Do we need thrombin generation assays for monitoring anticoagulation?

Thromb Haemost 2008; 100: 179–180 Protamine sulphate neutralizes the anticoagulant effect of unfractionated heparin (UFH), and this effect can easily be demonstrated by a variety of assay systems, including the traditional activated partial thromboplastin time (aPTT), as well as anti-factor Xa assays, or thrombin generation tests such as the Calibrated Automated Thrombogram (CAT) (1). Protamine also neutralizes the anticoagulant effect of low-molecular-weight heparins (LMWH), but to a lesser and variable extent (2). In this issue of Thrombosis and Haemostasis Gatt et al. (3) compared the effect of protamin sulphate and other drugs on the effect of UFH, LMWH, danaparoid and fondaparinux on aPTT, anti-factor Xa-level, and a thrombin generation test. Reversal of anticoagulation is an interesting model for testing the performance of monitoring assays for anticoagulant drugs. One important finding of the present paper by Gatt et al. is that protamine has little effect on apparent anti-factor Xa-activity levels of LMWH, but a considerable effect on the CAT results. This suggests that the success of protamine administration cannot be monitored by measuring anti-factor Xa activity.This is in some contrast to other investigations showing that anti-factor Xa activity of LMWH is affected by protamine (4, 5). Finally, protamine has no effect on the anticoagulant effect of danaparoid and fondaparinux, and this was evident in both anti-factor Xa-activity and CAT assays. Reversal of danaparoid and fondaparinux, and the conventional LMWH is seldom necessary, but these drugs display a considerably prolonged half-life in patients with impaired renal function and severe bleeding may occur if the dose is not adjusted appropriately (6). Monitoring of the anticoagulant effect in patients with impaired renal function and in critically ill patients in general is essential for prevention of bleeding complications, and an assay system that gives a more global image of haemostatic function than an anti-factor Xa assay might be advantageous (7). Recombinant factor VIIa (rFVIIa, NovoSeven®) has been suggested for treatment of patients with severe anticoagulant-induced bleeding (8), and several case reports have been published on the use of rFVIIa in bleeding fondaparinux-treated patients (9, 10). As shown by Gatt et al. (3), the anti-factor Xa levels are not influenced by rFVIIa, whereas the CAT results reflect the procoagulant effect of rFVIIa. FEIBA® is an alternative to rFVIIa in the treatment of haemophilia patients with inhibitors, and may also be helpful in anticoagulant-induced bleeding. In the present experiments of Gatt et al., FEIBA acted quite similar to rFVIIa, although rFVIIa seemed to be more efficient. Another interesting result is that fresh frozen plasma reduces the anti-factor Xa-activity of UFH, LMWH, danaparoid, and fondaparinux, but has little effect on the anticoagulant effect measured by CAT. The main effect of fresh frozen plasma is dilution, and it might be that the threshold concentration of the anticoagulants for an effect on the assay systems is different. Additional experiments might be needed to determine the threshold concentration of LMWH, danaparoid and fondaparinux for the CAT. From the present results, and experiments by other investigators it seems that thrombin generation tests more reliably reflect the anticoagulant effect of these drugs (11). Anticoagulant drugs delay clot formation and traditional methods for the monitoring of anticoagulation are based precisely on this effect.After addition of an activator, clot formation occurs when only about 5% of the prothrombin has been activated to form thrombin. There are many theories why additional thrombin is formed after clot formation.The additional thrombin may not only improve clot stability by activating factor XIII and TAFI, induce cellular invasion or activation of endothelial cells adjacent to the clot in vivo, but also serves to induce the anticoagulant protein C-dependent feedback control of thrombin generation. Thrombin generation assays not only detect delayed thrombin generation (leading to a prolonged clotting time), but also reduced thrombin activity (leading to changes in structural and functional properties of the clot). Both may be relevant for the therapeutic effect of anticoagulant drugs. The results of Gatt et al. (3) indicate that the CAT may be a prototype for global coagulation assays used for monitoring of anticoagulant therapy. New anticoagulants are entering the scene and one of the major advantages of these drugs is the reliable anticoagulant effect with little intraand inter-individual varia-