Elevated levels of thrombin-generating microparticles in stored red blood cells

Background During storage, red blood cells (RBCs) lose their membrane stability, leading to haemolysis and microparticle (MP) formation. The use of RBCs stored for more than 28days has been associated with an increased incidence of deep vein thrombosis. However, the exact mechanism by which coagulation activation is enhanced in stored RBCs is still unknown. Objectives To investigate the relevant potential procoagulant activities of MPs and study the relative procoagulant factors for initiating the coagulation on MPs in stored RBCs. Study Design and Methods MPs were isolated from the plasma of RBC units stored in citrate-phosphate-dextrose-adenine. At seven storage time-points (d0, d7, d14, d21, d28, d35 and d42), MPs were morphologically observed, quantified and analysed for tissue factor, factor XI (FXI) and their thrombin-generating potential. Results MPs were observed using electron microscopy. The size of the MPs ranged from 0 center dot 272m to 0 center dot 973m in diameter. During the storage of RBCs in plastic bags, the MP concentration increased from 3389 +/- 218/l at day 0 to 61586 +/- 2237/l at d42. Thrombin generation was dependent on the total number of MPs (r=0 center dot 987). Anti-human FXI antibody inhibited thrombin concentrations by 50 center dot 3% compared with control plasma, whereas antitissue factor and antitissue factor pathway inhibitor failed to reduce thrombin concentrations. Conclusions Our study provides evidence that MP formation due to RBC storage might propagate coagulation not only by exposing phosphatidylserine, but also by initiating thrombin generation independently of tissue factor in a FXI -dependent manner.