Mechanism of action the antithrombotic activity of fondaparinux is the result of atiii-mediated selective inhibition of factor xa. by selectively binding to atiii, fondaparinux potentiates (about 300 times) the neutralization of factor xa by atiii. neutralization of factor xa interrupts the blood coagulation cascade and thus inhibits thrombin formation and thrombus development. it is thought that fondaparinux is unlikely to induce thrombocytopenia via a heparin-induced thrombocytopenia (hit)-like mechanism given its chemical structure (pmid 19825921). as a result, fondaparinux has been used as an alternative anticoagulant in heparin-induced thrombocytopenia (hit) patients (pmid 19737996, 19432027, 18217156). however, it is important to note that rare cases of hit have been reported in patients treated with fondaparinux