Platelet preservation is restricted to 3 and 5 days in Japan and worldwide, respectively. This limit is based on the fact that platelets are sensitive to changes in temperature and pH: when samples are stored at 2 to 6 °C, platelets become unsuitable for production of platelet concentrates [3]. Preservation of platelet concentrates results in a drop of pH below 6.0 depending on the platelet count [15], and pH below 6.2 correlates with decreased in vivo efficacy of platelets [16]. Furthermore, it was recently demonstrated that growth factors in PRP degrade in the course of storage at 22 °C [17].

On the other hand, in general, WB can be stored in the presence of ACD or citrate phosphate and dextrose (CPD) at room temperature for a relatively long period (3 weeks or longer) before it is processed into blood components [1]. The WB storage has also been supported by recent developments in oxygen-permeable plastic bags. Nevertheless, out of concern about bacterial contamination, the maximal storage period is restricted to 8 h in some countries [3]. To minimize and prevent bacterial proliferation, it is recommended to maintain white blood cells in WB samples during the initial 16 to 20 h of storage to digest bacteria during storage [18, 19].

Here, it is worth discussing which functional states of platelets are expected to be maintained during storage for subsequent preparation of platelet concentrates (to be used for regenerative therapy). There is no doubt that the functional states observed in freshly isolated platelets are the best for preparation of platelet concentrates and for their best clinical performance. Nevertheless, given that platelet concentrates are expected to provide significant amounts of growth factors and fibrin(ogen) at implantation sites, stored platelets are not necessarily expected to function as fully as fresh ones (e.g., in terms of aggregation). Rather, stored platelets are expected not to lose growth factors during the storage period, while coagulation factors, especially those involved in the endogenous coagulation cascade, should maintain their activities to convert and polymerize fibrinogen to form well cross-linked fibrin fibers.