Platelet and platelet-derived microparticle surface factor V/Va binding inwhole blood: Differences between neonates and adults

Platelet-derived microparticles (PDMP) appear to play a major role in the g eneration of procoagulant activity. In this study, we describe a novel flow cytometric method that allows direct evaluation of the procoagulant activi ty of PDMP and platelets in the physiological milieu of whole blood. The pe rcent PDMP generated in response to calcium ionophore A23187 and calcium wa s increased in preterm neonates (67.5 +/- 3.4%, mean +/- S.E.M., n = 8, p < 0.05) and term neonates (67.2 + 2.7%, n = 7, p<0.05) compared with adults 149.5 +/- 3.4%, 13). However, in preterm neonates A23187/calcium-induced bi nding of factor V/Va to PDMP and platelets (22.8 +/- 5.6 fluorescence units ) was markedly reduced (p <0.05) compared to term neonates (58.2 +/- 7.2) a nd adults (50.6 + 6.3). In preterm blood, A23187/calcium-induced binding of factor V/Va to PDMP and platelets returned to adult levels when: a) adult plasma, rather than autologous preterm neonatal plasma, was added; or b) fa ctor V, but not factor VIII, was added to autologous preterm neonatal plasm a. In summary: 1) We have developed a flow cytometric method for the direct detection of procoagulant PDMP and platelets in whole blood. 2) Compared t o adults and term neonates, PDMP and platelets of preterm neonates bound ma rkedly less factor V/Va (reflecting reduced procoagulant activity), because of a relative lack of factor V in preterm neonates. 3) This procoagulant d efect in PDMP and platelets may contribute to the propensity of preterm neo nates, but not term neonates, to intraventricular hemorrhage. 4) The percen t PDMP does not necessarily reflect the degree of procoagulant activity of PDMP or platelets.