The qLabs® FIB system, a novel point-of-care technology for a rapid and accurate quantification of functional fibrinogen concentration from a single drop of citrated whole blood.

Hypofibrinogenemia is often associated with excessive bleeding and requires immediate treatment. The qLabs FIB® is a handheld and easy-to-use point-of-care (POC) device designed for the rapid measurement of functional fibrinogen concentration from a single drop of citrated whole blood. The aim of this study was to evaluate the analytical performances of the qLabs FIB system. Fibrinogen concentrations from 110 citrated whole blood specimen were measured by both the qLabs FIB and the Clauss laboratory reference methods (STA®-Liquid Fib assay on STA-R® Max from Stago). A three-laboratories comparison study was conducted to assess reproducibility and repeatability of the qLabs FIB using plasma quality control material. In addition, single-site assays were conducted to assess the repeatability from citrated whole blood specimen covering the qLabs FIB reportable range. A very strong correlation between the qLabs FIB and the Clauss laboratory reference method was observed (r = 0.95). Using a clinical cut-off value of 2.0 g/L, the area under the receiver operating characteristic curve (ROC) of citrated whole blood was 0.99 and sensibility and specificity were 100 % and 93.5 %, respectively. Percent CVs for reproducibility and repeatability assessed from quality control material, were both <5 %. Repeatability assessed from citrated whole blood specimen showed a CV of 2.6 to 6.5 %. In conclusion, the qLabs FIB system enables a rapid and reliable measurement of functional fibrinogen levels from citrated whole blood and exhibits a strong prediction power at the 2 g/L clinical cut-off when compared to the Clauss laboratory reference. Further clinical studies should demonstrate its ability to quickly confirm the diagnosis of acquired hypofibrinogenemia and help identify patients who may benefit from targeted hemostatic treatment.

[A case of bone marrow necrosis and fat embolism in a sickle-cell disease patient]. / Un cas de nécrose médullaire compliquée d'embolie graisseuse chez un patient drépanocytaire.

We report here a case of bone marrow necrosis and fat embolism syndrome in a 23-year-old sickle-cell disease (HbSS) patient. A brutal and severe bicytopenia conducted to suspect bone marrow necrosis, confirmed by bone marrow aspiration and analysis. This was the first life-threatening medical event for this patient. In the present case, a complex alloimmunization against blood group antigens complicated the treatment because of the risks associated with the transfusion strategy. These rare complications of sickle-cell disease may be fatal, but an efficient symptomatic treatment generally allows for recovery. Medical biologists should be aware of the danger of bone marrow necrosis in sickle-cell disease, so that they can help clinicians and accurately diagnose this serious complication.

Platelet glycoprotein VI binds to polymerized fibrin and promotes thrombin generation.

Fibrin, the coagulation end product, consolidates the platelet plug at sites of vascular injury and supports the recruitment of circulating platelets. In addition to integrin αIIbß3, another as-yet-unidentified receptor is thought to mediate platelet interaction with fibrin. Platelet glycoprotein VI (GPVI) interacts with collagen and several other adhesive macromolecules. We evaluated the hypothesis that GPVI could be a functional platelet receptor for fibrin. Calibrated thrombin assays using platelet-rich plasma (PRP) showed that tissue factor-triggered thrombin generation was impaired in GPVI-deficient patients and reduced by the anti-GPVI Fab 9O12. Assays on reconstituted PRP and PRP from fibrinogen-deficient patients revealed a fibrinogen-dependent enhancement of thrombin generation, which relied on functional GPVI. The effect of GPVI was found to depend on fibrin polymerization. A binding assay showed a specific interaction between GPVI-Fc and fibrin, inhibited by the Fab 9O12. This Fab also reduced platelet adhesion to fibrin at low (300 s(-1)) and high (1500 s(-1)) wall shear rates. Platelets adherent to fibrin displayed shape change, exposure of procoagulant phospholipids, and the formation of small clots. When hirudinated blood was perfused at 1500 s(-1) over preformed fibrin-rich clots, the Fab 9O12 decreased the recruitment of platelets by up to 85%. This study identifies GPVI as a platelet receptor for polymerized fibrin with 2 major functions: (1) amplification of thrombin generation and (2) recruitment of circulating platelets to clots. These so-far-unrecognized properties of GPVI confer on it a key role in thrombus growth and stabilization.

Platelet glycoprotein VI dimerization, an active process inducing receptor competence, is an indicator of platelet reactivity.

