BACKGROUND Using bacterial detection or pathogen reduction, extended platelet (PLT) storage may be licensed if PLT viability is maintained. The Food and Drug Administration (FDA)'s poststorage PLT acceptance guidelines are that autologous stored PLT recoveries and survivals should be 66 and 58% or greater, respectively, of each donor's fresh PLT data. METHODS Nonleukoreduced PLT concentrates were prepared from whole blood donations. Autologous PLT concentrates from 62 subjects were stored in 100% plasma (n=44) or 20% plasma/80% Plasmalyte (n=18), an acetate-based, non-glucose-containing crystalloid solution previously used for PLT storage. Fresh PLTs were obtained on the day the donor's stored PLTs were to be transfused. The fresh and stored PLTs were alternately radiolabeled with either (51) chromium or (111) indium, and in vitro measurements were performed on the stored PLTs. RESULTS The FDA's PLT recovery criteria were met for 7 days of plasma storage, but PLT survivals maintained viability for only 6 days. Plasmalyte-stored PLTs did not meet either acceptance criteria after 6 days of storage. After 7 days of storage, PLT recoveries averaged 43±4 and 30±4% and survivals 4.1±0.4 and 2.0±0.2 days for plasma- and Plasmalyte-stored PLTs, respectively (p=0.03 for recoveries and p<0.001 for survivals). Poststorage PLT recoveries correlated with the commonly used in vitro PLT quality measurements of hypotonic shock response and annexin V binding, while survivals correlated with extent of shape change, morphology score, and pH. CONCLUSIONS There is a progressive decrease in recoveries and survivals of plasma-stored PLTs over time. PLT viability is better maintained in plasma than Plasmalyte.