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Platelets coordinate blood coagulation at sites of vascular injury and play fundamental roles in a wide variety of (patho)physiological processes. Key to many platelet functions is the transport and secretion of proteins packaged within α-granules, organelles produced by platelet precursor megakaryocytes. Prominent among α-granule cargo are fibrinogen endocytosed from plasma and endogenously synthesized von Willebrand factor. These and other proteins are known to require acidic pH for stable packaging. Luminal acidity has been confirmed for mature α-granules isolated from platelets, but direct measurement of megakaryocyte granule acidity has not been reported.

Platelet α‐granule biogenesis in precursor megakaryocytes is critically dependent on VPS33B and VPS16B, as demonstrated by the platelet α‐granule deficiency seen in the rare multisystem disorder arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome associated with biallelic pathogenic variants in VPS33Band VIPAS39(encoding VPS16B). VPS33B and VPS16B are ubiquitously expressed proteins that are known to interact and play key roles in protein sorting and trafficking between subcellular locations. However, there remain significant gaps in our knowledge of the nature of these interactions in primary cells from patients with ARC syndrome.

Platelet α‐granule biogenesis in precursor megakaryocytes is critically dependent on VPS33B and VPS16B, as demonstrated by the platelet α‐granule deficiency seen in the rare multisystem disorder arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome associated with biallelic pathogenic variants in VPS33Band VIPAS39(encoding VPS16B). VPS33B and VPS16B are ubiquitously expressed proteins that are known to interact and play key roles in protein sorting and trafficking between subcellular locations. However, there remain significant gaps in our knowledge of the nature of these interactions in primary cells from patients with ARC syndrome. To use primary cells from patients with ARC syndrome to better understand the interactions and roles of VPS33B and VPS16B in platelets and precursor megakaryocytes. The proband and his male sibling were clinically suspected to have ARC syndrome. Confirmatory genetic testing and platelet phenotyping, including electron microscopy and protein expression analysis, was performed with consent in a research setting. We describe the first case of ARC syndrome identified in Costa Rica, associated with a novel homozygous nonsense VPS33Bvariant that is linked with loss of expression of both VPS33B and VPS16B in platelets. These results indicate that stable expression of VPS16B in platelets, their precursor megakaryocytes, and other cells is dependent on VPS33B. We suggest that systematic evaluation of primary cells from patients with a range of VPS33Band VIPAS39variants would help to elucidate the interactions and functions of these proteins.

Venous thromboembolism (VTE) is associated with excess mortality and morbidity in cancer, and is influenced by patient‐, tumor‐, and treatment‐related factors. We aimed to investigate the impact of such factors in a national cohort of patients with epithelial ovarian cancer (EOC).

Venous thromboembolism (VTE) is associated with excess mortality and morbidity in cancer, and is influenced by patient‐, tumor‐, and treatment‐related factors. We aimed to investigate the impact of such factors in a national cohort of patients with epithelial ovarian cancer (EOC). Patients in the Danish Gynecologic Cancer Database (DGCD) with EOC from 2005 to 2014 were followed from time of diagnosis to VTE, or censoring. Surgery, chemotherapy, and vascular epithelial growth factor (VEGF)‐inhibitors were included as time‐varying exposures in Cox proportional hazard regression models. A total of 551 VTE events were registered in 4991 EOC patients. Median follow‐up time was 2.9 years. The 2‐year cumulative incidence of VTE was 7.2%. Patients were at highest risk during the first year after EOC diagnosis. Previous VTE was associated with a hazard ratio (HR) of 3.26 (95% confidence interval [CI] 2.42–4.39). Exposure to major pelvic surgery was associated with a HR of 3.21 (95% CI 2.29–4.50). Exposure to chemotherapy or (VEGF)‐inhibitors were associated with HRs of 1.91 (95% CI 1.56–2.33) and 1.05 (95% CI 0.57–1.93), respectively. Hazard ratios for patients with clear cell histopathology was 1.46 (95% CI 0.97–2.20) and 2.42 for International Federation of Gynaecology and Obstetrics stage III‐‐IV (95% CI 1.93–3.03). EOC is associated with a high risk of VTE, particularly within the first year after diagnosis. Major pelvic surgery and chemotherapy were strongly associated with VTE. Person‐related risk factors were increasing age and previous VTE. Advanced stage was an independent tumor‐related risk factor. These findings support the indication for thrombosis prophylaxis during chemotherapy.

Defective complement inhibition can lead to the formation of membrane attack complexes (MAC; C5b‐9) on the plasma membranes of vascular endothelial cells, resulting in injury that drives the progression of thrombotic microangiopathy (TMA), a key pathology in kidney disease.

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Upon vessel injury, platelets adhere to exposed matrix constituents via specific membrane receptors, including the von Willebrand factor receptor GPIb-IX-V complex and integrins β1 and β3. In platelets, the F-BAR protein PACSIN2 associates with the cytoskeletal and scaffolding protein filamin A (FlnA), linking GPIbα and integrins to the cytoskeleton.

