Relative and absolute changes of activated platelets, microparticles and platelet aggregates after activation in vitro

Standard flow cytometers provide relative numbers of activated platelets, m icroparticles, and platelet aggregates. With fluorescent beads it is now po ssible to determine absolute numbers. Whole blood and platelet-rich plasma were incubated with agonists (ADP, collagen, thrombin). CD62p expression, m icroparticle and platelet aggregate formation were measured. Flow-Count Flu orospheres(R) were added to calculate absolute concentrations. After activa tion there was an increase in the percentage of CD62p-positive platelets. H owever, the total number of platelets decreased and therefore the absolute number of CD62p-positive platelets did not increase but decreased. The numb er of CD62p-positive platelets decreased not as much as the number of CD62p -negative platelets, which explains why the relative percentage of CD62p-po sitive platelets increased. A similar increase in percent and decrease in a bsolute counts was found for microparticles. Platelet aggregates increased both in relative and absolute numbers, These results suggest that the detec tion of activated platelets by flow cytometry has to be complemented by the determination of the absolute concentrations to avoid misinterpretation.