Validation of a mechanical freezer used for the cryopreservation of fresh frozen plasma: Thermodynamic and heat transfer evaluations

Plasma for clinical use should be frozen and stored in this state until use. In this study we revised our standard procedure for the cryopreservation of plasma to verify its compliance to Italian, European, and American norms, which require a specific cooling rate. To this aim, we performed three tests. In test 1 we recorded the temperature of the freezing chamber of our mechanical blast freezer; in tests 2 and 3 we recorded and certified with thermo-couples the freezing curve of plasma units frozen without and with protective packaging (soft polyethylene bag and cardboard box). In test 2 the freezer was loaded in a single operation, whereas in test 3 we simulated the ordinary loading operation that requires the opening of the loading door on two occasions, which decreases thermal efficiency. The validation of the cooling system, which included a thermal analysis, allowed us to characterize the critical steps of the plasma freezing process. The thermal analysis was conducted under ideal conditions in order to evaluate the theoretical cooling output necessary for our freezing process. We found that in spite of high theoretical and actual device power, the efficiency of the heat transfer between the chamber and our samples was not sufficient, particularly for the packaged ones, to match the requirement prescribing that plasma is frozen to below -30°C within 1 h (European guidelines). In fact, at 90 min after the onset of the freezing procedure, the average core temperatures of the unpackaged and packaged plasma units were -16°C and -0.5°C, respectively. We are currently researching less insulated packaging systems and studying a different loading procedure to comply with the European guidelines. Our study underlines the importance of a careful validation procedure, even of an apparently simple process, before routine procedure implementation. Moreover, it would be helpful if freezing devices were validated together by manufacturers and customers for specific processes.