Factor V Leiden
Background
- Most common inherited thrombophilia (3-8% of Caucasians)
- Point mutation in Factor V making it resistant to cleavage by activated protein C → hypercoagulable state
- Heterozygous: 3-8× increased risk of VTE; Homozygous: 50-80× increased risk
- Most individuals with FVL will never develop a venous thromboembolism (VTE)
- EM relevance: Presents when a provoking factor (OCP use, surgery, immobilization, pregnancy) unmasks the predisposition
Clinical Features
- DVT, pulmonary embolism
- VTE at young age (<50 years) without clear provoking factor
- VTE in atypical locations (mesenteric, cerebral venous sinus)
- Recurrent VTE
- Family history of VTE or known thrombophilia
Differential Diagnosis
Evaluation
- Activated protein C resistance assay — screening test
- Factor V Leiden genetic testing — confirmatory
- Do NOT order thrombophilia workup in the acute ED setting — anticoagulation and acute illness affect results
- Thrombophilia testing should be deferred to outpatient hematology follow-up
Management
- Acute VTE: Standard anticoagulation per DVT or pulmonary embolism guidelines (same as non-FVL patients)
- Duration of anticoagulation (hematology decision):
- First provoked VTE: 3-6 months
- Unprovoked VTE or recurrent VTE: consider indefinite anticoagulation
- Asymptomatic FVL carriers: no prophylactic anticoagulation; counsel on risk reduction
Disposition
- Per underlying VTE management
- Ensure hematology follow-up for thrombophilia evaluation after acute treatment