HIT antibodies activate PF4-treated cryopreserved platelets, including 1-year-old platelets, and TSP1 release correlates with serotonin release. (A) TRAs were performed with 3 platelet-activating HIT (closed circles) and 3 NC (open circles) samples using 20 lots of cryopreserved platelets. Each data point represents the mean of technical triplicates. (B) TSP1 release from 7 of the 20 cryopreserved platelet lots stored for >4 weeks at −80°C (4W) were compared with activation of platelets stored for ≤1 week at −80°C (baseline [B]). Four-week-old platelets ranged in storage age from 29 to 36 days, whereas baseline platelets ranged in storage age from 1 to 7 days. Each data point represents the mean of technical triplicates. (C) PF4-dependent platelet activation induced by a HIT (closed circle) and NC sample (open circle) with 3 cryopreserved platelet lots stored at −80°C for 12 months. Each data point represents the mean of technical triplicates. (D) Platelet activation induced by TRAP. Data are from technical triplicates with mean ± SD displayed. Groups are compared using two-tailed, unpaired t test with Welch’s correction. ∗∗P < .005. Studies were performed at Retham Technologies (Wauwatosa, WI) on cryopreserved platelets that were not subject to shipping. NC, normal control.