Among the so-called permanent risk factors (i.e., risks always present even without clinical evidence) are congenital factors such as anti-thrombin III, protein C, and protein S deficits. Anti-thrombin III deficit seems to be related more to venous thromboembolic risk than the two others. