A 62-year-old man with atrial fibrillation is started on warfarin for stroke prevention. After 5 days of therapy, he develops sudden severe left leg pain and swelling. On examination, there is cyanosis of the toes and a palpable cord in the calf. Which investigation is most appropriate to confirm the diagnosis of warfarin-induced thrombosis?

## Warfarin-Induced Thrombosis: Diagnostic Approach ### Clinical Context Warfarin-induced thrombosis (WIT), also known as warfarin-induced skin necrosis or venous thromboembolism paradox, occurs early in anticoagulation when protein C (a natural anticoagulant with short half-life) drops faster than vitamin K-dependent procoagulants (factors II, VII, IX, X). This creates a transient hypercoagulable state. ### Investigation of Choice **Key Point:** Compression ultrasonography (B-mode) is the gold standard for diagnosing deep vein thrombosis (DVT) and venous thromboembolism in warfarin-induced thrombosis. **High-Yield:** Compression ultrasonography has: - Sensitivity: 95–98% for proximal DVT - Specificity: 98–99% - Non-invasive, no radiation, no contrast needed - Can be repeated serially if clinical suspicion remains high - Directly visualizes thrombus and assesses compressibility ### Why This Investigation? | Feature | Compression Ultrasound | Alternative Tests | |---------|------------------------|-------------------| | **Directly visualizes thrombus** | Yes | No | | **Confirms DVT diagnosis** | Yes | No | | **Bedside availability** | Yes | Varies | | **Repeatable safely** | Yes | Limited | | **Cost-effective** | Yes | Yes (D-dimer) but less specific | **Clinical Pearl:** In warfarin-induced thrombosis, the paradoxical thrombosis occurs despite elevated INR, making PT/INR alone insufficient for diagnosis. The clinical picture (acute limb ischemia, cyanosis, cord-like vein) demands imaging confirmation. **Mnemonic: DVT Diagnosis = DUST** - **D**-dimer (screening, not confirmatory) - **U**ltrasonography (gold standard) - **S**can (CT angiography for PE) - **T**hrombin time (not for DVT diagnosis) ### Pathophysiology Timeline ```mermaid flowchart TD A[Warfarin started]:::action --> B[Protein C drops rapidly<br/>Half-life: 8-12 hours]:::outcome B --> C[Factors II, VII, IX, X<br/>still elevated initially<br/>Half-life: 24-72 hours]:::outcome C --> D{Transient hypercoagulable state}:::decision D --> E[DVT/PE/Skin necrosis]:::urgent E --> F[Compression ultrasound<br/>confirms DVT]:::action ``` ### Management Implications Once DVT is confirmed by ultrasound, management includes: 1. Immediate anticoagulation with LMWH or unfractionated heparin (bridge therapy) 2. Vitamin K supplementation 3. Consider protein C concentrate if available 4. Continue warfarin only after INR therapeutic and heparin overlap complete [cite:Harrison 21e Ch 297]