Donor and Storage Duration Impact Cold-Stored Platelet Function More Than Handling Protocols

Cold storage of platelets has the potential to mitigate the logistical and financial burdens associated with platelet inventory management. In addition, cold-stored platelets (CS-PLT, stored at 2-6°C) have better preserved hemostatic function than room temperature (RT)-stored platelets (RT-PLT), suggesting that CS-PLT may provide improved hemostatic resuscitation in actively bleeding patients as compared to RT-PLT when transfused. With CS-PLT clinical use on the rise, questions regarding CS-PLT inventory management have been raised - specifically how should CS-PLT be stored and handled over the course of storage (after collection and prior to transfusion). RT-PLT are currently stored flat on specialized porous racks and agitated at 18-22°C, which allows for oxygenation. These products are stored out to 5 days, with 7 days allowed for large volume delayed sampling collections. In contrast, refrigerated blood products (i.e., packed red blood cells and whole blood) are stored vertically in pull out drawers, with the goal of reducing harmful oxidizing damage to red blood cells. Notably, refrigeration is known to induce aggregation of platelets, a feature historically associated with bacterial contamination in RT-PLT. In order to diminish concerns regarding aggregate formation, it may be beneficial to massage CS-PLT briefly each day to reduce aggregate formation, yet how this may impact hemostatic function is unknown. To this end, optimal storage and handling conditions for CS-PLT remain to be determined.

The objective of our study was to measure CS-PLT hemostatic function in response to two major storage and handling variables: agitation and massage to reduce aggregation in the bag. Single donor apheresis platelet units (single Trima collection in plasma, split equally into two bags to control for donor variability; n=20 donors in 40 bags) were purchased from our regional blood center and delivered to our lab on day of collection. Upon arrival, units were spiked and sampled under sterile conditions for baseline (day 0) profiling, assigned to one of the study groups, and stored accordingly. Our study groups were as follows: "Agitated" (n=10 units), "Not Agitated" (n=10 units, paired with Agitated donors), "Massaged Daily", (massaged 60s daily, n=10 units), and "Massaged at Sampling" (massaged only at sampling, n=10 units, paired with Massaged Daily donors). "Agitated" platelet units were stored in a custom refrigerated shaker (courtesy Helmer Scientific). For the other groups, platelet units were stored in a walk-in cold room. All units were stored on the same style perforated agitator rack, with agitation either powered on or off as assigned. Units from all groups were sampled under sterile conditions at days 2, 5, 7, 14, and 21 of storage, using 8 mL draws at each time point to ensure equal volume removal over the course of storage. Hemostatic function was assayed using light transmission aggregometry (LTA; ADP, collagen, epinephrine agonists), rotational thromboelastometry (ROTEM; ExTEM, InTEM agonists), and thrombin generation in response to 5 pM tissue factor. Platelet counts (x10 ³/µL) were obtained using a hematology analyzer.

During the first week of storage, there were no significant differences in the hemostatic profiles between study groups. While platelet counts, endogenous thrombin potential, and ROTEM clot formation time and maximum clot firmness were fairly stable over the first 7 days of storage across all groups, there was a > 50% reduction from baseline in aggregation responses to both ADP and collagen across all 4 groups, suggesting that platelet aggregation, as detected by LTA, may not be the best representative of CS-PLT function. By day 21 of storage, there was a robust increase in endogenous thrombin potential in all study groups when compared to baseline (Agitated, 22%; Not Agitated, 70%; Massaged Daily, 51%; Massaged at Sampling, 47%). Despite this increase in thrombin generation, after extrinsic activation day 21 CS-PLT in all groups had two-fold increased clot formation times compared to baseline (Agitated, 185%; Not Agitated, 223%; Massaged Daily, 236%; Massaged at Sampling, 190%), and reduced maximum clot firmness compared to baseline (Agitated, -44%; Not Agitated, -36%; Massaged Daily, -54%; Massaged at Sampling, -43%). These data suggest that storage duration, and not agitation nor massaging to reduce aggregates, has the most impact on CS-PLT hemostatic function.