Aggregates in cold-stored platelet units have diverse phenotypes and are removed by pre-transfusion filters.

Background and objectives: The US Food and Drug Administration guidance for cold-stored platelets (CSPs) permits storage of apheresis platelets at 1-6°C for ≤14 days. During a pilot programme, CSPs were evaluated in a large US blood centre over a 10-month period (September 2023 to July 2024) to better understand the formation of aggregates under routine use.

Materials and methods: Platelets collected in 100% plasma were moved into cold storage within 4 h of collection and shipped to local hospitals. Hospitals returned any units with aggregates to the blood centre. Units were visually inspected and tested for platelet concentration, pH, metabolism and activation parameters. At expiration (Day 14), units were passed through a transfusion filter (170-260 μm) and assessed post-filtration and after 24 h.

Results: Aggregation rates were high initially but decreased to 9% of released CSPs by study conclusion. Fifty-five aggregated units were returned. The majority of units (42%) had a small, flaky phenotype, which did not change over storage. Platelet counts significantly decreased with a 50% drop by expiry. Lactadherin and P-selectin increased, CD63 and CD41/61 complex decreased and there was slower metabolism. Filtration removed all aggregates, except for one unit with small aggregates being observed 24 h post-filtration. Platelet recovery after filtration was 94% ± 26%. Seven repeat donors donated 6 units with no aggregates and 17 units with aggregates, with variable aggregate phenotypes between donations.

Conclusion: This study demonstrates that aggregate phenotypes in CSPs are highly variable and removed effectively by filtration; they appear to arise more from external handling conditions than from any intrinsic product property.