Materials and methods Blood samples The optimization of microparticle detection was performed using healthy controls. Written informed consent was obtained from 10 healthy donors (six female and four male donors) under a study protocol approved by the local Institutional Review Board. Citrated blood (18 mL) was collected in an atraumatic fashion using a 21-gauge needle. The first 3 mL were discarded. The samples were processed within two hours.
Preparation of platelet-poor plasma Platelet-poor plasma was obtained using three different centrifugation protocols (Table 1). Each centrifugation protocol included two steps. The initial step for protocol 1 consisted of two centrifugations at 5000 × g for five minutes at room temperature. After the first centrifugation at 5000 × g for five minutes, the supernatant was transferred into a new tube, leaving 200 µL above the cell pellet, and was centrifuged again for five minutes at 5000 × g. In protocol 2, platelet-poor plasma was obtained by an initial single centrifugation at 5000 × g for 15 minutes, and in protocol 3 by centrifugation at 1500 × g for 15 minutes. After centrifugation, the supernatant was transferred into a new tube, while discarding the last 500 µL at the base of the centrifuged tube. Aliquots of 500 µL were stored at −80°C. After thawing quickly at 37°C, a microparticle pellet was obtained from the platelet-poor plasma by a second centrifugation step at 17,000 × g for either 20 or two minutes. Subsequently, the supernatant was discarded and the microparticle pellet was reconstituted in Annexin V buffer (Becton Dickinson, Franklin Lakes, NJ) at 4°C. All buffers were sterile-filtered with a 0.2 µm filter (Whatman, Piscataway, NJ).