Further evaluation of a new standard of efficacy for stored platelets

BACKGROUND: The proposal to assess the viability capabilities of platelets (PLTs) collected, treated, or stored in a developmental system against "fresh" PI-Ts from the same subject poses several important methodologic issues pertaining to the timing and manner of the collecting and separating the fresh PLTs. This study extended the previous validation of this method of comparing fresh and stored PLTs, applying it to an assessment of apheresis PLTs stored for 7 days with a newly standardized radiolabeling protocol. STUDY DESIGN AND METHODS: Eighteen normal subjects donated 1 unit of leukoreduced PLTs, pheresed with a standard, approved system. They received an aliquot radiolabeled with Cr-51 on Day 7 simultaneously with In-111-labeled fresh PLTs that had been separated by a manual method. Recovery and survival were compared to determine whether the stored PLTs were not inferior to the criterion of 67 percent of recovery and 50 percent of survival of fresh PLTs. Separate studies were undertaken to document the similarity of recovery and survival with Cr-51 and In-111 radiolabeling in PLTs stored to 8 days and to determine the importance of correcting the radioactivity in timed samples for the activity remaining in blood beyond the life span of the retransfused PLTs. RESULTS: PI-Ts stored for 7 days demonstrated 88.7 +/- 35.2 percent of the recovery and 89.9 +/- 21.2 percent of the survival of PLTs collected via a nonproprietary, manual system and thus met the comparative criterion. In a separate study (n = 12), labeling Day 8 PLTs with Cr-51 or In-111 resulted in recoveries and survivals that were not different. Radiolabel eluted from labeled PLTs in vitro was taken up by cellular blood elements in a reuptake incubation. CONCLUSION: Apheresis PLTs stored for 7 days met the criterion proposed for comparison with fresh PLTs. This analytic approach is feasible with PLTs collected and prepared via a manual method. A standardized protocol for radiolabeling PLTs with Cr-51 and In-111 and analyzing the results in a standardized fashion was employed successfully, with the two radioisotopes yielding similar results. The importance of correcting for residual activity after disappearance of injected cells was noted.