SummarySickle cell disease (SCD) is linked to hypercoagulability and is characterised by high concentrations of erythrocyte-derived microparticles (Ed-MPs). However, the impact of procoagulant cell-derived microparticles on the thrombin generation process remains unclear. We analysed the alterations of each phase of thrombin generation (TG) in relation to the concentration of erythrocyte- or platelet-derived microparticles (Ed-MPs and Pd-MPs) in a cohort of patients with steady-state SCD. We studied 92 steady-state SCD patients, 19 of which were under treatment with hydroxyurea, and 30 healthy age- and sex-matched individuals. TG was assessed by calibrated automated thrombogram. Ed-MP and Pd-MP expressing or not phosphatidylserine (PS) were determined by means of flow cytometry. Procoagulant phospholipid-dependent activity in the plasma was evaluated by the Procoag-PPL assay. Levels of thrombomodulin and haemoglobin in the plasma as well as red blood cell and reticulocyte counts were measured. SCD patients, independently of the administration of hydroxyurea, were marked by a significant acceleration in the propagation phase of TG which correlated with the Ed-MP/PS+ concentration. TG was significantly attenuated in hydroxyurea-treated patients. In conclusion, the acceleration of the propagation phase of TG, driven by Ed-MP/PS+, is a major functional alteration in blood coagulation in patients with steady-state SCD. Treatment with hydroxyurea, in addition to the regulation of haemolysis, lowers Ed-MPs and attenuates thrombin generation. The thrombogram could be a useful tool for the diagnosis of hypercoagulability and optimisation of the treatment in patients with SCD.

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Identification of outpatients with high platelet reactivity (HPR) on antiplatelet treatment is an unmet need. The present study was conducted in healthy individuals (n = 50) and in outpatients with coronary artery disease (CAD) at a distance from the acute ischemic episode (aspirin group, n = 71; aspirin/clopidogrel group, n = 106). We studied the feasibility and the precision of whole blood multiple electrode aggregometry (MEA) after triggering platelet aggregation by arachidonic acid or adenosine diphospate (ADP). The MEA can be performed on whole blood within 2 hours after sample venipuncture. The threshold for the diagnosis of HPR is situated at 55 and 50 U for the arachidonic acid and ADP test, respectively. Frequency of HPR was 7% and 20% in aspirin and aspirin/clopidogrel groups, respectively. In 3.8% of patients in aspirin/clopidogrel group, combined HPR on aspirin and clopidogrel was found. In outpatients with CAD, use of MEA is feasible for the diagnosis of HPR.

4001 Poster Board III-937 Introduction Patients with Sickle Cell Anemia (SCA) are at risk of thrombosis, but this clinical manifestation is variable and it is probably not associated with the frequency of vaso-occlusive crisis. Increased plasma levels of platelet- and erythrocyte-derived microparticles and hypercoagulability markers have been reported in steady state SCA patients. However, the link between hypercoagulability and SCA is not completelty understood. Aim of the study We determined erythrocyte-derived (Ed-MP) and platelet-derived microparticles (Pd-MP) levels in patients with steady state SCA. We studied their relationship with hemolysis markers and their impact on thrombin generation process. Materials and methods Consecutive out-patients with steady state SCA (n=78) and 20 healthy age and sex-matched controls were included. They were free of any acute episode of SCA for at least one month prior inclusion. Microparticles were assessed with standardized whole blood flow cytometry assay. Ed-MP and Pd-MP were identified using respectively anti-CD235a and anti-CD41 monoclonal antibody and annexine V. Thrombin generation (TG) in citrated platelet poor plasma was assessed with Calibrated Automated Thrombogram® (Stago, France) using PPP-reagent 5pM® (Thrombinoscope BV, Nederlands). The following TG parameters were analyzed: lag-time (LT), time to peak of thrombin (ttpeak), peak of thrombin (peak), mean velocity rate index of the TG (MRI) and endogenous thrombin potential (ETP). Results Mean patient age was 25±8 (range 17-58 ys). In the patients group, Ed-MP and Pd-MP, expressing or not phosphatidyl-serine (PS), were significantly increased compared to the control group. Thrombogram parameters were not significantly different in both groups (Table 1). There was a slight though significant inverse correlation between Ed-MP and both LT and ttpeak (r=-0.235, r=-0.315 respectively; p<0.05). Ed-MP levels were correlated with MRI increase (r=0.241; p<0.05). Ed-MP values were inversely correlated with Hb levels and well correlated with reticulocytes count (r=-0.427, r=0.520 respectively; p<0.05). No relationship was found between Ed-MP and ETP values. The sub-population of Ed-MP/PS+ (expressing PS) showed also an inverse correlation with both ttPeak (r=-0.315, p<0.05) and ETP (r=-0.236; p<0.05), and a positive correlation with MRI (r=0.306 ; p<0.05). Pd-MP concentration was inversely correlated with Hb levels (r=-0.273 ; p<0.05). Only Pd-MP/PS+ plasma concentration was slightly by significantly correlated with ETP (r=0.225; p=0,049). Conclusion Patients with steady state SCA presented a significant increase of Ed-MP and Pd-MP plasma levels which seems to be linked to haemolysis degree. Each type of microparticles had different impact on TG process. Ed-MP induced acceleration of TG kinetics without increase of ETP whereas Pd-MP/PS+ affected mainly ETP. However the increase of Ed-MP and Pd-MP plasma concentration does not appear to be by itself a sufficient condition to induce a significant increase of TG. Disclosures: No relevant conflicts of interest to declare.