Blood preservation is generally and widely used in the fields of blood transfusion and surgery for either autologous or allogeneic blood [1–3]. In case of small lots of blood-derived materials used in regenerative therapy, such as platelet concentrates, it is generally accepted that autologous blood samples should be collected on-site and immediately centrifuged for processing [4]. Accordingly, it is officially recommended to use thus prepared materials immediately. The advantages of this preparation protocol are the zero cost of preservation and no risk of degradation and contamination.

In Niigata University Hospital, when relatively severe surgical operations (e.g., large bone defects that require hospitalization for alveolar ridge augmentation and sinus floor elevation) are planned, relatively large volumes of blood samples are usually collected the day before the operation, and platelet-rich plasma (PRP) is prepared and stored at ambient temperature until use [5]. Nevertheless, there are no established methods for preparation of self-clotted platelet concentrates from stored whole-blood (WB) samples. This may be another reason why platelet-rich fibrin (PRF) should be prepared on-site and used immediately.

On the other hand, if PRF can be prepared from stored WB samples on the next day or later without significant reduction in the bioactivity, clinical applications of PRF will expand. In this study, we developed a method for preparation of PRF from stored WB samples by adding CaCl₂ and evaluated the quality in terms of suitability for regenerative therapy. As a result, we successfully validated the method and ensured the quality of PRF prepared from stored WB samples.