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

• Disseminated Intravascular Coagulation (DIC)
• Factor V Leiden
• Nephrotic syndrome

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

See Also

• DVT
• Pulmonary embolism
• Coagulopathy (Main)