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Venous thromboembolism (VTE) is associated with increased coagulation activity, which in part can be attributed to the contact pathway of coagulation. Evidence from pre-clinical and epidemiological studies suggests that deficiency in factors of contact activation (e.g. coagulation factors (F) XI and FXII) protects against VTE. However, limited information exists regarding the activation of the contact system in the setting of acute VTE. In the current study, patients with confirmed VTE events (n=321) from the VTEval study and controls (n=300) from the population-based PREVENT-it pilot study were included. Plasma samples were collected from patients after confirmed VTE events or controls upon inclusion in the study. FXI as well as FXIa and plasma kallikrein (PKa) levels were assessed in plasma samples from all subjects using an activated thromboplastin time-based assay (FXI:c), a thrombin generation-based assay (CAT:FXIa) and by measuring inhibitory complexes (FXIa:antithrombin (AT), FXIa:alpha-1-antitrypsin (α1AT), FXIa:C1 esterase inhibitor (C1Inh) and PKa:C1Inh) using enzyme-linked immunoassay (ELISA). After a 2-year follow up period, a composite endpoint of recurrent VTE or death was determined. Increased FXI:c levels were determined in VTE patients compared to control individuals (124.08 ± 37.48% vs. 113.55 ± 27.99%), whereas CAT:FXIa levels were reduced in VTE patients (0 pM [IQR, 0-0.56] vs 0.56 pM [IQR, 0-0.88]). Levels of FXIa:α1AT and FXIa:AT inhibitory complexes were increased in VTE patients compared to controls (median[IQR]; 311.8 pM [238.2-424.0] vs. 202.5 pM [143.7 - 287.5] and 29.1 pM [23.4-38.3] vs 23.2 pM [19.7-29.8], respectively). Considering that 86% of the VTE patients were already on anticoagulant treatment (Table 1), investigation of their possible effect on the biomarkers revealed that only the CAT:FXIa was influenced by the presence of anticoagulants. Logistic regression models revealed a good discriminatory value for FXI:c and FXIa:α1AT (AUC=0.64 [0.6/0.69] and AUC=0.67 [0.62/0.71], respectively) to distinguish VTE from controls, whereas the other biomarkers were not able to distinguish between groups. The outcome recurrent VTE or death could be predicted by the inhibitory complexes, but not by the FXI(a) levels (Figure 1). Only the FXIa:α1AT complexes were able to both detect the presence of VTE (OR per SD [95%CI]: 1.28 [1.01-1.63], p=0.04) and predict recurrent VTE or death (HR per SD [95%CI]: 1.40 [1.2-1.62], p In summary, acute VTE is associated with both elevated FXI:c levels and increased activation of FXI and plasma prekallikrein, the latter specifically indicating contact activation. The generation of FXIa during acute VTE and its association with recurrent VTE suggests an important risk contribution of FXI activation. This study has added evidence favouring the utility of FXIa inhibition in the setting of acute VTE. Figure 1 Figure 1. Disclosures Knoeck: Bayer AG: Consultancy. ten Cate: Bayer AG: Other; Pfizer: Other; LEO Pharma: Other; Gideon Pharmaceuticals: Other; Alveron Pharma: Other. Wild: Bayer AG: Other, Research Funding; Boehringer Ingelheim: Other, Research Funding; Novartis Pharma: Other, Research Funding; Sanofi-Aventis: Other, Research Funding; Astra Zeneca: Other, Research Funding; Daiichi Sankyo Europe: Other, Research Funding.

