Studies of fibrin formation and fibrinolytic function in patients with the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is defined by persistent antiphospholipid antibodies together with thrombosis and/or pregnancy morbidity. We investigated the tightness of fibrin clot and fibrinolytic function in plasma samples from APS patients compared with two control groups.

APS patients (n=49), healthy controls (HC) (n=19) and warfarin-treated nonAPS thrombosis controls (nonAPS-TC) (n=39) were investigated. Fibrin permeability was assessed as the permeability coefficient (Ks) by a flow measurement technique. Additionally, clot density and fibrinolytic function was analysed by a turbidimetric clotting and lysis assay. Fibrin structure was visualised using scanning electron microscopy. Finally, the number of cell-derived microparticles (MPs) in the samples were correlated to fibrin permeability

The Ks value was lower in samples from APS-patients compared to HC and nonAPS-TC (p<0.0001 for both) indicating a less permeable fibrin clot in APS patients. Scanning electron microscopy images confirmed compact fibrin with smaller intrinsic pores and thinner fibers in samples from APS patients as compared to HC. Prolonged fibrinolysis (clot lysis) times were present in the subgroup of APS patients with previous arterial thrombosis (n=15) as compared to HC and to nonAPS-TC (all p-values<0.05). In conclusion, tighter fibrin clots were formed in plasma from APS patients compared with healthy controls and warfarin treated patients with thrombosis of "nonAPS origin". This new observation presents a possible mechanism contributing to the thrombotic predisposition of APS patients. Impaired fibrinolysis, selectively present among APS patients with previous arterial thrombosis, may further aggravate the pro-thrombotic state in this APS subgroup.