Anticoagulation (Warfarin and DOACs)
Warfarin and the direct oral anticoagulants interrupt the clotting cascade at different points (vitamin K recycling versus a single activated factor), and that mechanistic difference dictates monitoring, dosing and how bleeding is reversed.
In a nutshell
Warfarin blocks vitamin K recycling, so new clotting factor synthesis falls and effect builds slowly, monitored by INR. DOACs directly inhibit a single active clotting factor (Xa or thrombin), giving fast, predictable, unmonitored anticoagulation. Reversal strategy in each case mirrors the underlying mechanism.
Classic presentation
A patient on long-term anticoagulation presenting either with bleeding (bruising, epistaxis, GI or intracranial haemorrhage) after a precipitant, or with a new thromboembolic event suggesting under-treatment.
Key points
- INR only reflects warfarin; it does not measure DOAC effect, which needs a specific assay if quantification is required.
- Factor VII has the shortest half-life of the vitamin K-dependent factors, so INR rises before warfarin's antithrombotic effect is complete; a bridging anticoagulant may be needed initially.
- Warfarin reversal is slow (vitamin K) or fast (PCC); DOAC reversal uses a drug-specific antidote (idarucizumab, andexanet alfa) or PCC if unavailable.
- Mechanical heart valves require warfarin: DOACs are inadequate in this setting despite being effective in atrial fibrillation and VTE.
First-line investigation
INR for a patient on warfarin; a drug-specific anti-Xa or dilute thrombin time assay if DOAC effect must be quantified.
First-line management
Stop the anticoagulant and assess bleeding severity; reverse with vitamin K/PCC for warfarin or a drug-specific/PCC agent for DOACs if bleeding is major.
Exam traps
- A normal INR does not exclude a clinically significant DOAC effect: INR is not a reliable DOAC test.
- Vitamin K alone is too slow for life-threatening bleeding on warfarin; PCC is needed for immediate reversal.
- Mechanical valves are an exception to DOAC-first prescribing: warfarin remains mandatory.
Educational content pending clinical review. Not medical advice.