Background Although combined oral contraceptives (COC) are considered a transient risk factor for venous thromboembolism (VTE), the risk of recurrence after discontinuation of anticoagulation is unclear. Few studies have focused on the risk of recurrence in this group; studies report variable results and are limited by small sample size. The risk of recurrence appears to be low, but this could relate to the young age of affected women. Deciphering the absolute VTE recurrence risk after a COC-associated VTE is crucial in helping clinicians and patients decide if anticoagulation could be discontinued after the initial treatment period. Objectives The objectives of this systematic review and meta-analysis are to estimate the incidence of recurrent VTE among women with COC-associated VTE, compared with women with unprovoked VTE. Methods We searched the following databases: Cochrane Central Register of Controlled Trial, Cochrane Database of Systematic Reviews, Embase Classic +Embase, and Medline ALL, all from the OvidSP platform, from the database's inception to July 2020. Additional studies were identified by screening citations from included studies. Prospective cohort studies, randomized controlled trials (RCTs), and meta-analyses of prospective cohort studies or RCTs were reviewed by two authors for study inclusion (Figure 1). Studies were included if women had objectively confirmed COC-associated VTE, received a minimum of three months of anticoagulation, discontinued COC prior to or at time of discontinuation of anticoagulation, time of follow-up began after anticoagulation was stopped, and recurrent VTE data was available. Studies were excluded if patients were systematically treated with an alternative pharmacologic agent intended to reduce the risk of recurrent VTE such as aspirin. Authors of identified papers were contacted for additional data on critical variables. If there were multiple publications from a cohort, the study with the longest follow up was included. Two authors extracted study data and assessed included studies for risk of bias using the Newcastle Ottawa Scale. Meta-analysis was done using a random effects Poisson regression model. Heterogeneity was assessed using the I-squared measure. Results Our systematic review included 19 studies with a total of 1,537 women (5,828 patient years of follow up) with an index COC-associated VTE, and 1,974 women (7,798 patient years of follow up) with an index unprovoked VTE. Authors contributed additional unpublished data in 16 of the 19 studies. Overall, studies were at low risk of bias, with a mean of 7 stars (out of a possible 9) in the Newcastle Ottawa Scale. Among the 19 studies, the incidence rate of VTE recurrence in women with COC-associated VTE was 1.22 per patient year (95% confidence interval (CI) 0.94 to 1.59, I 2 = 6.4%, 95% prediction interval (PI) 0.81 to 1.85) (Figure 2). The incidence rate of VTE recurrence in women with an index unprovoked VTE not associated with COC was 3.89 per patient year (95% CI 2.98 to 5.07, I 2 =74.2%, 95% PI 1.37 to 11.03). The unadjusted incidence rate ratio of recurrent VTE comparing women with COC-associated events to women with unprovoked events was 0.34 (95% CI 0.26 to 0.45, I 2 = 2.6%, 95% PI 0.26 to 0.46). Only three studies had age-adjusted comparisons, but each with a different effect measure so they could not be combined, with a relative risk ratio 0.4 (95% CI 0.2 to 0.8) (also adjusted for site of VTE and congenital thrombophilia) (Eischer 2014), a hazard ratio of 0.11 (95% CI 0.01 to 0.85) (Kearon 2019), and an incidence rate ratio of 1 (95% CI 0.3 to 3.2) (Le Moigne 2013). Conclusions The estimated risk of VTE recurrence after a COC-associated VTE is low, and is lower compared to women with unprovoked VTE, however this comparison may be confounded by age. With only a minority of studies providing age adjusted analyses, the true difference remains unknown. Our meta-analysis is strengthened by the substantial contribution of unpublished data from individual study authors. This can help to guide clinicians and patient shared decision-making on the duration of anticoagulation. Figure 1 Figure 1. Disclosures Le Gal: LEO Pharma: Honoraria; BMS: Honoraria; Pfizer: Honoraria; Bayer: Honoraria; Aspen: Honoraria; Sanofi: Honoraria. Schulman: Boehringer-Ingelheim: Research Funding; Octapharma: Research Funding. Skeith: CSL Behring: Research Funding; Leo Pharma: Honoraria; Sanofi: Honoraria.

Immunoassays are essential in the workup of patients with suspected heparin-induced thrombocytopenia. However, the diagnostic accuracy is uncertain with regard to different classes of assays, antibody specificities, thresholds, test variations, and manufacturers. We aimed to assess diagnostic accuracy measures of available immunoassays and to explore sources of heterogeneity. We performed comprehensive literature searches and applied strict inclusion criteria. Finally, 49 publications comprising 128 test evaluations in 15 199 patients were included in the analysis. Methodological quality according to the revised tool for quality assessment of diagnostic accuracy studies was moderate. Diagnostic accuracy measures were calculated with the unified model (comprising a bivariate random-effects model and a hierarchical summary receiver operating characteristics model). Important differences were observed between classes of immunoassays, type of antibody specificity, thresholds, application of confirmation step, and manufacturers. Combination of high sensitivity (>95%) and high specificity (>90%) was found in 5 tests only: polyspecific enzyme-linked immunosorbent assay (ELISA) with intermediate threshold (Genetic Testing Institute, Asserachrom), particle gel immunoassay, lateral flow immunoassay, polyspecific chemiluminescent immunoassay (CLIA) with a high threshold, and immunoglobulin G (IgG)-specific CLIA with low threshold. Borderline results (sensitivity, 99.6%; specificity, 89.9%) were observed for IgG-specific Genetic Testing Institute-ELISA with low threshold. Diagnostic accuracy appears to be inadequate in tests with high thresholds (ELISA; IgG-specific CLIA), combination of IgG specificity and intermediate thresholds (ELISA, CLIA), high-dose heparin confirmation step (ELISA), and particle immunofiltration assay. When making treatment decisions, clinicians should be a aware of diagnostic characteristics of the tests used and it is recommended they estimate posttest probabilities according to likelihood ratios as well as pretest probabilities using clinical scoring tools.

