Platelet polyphosphate (polyP) can be conveniently quantified by exploiting a recent methodological breakthrough using 4′,6-diamidino-2-phenylindole (DAPI). However, the preservation of these biological samples has not yet been standardized. In a preliminary study, potential protocols were screened, while accepted protocols were further tested in this study. Pure-platelet-rich plasma (P-PRP) samples and washed platelet suspensions were prepared using blood obtained from non-smoking healthy male donors and were fixed with ThromboFix for 20-24 h at 4 °C. Mass polyP levels were determined using a fluorometer at wavelengths of 425 and 525 nm. Platelet polyP levels were normalized to platelet counts. Statistical analyses were performed using non-parametric tests. Platelet polyP levels significantly decreased by 20% after 7 days in the platelet suspension maintained under fixed conditions at 4 °C (control). In contrast, the platelet polyP levels in both the P-PRP and washed platelet suspensions were maintained without a significant reduction for up to 6 weeks by removing ThromboFix after fixation and subsequent freezing in pure water at -80 °C. Fluorometric polyP quantification often interferes with the low specificity of DAPI binding and the wavelength used. Our validated protocols will enable long-term preservation and high-throughput polyP quantification and can be applied to relatively large cohort studies.