1 Anticoagulation and Antiplatelet Therapy Principles

How antithrombotic drugs differ

Antithrombotic drugs reduce clot formation but act at different stages of hemostasis. Antiplatelet drugs mainly limit platelet activation or aggregation, while anticoagulants mainly reduce coagulation-factor activity and fibrin formation. Neither class directly dissolves an existing clot, and both can cause bleeding, so treatment and reversal decisions require clinical assessment.

From platelet plug to stable clot

Hemostasis starts with primary hemostasis: platelets adhere to an injured vessel, activate, and aggregate to form an initial platelet plug. Secondary hemostasis strengthens that plug through coagulation. The intrinsic and extrinsic pathways converge at factor X; factor Xa helps generate thrombin, which converts fibrinogen to fibrin. Fibrin stabilizes the clot.

Antiplatelet drugs primarily affect the platelet plug. Anticoagulants primarily reduce thrombin or fibrin generation.

Drug targets

Different medicines target different parts of clot formation:

• Heparin enhances antithrombin activity, inhibiting thrombin and factor Xa. The relative effects depend on the heparin preparation.

• Warfarin inhibits vitamin K recycling, reducing production of functional factors II, VII, IX, and X, as well as proteins C and S.

• Direct oral anticoagulants (DOACs) directly inhibit thrombin or factor Xa. Dabigatran inhibits thrombin; apixaban, edoxaban, and rivaroxaban inhibit factor Xa.

• Aspirin inhibits platelet cyclooxygenase, reducing thromboxane A₂ production and platelet activation.

• P2Y12 inhibitors, such as clopidogrel, reduce ADP-mediated platelet activation.

Monitoring treatment

Monitoring depends on the medicine. IV unfractionated heparin is commonly assessed with aPTT or a heparin-calibrated anti-Xa assay; the method and target depend on the clinical protocol. Platelet counts are also monitored because heparin can cause thrombocytopenia.

Routine warfarin dosing is guided by the INR, which standardizes the prothrombin-time result. The intended INR range depends on the reason for treatment.

DOACs are not usually adjusted through routine INR-based dosing. Clinical follow-up remains important, including assessment of kidney function when appropriate. Specialized testing may be considered in selected urgent situations.

Bleeding precautions

Patients taking an anticoagulant should tell clinicians and pharmacists about it and check before starting other medicines or supplements. Medicines such as NSAIDs and some supplements may increase bleeding risk or interact. Do not stop or change an anticoagulant without consulting the care team, including before dental work, and do not double a dose after a missed dose.

Seek urgent medical help for severe or uncontrolled bleeding, blood in vomit or stool, or a sudden severe headache, especially after an injury. Anticoagulant treatment can increase bleeding risk, so symptoms and possible interactions require attention.

Reversal and emergency management

Reversal in a serious bleeding emergency depends on the anticoagulant and the clinical situation:

• Unfractionated heparin: protamine sulfate.

• Warfarin: vitamin K plus four-factor prothrombin complex concentrate (PCC), when indicated.

• Dabigatran: idarucizumab.

• Factor Xa inhibitors, such as apixaban or rivaroxaban: specialist-guided reversal or supportive management, following current local guidance.

The supplied material notes that andexanet alfa was withdrawn from the U.S. market in December 2025 and discontinued in January 2026. For bleeding associated with a factor Xa inhibitor, follow current institutional and specialist guidance rather than assuming that this agent is available.