Introduction: During the last 50 years vitamin K antagonists (VKAs) have been widely used for the primary prevention of thromboembolism in patients with atrial fibrillation, or for the secondary prevention in patients with a history of venous thromboembolism. The most prevalent adverse effect of anticoagulant therapy is an increased risk of bleeding. Annually approximately 1-4% of patients treated with VKAs suffer from major bleeding episodes. In the past several attempts have been made to predict the bleeding risk for these patients. Among the methods used at this moment are clinical decision scores, of which the HAS-BLED score is the most common one. Up to this point there are no laboratory methods available to predict patients at risk for bleeding. Thrombin generation (TG) tests have been shown to provide an intermediate phenotype for thrombosis. Additionally, TG has the capacity to detect the anticoagulant effect of many, if not all, anticoagulants, including VKAs. This method was only applicable in plasma, but recently this TG test has been introduced in whole blood (WB). With this study we aimed to investigate whether TG, in plasma or WB, could detect bleeding in patients taking VKAs. Materials & methods: Blood samples were collected in citrate tubes from 129 patients taking VKAs for over 3 months, who were older than 18 years of age and followed over time for bleeding episodes. The patients signed informed consent forms and the study was approved by the local medical ethical committee. TG was determined in WB, platelet rich plasma (PRP) and platelet poor plasma (PPP) by means of calibrated automated thrombinography (CAT). Tissue factor was used as a trigger at a final concentration of 1 pM in WB and PRP and at both 1 pM and 5 pM with 4µM phospholipids in PPP. Hematocrit and the hemoglobin concentration were determined in WB. The PPP of the patients was used to define the International normalized ratio (INR). Results: In our study the average INR value of all patients was 2.95 (± 0.92 (SD)). Twenty six patients (20.2 %) suffered from 44 clinically relevant bleeding episodes during a mean follow-up of 15.5 months. Applying TG in PPP and PRP we found no differences in either endogenous thrombin potential (ETP), an indication for the total amount of thrombin that can be converted, nor peak height (the highest amount of active thrombin present during coagulation). Interestingly, when we applied TG in whole blood, so including the blood cells, we found a significantly lower ETP (p < 0.01) and peak (p < 0.05) in the patients that suffered from bleeding (median: 182.5 nM.min and 23.9 nM, respectively) compared to patients that did not bleed (median: 256.2 nM.min and 39.1 nM). When examining the INR, hematocrit and hemoglobin levels, no significant differences were detected. A receiver operating curve (ROC) was constructed for the ETP and peak (figure 1). By determining the area under the curve (AUC) of the ROC we found that the ETP and peak are significantly (p < 0.05) associated with the tendency to bleed (ETP, AUC: 0.700; peak, AUC: 0.642). This compares favorably with the AUC reported for the HAS-BLED score related to clinically relevant bleeding in patients on VKAs (0.60). Conclusions: The TG test in whole blood was only recently developed and this is the first study implementing it to test patients using VKAs. In our study TG measured in WB proved to be the first laboratory test that is able to detect patients at risk of bleeding when treated with VKAs. The INR and plasma TG (CAT-based method), on the other hand, did not discriminate between bleeding and non-bleeding patients. For WB TG, based on the AUC of the ROC, the ETP and peak in WB have a predictive value for bleeding in patients taking VKAs superior over the currently used HAS-BLED score. Figure 1: Analysis of patients with and without bleeding symptoms. Receiver operating curves (ROC) of the ETP and peak in WB TG. Figure 1:. Analysis of patients with and without bleeding symptoms. Receiver operating curves (ROC) of the ETP and peak in WB TG. Disclosures No relevant conflicts of interest to declare.

