Expansion
Factor V Leiden is commonest inherited; antiphospholipid syndrome commonest acquired
Expansion
Inherited
- Factor V Leiden: activated protein C resistance; commonest, present in about 5 per cent of Europeans
- Prothrombin G20210A mutation
- Protein C, protein S and antithrombin deficiency: rarer but higher risk
Acquired
- Antiphospholipid syndrome: lupus anticoagulant, anticardiolipin and anti-beta2 glycoprotein antibodies. Causes both arterial and venous thrombosis and recurrent miscarriage, and paradoxically prolongs the APTT in vitro
- Malignancy, pregnancy, oestrogens, nephrotic syndrome, myeloproliferative disease, paroxysmal nocturnal haemoglobinuria
- Immobility, surgery, obesity, smoking
Virchow’s triad remains the organising principle: stasis, endothelial injury and hypercoagulability.
Testing caveats: protein C and S are vitamin K dependent so are falsely low on warfarin; antithrombin is consumed in acute thrombosis and reduced by heparin; and factor V Leiden and prothrombin gene testing are unaffected by either. Most tests should be deferred until at least 3 months after the event and off anticoagulation.