• Active cancer
• Prolonged immobility (long journeys, lower limb fracture)
• Recent major surgery
• Pregnancy or recent childbirth

Uncommon or rare

• Factor V Leiden
• Antithrombin III deficiency
• Protein C deficiency
• Protein S deficiency
• Antiphospholipid syndrome
• Active inflammatory bowel disease
• Myeloproliferative disorders
• Oestrogen-containing oral contraceptive (especially when combined with Factor V Leiden)
• Nephrotic syndrome
• Homocystinuria
• Paroxysmal nocturnal haemoglobinuria
• Hyperviscosity syndrome
• Behcet's disease