A method for the quantitative assessment of platelet-induced clot retraction and clot strength in fresh and stored platelets

Background and Objectives: The changes that occur in platelets as they undergo storage have been documented by aggregometry as well as by flow cytometry. However, one of the most essential platelet functions, the induction of clot retraction, has not been quantitatively assessed in stored platelets. We describe two potentially useful methods, platelet-induced clot retraction and clot strength, to assess effect of storage of platelets in blood banks or of platelet preparations subjected to freezing or freeze-drying. These methods have previously been developed for bedside monitoring of patients receiving c7E3 (Reopro®). Materials and Methods: Platelet-induced clot retraction (PICR) and clot strength were measured with the Hemodyne® and Thromboelastograph®, respectively. Paired Study: Fresh platelet concentrates (n = 3) were obtained from leukapheresis donors and divided into two equal units; one unit was tested within 4 h of collection and the other stored for 5 days at 22°C in a platelet incubator and tested. Unpaired Study: Fresh platelet concentrates (n = 15) were obtained from leukapheresis donors and tested within 4 h of collection and compared to outdated platelets (n = 30; random or single donor) that had been stored for 5 days at 22°C in a platelet incubator. Alternative Presevation Methods: Lyophilized platelets, platelets chilled to 4°C, platelets frozen at -70°C in 5% dimethyl sulfoxide (DMSO) or in the absence of a cryoprotectant. Results: Paired Study: Stored platelets demonstrated an increase in PICR; the difference was not significant (p = 0.55). There was no difference in clot strength between fresh and outdated platelets (p = 0.90). Unpaired Study: When compared to fresh platelets, stored platelets demonstrated a 2-fold higher PICR (p = 0.0011). On the other hand, there was no difference in the time to onset of PICR (p = 0.08) and there was no difference in clot strength between fresh and outdated platelets (p = 0.14). Alternate Preservation Methods: In contrast, PICR and clot strength were reduced in platelets frozen at -70°C in 5% DMSO and absent in lyophilized platelets, in platelets frozen at -70°C in the absence of cryoprotectants or stored at 4°C. Conclusion: The data indicate that the ability of platelets to indcue clot retraction and to enhance clot strength is not altered by storage, despite functional abnormalities in aggregation and agglutination. These data suggest that quantitative measurements of PICR and clot strength may be simple, useful tools for assessing the function of stored platelet concentrates, platelets that have undergone freezing or exposure to alternative buffers and for evaluating platelet functions relevant to PICR.