OBJECTIVE: The immune receptor homologue glycoprotein VI (GPVI)/FcR receptor γ chain complex is primarily responsible for platelet activation by collagen. There is growing evidence that optimal binding of GPVI to collagen depends on the assembly of GPVI dimers. The valence of GPVI on resting platelets needs to be clearly established because platelet avidity for collagen would be greater if GPVI is constitutively expressed as a dimer than as a monomer. METHODS AND RESULTS: Using a monoclonal antibody (9E18) that preferentially binds to GPVI dimers, we found that GPVI was maintained in a monomeric form on human resting platelets under the control of intraplatelet cAMP concentration. Activation by soluble agonists or von Willebrand factor induced a shift toward GPVI dimerization related to increased platelet adhesion to collagen. A correlation between platelet binding of 9E18 and P-selectin exposure was observed in patients experiencing coronary artery disease, and antagonists of the ADP receptor P2Y12 limited ADP-induced GPVI dimerization. CONCLUSION: The rapid assembly of highly competent dimers of GPVI at sites of vascular lesion represents an important step in the sequence of events leading to platelet activation by collagen. GPVI dimers could represent a new marker to analyze platelet reactivity.

A severe neonatal presentation of factor II deficiency.

Prothrombin deficiency is an autosomal recessive disorder associated with moderate or severe bleeding tendency. In this study, a three-month-old boy with non-consanguineous parents was referred for convulsions because of intracerebral hemorrhage. Standard coagulation tests revealed that the patient's plasma prothrombin activity was 12%, while his father's and mother's levels were 55% and 70%, respectively. Analysis of the prothrombin gene revealed that this patient is a compound heterozygote for two missense mutations: one maternally inherited point mutation in the propeptide (p.Arg4Gln) and one paternally inherited mutation in the kringle-2 (p.Arg220Pro) domain. Structural analysis was performed and confirmed that the resulting mutations were inferred to respectively affect the cleavage of the propeptide from the Gla domain, and the stability of the kringle-2 domain, both resulting in a severe hypoprothrombinemia. In unusually bleeding newborn of non-consanguineous parents, rare severe homozygous bleeding disorders need to be considered to facilitate early diagnosis and treatment.

[New oral anticoagulant drugs: dabigatran, rivaroxaban and apixaban. Present and future]. / Les nouveaux anticoagulants oraux: utilisation actuelle et avenir.

For years, prevention and treatment of thromboembolic events have been restricted to the use of heparins and vitamin K antagonists. These treatments, in spite of their unquestioned efficacy, present numerous limits (hemorrhagic risk, need for regular laboratory controls). These limits call for the development of new antithrombotic drugs. This review briefly reports on three new molecules, in very advanced phases of clinical research: dabigatran (Pradaxa®), rivaroxaban (Xarelto®) and apixaban. These molecules represent new oral anticoagulants, which directly inhibit a coagulation factor (thrombin for dabigatran, factor Xa for rivaroxaban and apixaban) and do not need regular anticoagulant monitoring or dose adjustment. The approval is still restricted in France to the prophylaxis of venous thromboembolism in orthopaedics. Dabigratran will be soon available in the prevention of stroke in atrial fibrillation. With the forthcoming phase III studies to prevent and treat venous thromboembolism, anticoagulant therapy management will be most probably improved in the coming years.

Absence of collagen-induced platelet activation caused by compound heterozygous GPVI mutations.

The glycoprotein VI (GPVI)/FcRgamma complex is a key receptor for platelet activation by collagen. We describe, for the first time, 2 genetic abnormalities in one patient. This 10-year-old girl presented ecchymoses since infancy, a prolonged bleeding time despite a normal platelet count and no antiplatelet antibodies. Collagen-induced platelet activation was null, whereas GPVI quantification by flow cytometry evidenced an incomplete deficiency. Immunoblotting showed an abnormal migration of residual GPVI, and no FcRgamma defect. GPVI DNA sequencing revealed (1) an R38C mutation in exon 3 of one allele and (2) an insertion of 5 nucleotides in exon 4 of the other allele, leading to a premature nonsense codon and absence of the corresponding mRNA. Introduction of the R38C mutation into recombinant GPVI-Fc resulted in abnormal protein migration and a loss of collagen binding. Thus, this composite genetic GPVI deficiency and dysfunction cause absence of platelet responses to collagen and a mild bleeding phenotype.

Chimeric Fc receptors identify ligand binding regions in human glycoprotein VI.

Glycoprotein (GP) VI, a key receptor for collagen-induced platelet activation, recently emerged as a major target for developing new antithrombotics. However, little is known about its functional domains, which is a disadvantage for the rational development of antagonists. Our aim was to identify the structures determining GPVI specificity. GPVI presents homologies with members of the Ig superfamily (in particular with FcalphaRI) whose extracellular parts present two domains, D1 and D2 linked by a hinge interdomain. To identify the respective role of these domains in GPVI, we have substituted D1 and D2 by their FcalphaRI homologue in a soluble GPVI fusion protein (GPVI-Fc) and have modified the linker motif by mutagenesis. Proteins were tested for their binding to ligands and antibodies specific for GPVI and FcalphaRI. We demonstrate for the first time that D2 plays a specific and significant role in GPVI binding to collagen and that the hinge interdomain is critical for the binding to convulxin. Furthermore, binding to CRP requires elements of D1 and of the linker motif. Our results indicate that GPVI is unique amongst the receptors of its family as it uses different structural domains to interact with several agonists and provide evidence that different sites on GPVI constitute targets to develop antagonists of GPVI.