VKAs (Warfarin, Acenocoumarol) inhibit Vitamin K Epoxide Reductase, preventing the activation of clotting factors II, VII, IX, and X. However, they also rapidly deplete Protein C and Protein S (endogenous natural anticoagulants) which have much shorter half-lives than Factor II. Consequently, the patient enters a transient hypercoagulable state during the first 3-5 days of therapy. This is why bridging with LMWH or unfractionated heparin is physiologically mandatory until the INR reflects true therapeutic Factor II depletion.
| Pharmacokinetic Feature | Warfarin | Acenocoumarol (Acitrom) |
|---|---|---|
| Half-Life (T½) | Long (36 - 42 hours) | Short (8 - 11 hours) |
| Time to Steady State | 5 to 7 days | 2 to 3 days |
| Clinical Implication | More stable INRs; missing a single dose causes minor fluctuations. | Faster onset/offset. Missing a single dose causes a rapid drop in INR, increasing immediate stroke/clot risk. |
| Equivalent Dose Ratio | 5 mg | 2 mg (Acitrom is ~2.5x more potent per mg) |
The target INR for almost all standard indications is 2.5 (Range 2.0 - 3.0), including DVT/PE, AFib, and Bioprosthetic valves. The major exception requiring a target of 3.0 (Range 2.5 - 3.5) is the Mechanical Mitral Valve, due to the high-flow, high-thrombogenicity environment of the mitral position.
| Increase INR (Bleeding Risk) | Decrease INR (Clotting Risk) |
|---|---|
| Amiodarone (Reduces VKA dose requirement by ~30-50%) | Rifampicin (Rifampin) |
| Macrolides, Fluoroquinolones | Carbamazepine, Phenytoin |
| Azole antifungals, Metronidazole | Phenobarbital |
| NSAIDs (Increase bleeding risk independent of INR) | Cholestyramine |
AMA Style:
Umakanth S. VKA & Warfarin Dosing Pathway. MEDiscuss. Published 2026. Accessed .
Vancouver Style:
Umakanth S. VKA & Warfarin Dosing Pathway [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:
