antiXa

Anti-Xa broadly denotes activity that inhibits coagulation factor Xa or laboratory measures of such activity. In hemostasis, factor Xa sits in the coagulation cascade converting prothrombin to thrombin; reducing Xa activity lowers thrombin generation and clot formation. Anti-Xa activity can arise from direct inhibitors of Xa or from indirect inhibition that works through antithrombin.

Direct Xa inhibitors are a class of oral anticoagulants that include rivaroxaban, apixaban, edoxaban, and betrixaban. They bind to the active site of factor Xa, preventing conversion of prothrombin to thrombin and thereby reducing thrombin generation. Indirect Xa inhibition is achieved by heparins (unfractionated and low-molecular-weight heparin) which potentiate antithrombin. Through this pathway, they inhibit Xa along with thrombin to varying extents. The anti-Xa activity of some agents can be used to monitor therapy in specific clinical contexts, though routine measurement is not required for direct Xa inhibitors.

Anti-Xa assays are laboratory tests that quantify the activity of factor Xa using chromogenic substrates. They are used to monitor unfractionated heparin and LMWH therapy and may be used to estimate DOAC levels in overdose, organ impairment, or periprocedural management. Results require drug-specific calibration and can be affected by reagent type and hematocrit. Clinically, interpretation considers the pharmacokinetics of the agent and the patient’s condition, rather than relying solely on anti-Xa values.