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Shielding of procoagulant phosphatidylserine (PS) with annexin A5 attenuates thrombosis, but annexin A5 (35.7 kDa) is rapidly cleared from the circulation. In contrast, Diannexin, a 73.1 kDa homodimer of annexin A5, has an extended half‐life.

Background: Unfractionated heparin (UFH) is a widely used anticoagulant. Current American College of Chest Physicians guidelines for infants extrapolated from adults recommend 28 U kg−1 h−1of UFH to achieve an anti‐factor Xa level of 0.35–0.7 IU mL−1. Objective: To assess the profile of anti‐FXa‐based UFH dosing guidelines in infants. Patients/Methods: We included all infants aged < 6 months treated with per‐protocol intravenous UFH at the Hospital for Sick Children, Toronto, over a 3.5‐year period. Results: Of 100 infants, 11% achieved sustained therapeutic anti‐FXa levels with current dose recommendations. Only 15% achieved target anti‐FXa levels within 24 h with per‐protocol dose escalations. Seventeen per cent of patients never achieved therapeutic anti‐FXa levels, despite up to 60 days of therapy and triple the recommended dose. The median dose needed to achieve therapeutic anti‐FXa levels in the remaining 83 infants was 33 U kg−1 h−1(interquartile range, 30–36). Two in three infants had decreased thrombus size at completion of therapy and no thrombus progression/recurrence, and 11/100 infants suffered major bleeding. Without exclusion of extracorporeal membrane oxygenation patients, an activated partial thromboplastin time (APTT) of > 180 s was detected as a risk factor for major bleeding. Conclusions: UFH monitoring is challenging in infants. Despite their delay in reaching therapeutic anti‐FXa levels, infants monitored with the adult‐based anti‐FXa range have a high thrombus resolution rate, no thrombus progression, but a relatively high bleeding rate. Extreme APTT elevation may contribute to this bleeding risk, particularly in critically ill patients. Current UFH guidelines for young infants may still be inadequate, and laboratory methods with age‐appropriate ranges may be required to further improve clinical outcomes within this population.

Background: Childhood pulmonary embolism (PE) causes significant mortality and evidence suggests that it is under‐diagnosed. Clinical probability scores and D‐dimer estimation to assess pre‐test probability have not been studied in children with suspected PE. Patients/Methods: This retrospective cohort study evaluated Wells simplified probability score for PE in 50 children with PE and 25 PE negative control patients, and D‐dimer values in 27 PE positive and 12 PE negative children. Results: PE positive and PE negative groups had similar rates of risk factors for venous thromboembolism (VTE). Wells simplified probability score showed a small difference between PE positive and PE negative children (median score: PE positive, 4.5; PE negative, 4; P=0.009), children with PE are more likely to obtain a ‘PE likely’ score (score > 4), P=0.012. The difference was of slightly greater significance when the Wells score was adjusted to account for pediatric normal ranges for heart rate, P=0.007, and signs/symptoms of upper limb DVT, P=0.006. Children with PE were as likely as PE negative patients to have a D‐dimer value within the normal range (PE positive, 15%; PE negative, 25%; P=0.654). A combination of a ‘PE unlikely’ score and normal D‐dimer value occurred in 1/12 (8%) of PE negative children. Conclusions: The Wells clinical probability score and D‐dimer estimation may lack utility in the determination of pre‐test probability of PE in children. Validation of a pediatric clinical probability score, incorporating D‐dimer estimation, by prospective study, would be difficult as a result of the rarity of childhood PE.

Background: The Quebec platelet disorder (QPD) is inherited and characterized by delayed‐onset bleeding following hemostatic challenge. Other characteristics include increased expression and storage of active urokinase‐type plasminogen activator (u‐PA) in platelets in the setting of normal to increased u‐PA in plasma. There is also consumption of platelet plasminogen activator inhibitor‐1 and increased generation of plasmin in platelets accompanied by proteolysis of stored α‐granule proteins, including Factor V. Aims and Methods: Although fibrinolysis has been proposed to contribute to QPD bleeding, the effects of QPD blood and platelets on clot lysis have not been evaluated. We used thromboelastography® (TEG®), biochemical evaluations of whole blood clot lysis, assessments of clot ultrastructure, and perfusion of blood over preformed fibrin to gain insights into the disturbed hemostasis in the QPD. Results: Thromboelastography was not sensitive to the increased u‐PA in QPD blood. However, there was abnormal plasmin generation in QPD whole blood clots, generated at low shear, with biochemical evidence of increased fibrinolysis. The incorporation of QPD platelets into a forming clot led to progressive disruption of fibrin and platelet aggregates unless drugs were added to inhibit plasmin. In whole blood perfusion studies, QPD platelets showed normal adherence to fibrin, but their adhesion was followed by accelerated fibrinolysis. Conclusions: The QPD is associated with ‘gain‐of‐function’ abnormalities that increase the lysis of forming or preformed clots. These findings suggest accelerated fibrinolysis is an important contributor to QPD bleeding.