Fondaparinux is a synthetic analog of pentasaccharide mimicking the site of heparin that binds to antithrombin and enhances the inactivation of factor Xa without interaction with factor II (prothrombin):
Achieves steady state antithrombotic activity 3-4 days after administration. Almost completely dependent on kidneys for clearance; contraindicated in patients with severe renal impairment (creatinine clearance 30 mL/minute). Half-life ranges from 17 hours in young patients to 21 hours in older adults, and is further increased in patients with renal insufficiency.  Predictable anticoagulant effect so monitoring of anti-Xa levels not usually necessary but anticoagulant effect can be measured by fondaparinux-specific anti-Xa assays.