Introduction We tested whether the recently introduced measurement of thrombin generation (TG) in whole blood can be used to evaluate the clotting status of patients on vitamin K antagonist (VKA) prophylaxis. The prothrombin time, and hence the International Normalized Ratio (INR), only evaluates the vitamin K dependent factors II, VII and X but not the anticoagulant factors, protein C and S as well as factor IX. In TG all factors play their role and when thrombomodulin (TM) is added the function of proteins C and S is stressed. The thrombotic tendency in congenital protein C resistance proves the importance of this protein C pathway. Aim To compare the INR in samples from patients under VKA prophylaxis to TG in whole blood and in platelet rich and platelet poor plasma (PRP, PPP) both in the presence and in the absence of TM. Materials & Methods Blood samples were collected from 123 consenting patients on VKA. In two thirds (67%) the indication for prophylaxis was atrial fibrillation. Other indications included prosthetic valves, lung embolisms or thrombosis. The INR was determined in the PPP of the samples and the patients were stratified into 5 groups: INR of 1.0 to 1.5, 1.5 to 2.5, 2.5 to 3.5, 3.5 to 4.5 and higher than 4.5. Thrombin generation (TG) was measured via Calibrated Automated Thrombinography (CAT) in whole blood and in PRP and PPP, with and without 20 nM added TM. From the TG curve lag time and time to peak were obtained as well as the maximal thrombin concentration (peak) and the area under the curve (endogenous thrombin potential: ETP). Also red and white blood cells and platelets were counted. Results With increasing INR values, the ETP and peak height decrease and lag time and time to peak prolong. All TG parameters measured in whole blood were significantly correlated (p-values< 0.01) with the values determined in both PRP and PPP. INR was linearly correlated with lag time and time to peak (p-value< 0.01), whereas for the concentration dependent parameters (ETP and peak height) the correlation with the INR was hyperbolical (p-value< 0.01). In plasma, 20 nM TM causes a diminution of ETP and peak of 50-60 % in normals and in patients in the INR 1 – 1.5 group. At higher INR values inhibition is between 25 and 40%, independent of the INR value. In whole blood, on the contrary, the same concentration of TM causes around 30 % of inhibition in normals and in all patients alike. Conclusions Whole blood TG data correlate well with INR and reflect more of the coagulation mechanism than the INR does. Like the INR it does not reflect the function of the VKAs on the natural anticoagulant factors, however. In PPP and PRP addition of TM shows that VKA treatment induces TM resistance in patients with an INR value higher than 1.5. Disclosures: No relevant conflicts of interest to declare.

Prior studies have evaluated the safety and effectiveness of a diagnostic work up based on combined use of a clinical decision rule and a D-dimer laboratory test in patients with suspected deep vein thrombosis in secondary care facilities. The overall outcome of these studies show that approximately 30% of additional objective evaluation can be avoided without compromising the safety, resulting in failure rates of less than 1%. The objective of our study was to assess the effectiveness of a modified algorithm using a dichotomized clinical decision rule including a point of care D-dimer test in patients suspected of deep vein thrombosis in primary care. The AMUSE study is a prospective cohort study of consecutive patients with clinically suspected acute deep vein thrombosis conducted in 400 general practices coordinated by 3 university affiliated centers in the Netherlands from March 2004 through January 2007. The study population of 1029 patients included 63% women. Patients were categorized as “deep vein thrombosis unlikely” or “deep vein thrombosis likely” using a dichotomized version of a for general practice adapted clinical decision rule based on the Wells rule. Patients classified as unlikely were excluded from further testing. All other patients underwent ultrasonography. Anticoagulants were withheld from patients classified as unlikely. All patients were followed up for 3 months. The main outcome measure was symptomatic or fatal venous thromboembolism (VTE) during 3 months follow-up. Deep vein thrombosis was classified as unlikely in 500 patients (48.6%), all of whom were not treated with anticoagulants; subsequent non fatal VTE occurred in 8 patients (1.6% [95 CI 0.7–3.1%]). In 500 patients categorized as likely, deep vein thrombosis was confirmed in 124 patients (24.8%), 3 patients did not receive objective testing (0.6%) and 373 patients were tested negative (74.6%). Two out of 4 patients not receiving objective testing had confirmed deep vein thrombosis in the 3 month follow-up period. In patients that tested negative 4 non fatal events occurred during the 3 month follow-up (1.1%). The algorithm was completed and allowed a management decision in 97.2% of patients. We conclude that this management strategy is both safe and highly effective in patients with suspected deep vein thrombosis in general practice. Its use is associated with low risk for subsequent fatal and nonfatal VTE and a further increase in the yield of objective diagnostic tests.