dependent monoclonal anti-PF4/H Abs 5B9 and KKO, and polyclonal Abs from patients with typical HIT (group-2) and autoimmune HIT (group-3). METHODS: Interactions of Abs with PF4 and PF4/H were studied by enzyme-linked-immunosorbent assay, single-molecule force spectroscopy, isothermal titration calorimetry, and dynamic light scattering. Serotonin release assay and heparin-induced platelet activation assay were used to assess platelet activation. The binding sites of monoclonal Abs on PF4 were predicted in silico (MAbTope method). RESULTS: 1C12, 1E12, and 2E1 displayed higher affinity for PF4/H complexes than 5B9 and KKO, comparable to human group-3 Abs. Only 1C12, 1E12, 2E1, and group-3 Abs formed large complexes with native PF4, and activated platelets without heparin. The predicted binding sites of 1C12, 1E12, and 2E1 on PF4 differed from those of KKO and 5B9, but were close to each other. 2E1 exhibited unique bivalent binding, involving its antigen recognition site to PF4 and charge-dependent interactions with heparin. CONCLUSION: 1C12, 1E12, and 2E1 are tools for studying the pathophysiology of autoimmune HIT. 2E1 provides evidence for a new binding mechanism of HIT Abs.