Recovery and composition of microparticles after snap-freezing depends on thawing temperature

Consent regarding the correct processing and storage of blood microparticles is lacking and different protocols for the freeze-thaw cycle exist. Therefore, three different thawing procedures were evaluated regarding their influence on recovery and composition of microparticles. Microparticles were prepared by TRAP-6 or A23187 stimulation of platelet-rich plasma from smokers and nonsmokers (n = 8), from an endothelial cell line or directly obtained from platelet-free plasma of septic patients (n = 5). After snap-freezing in liquid nitrogen platelet-free samples were thawed at 37 degrees, at room temperature or on ice and staining of microparticles was carried out with Annexin V-Cy5 as well as fluorescein isothiocyanate (FITC) or phycoerythrin (PE) labelled antibodies or isotype controls. Microparticle concentrations were determined by means of Trucount tubes. Recovery of platelet microparticles was significantly reduced when samples were thawed on ice (P = 0.001 for all antigens) compared with the two other techniques (P = 0.6 for 37 degrees and P = 0.7 for room temperature, respectively) whereas microparticles of endothelial origin appeared to be less influenced. There was a strong trend towards altered microparticle composition as microparticle counts detected by CD41 staining showed a stronger decrease on ice than Annexin V enumeration (P = 0.07). For microparticle detection thawing of snap-frozen, platelet-free plasma samples should be carried out at room temperature or at 37 degrees C in a water bath but not on ice.