Summary.Epidemiological studies have shown that women who use third-generation oral contraceptives (OC) containing desogestrel, gestodene or norgestimate have a higher risk of venous thrombosis than women who use second-generation OC containing levonorgestrel. It is also known that a mutation in factor V (factor V Leiden ), which results in resistance to activated protein C (APC) and which is the most common cause of hereditary thrombophilia, potentiates the prothrombotic effect of OC.Effects of APC on thrombin generation in the plasma of women using OC were compared to the response to APC in non-OC users and in individuals that were heterozygous or homozygous for factor V Leiden . The response towards APC was evaluated on basis of the ratio (APC-sr) of the time integrals of thrombin formation determined in the presence and absence of APC.Compared with women not using OC, women who used OC exhibited a significantly decreased sensitivity to APC (P < 0 . 001), independent of the kind of OC used. Women who used third-generation monophasic OC were significantly less sensitive to APC than women using second-generation OC (P < 0 . 001) and had APC-sr that did not significantly differ from heterozygous female carriers of factor V Leiden who did not use OC. Women who were heterozygous for factor V Leiden and used OC had APC-sr in the range of homozygous carriers of factor V Leiden . Two women who started OC therapy had significantly elevated APC-sr within 3 d.Acquired APC resistance may explain the epidemiological observation of increased risk for venous thrombosis in OC users, especially in women using third-generation OC.

The lag time, ETP and peak height of thrombin generation depend on the levels of multiple coagulation factors and inhibitors. The specific assay determinants vary with the experimental conditions.

SwedenTo cite this article: van der Meijden PEJ, van Schilfgaarde M, van Oerle R, Renné T, ten Cate H, Spronk HMH. Platelet-and erythrocyte-derived microparticles trigger thrombin generation via factor XIIa. J Thromb Haemost 2012; 10: 1355-62.See also Shapiro S, Laffan M. Making contact with microparticles. This issue, pp 1352-4.Summary. Background: The procoagulant properties of microparticles (MPs) are due to the of the presence of phosphatidylserine (PS) and tissue factor (TF) on their surface. The latter has been demonstrated especially on MPs derived from monocytes. Objectives: To investigate the relative contribution of TF and factor (F)XII in initiating coagulation on MPs derived from monocytes, platelets and erythrocytes. Methods: Microparticles were isolated from calcium ionophore-stimulated platelets, erythrocytes and monocytic THP-1 cells. MPs were quantified, characterized for cellspecific antigens and analyzed for TF, PS exposure and their thrombin-generating potential. Results: The MP number was not proportional to PS exposure and the majority of the MPs exposed PS. TF activity was undetectable on platelet-and erythrocyte-derived MPs (< 1 fM nM )1 PS), whereas monocyte-derived MPs exposed TF (32 fM nM )1 PS). Platelet-, erythrocyte-and monocyte-derived MPs, but not purified phospholipids, initiated thrombin generation in normal plasma in the absence of an external trigger (lag time < 11 min). Deficiency or inhibition of FVII had no effect on thrombin generation induced by platelet-and erythrocyte-derived MPs, but interfered with monocyte MP-triggered coagulation. Platelet-and erythrocyte-derived MPs completely failed to induce thrombin generation in FXII-deficient plasma. In contrast, monocyte-derived MPs induced similar thrombin generation in normal vs. FXII-deficient plasma. Conclusion: MPs from platelets and erythrocytes not only propagate coagulation by exposing PS but also initiate thrombin generation independently of TF in a FXII-dependent manner. In contrast, monocyte-derived MPs trigger coagulation predominantly via TF.

Thrombin generation was greatly influenced by preanalytic conditions, demonstrating the need for an international standardized protocol.