Stored platelets contain residual amounts of tissue factor: Evidence from studies on platelet concentrates stored for prolonged periods

BACKGROUND: The advent of new strategies for pathogen reduction has raised the question of whether platelet (PLT) concentrates (PCs) exposed to longer periods of storage retain adequate hemostatic function. STUDY DESIGN AND METHODS: The effects of prolonged storage on adhesive and procoagulant functions of PLTs in PCs have been analyzed. The ability of PLTs to interact with surfaces was assessed by en face electron microscopy. Exposure of anionic phospholipids or tissue factor (TF) antigen on PLTs was assessed by flow cytometry and by immunocytochemical methods at the ultrastructural level. Studies were performed in six different PCs followed 0, 3, 5, 7, and 11 days of storage RESULTS: A progressive impairment of PLT-adhesive functions was observed after 5 days of storage. A progressive increase in expression of anionic phospholipids and development of procoagulant activity (PCA) measured by a modified prothrombin time (mPT) was observed along the storage. Incubation of PLTs with a specific anti-TF resulted in prolongation of the mPT by approximately 10 to 15 percent. Flow cytometry revealed minimal TF expression at later storage times. Immunocytochemical studies showed minimal TF labeling when studies were performed on PLT whole mounts. Labeling was markedly improved when PLTs were previously exposed to sonication. CONCLUSION: Prolonged storage of PCs was associated with decreased PLT-adhesive capacities and enhanced PCA. Current preparation procedures and storage media have important limitations for preserving PCs for longer than 1 week. PLTs in PCs retain residual amounts of TF as assessed by immunocytochemical and functional assays. The origin and hemostatic significance of this TF should be investigated further.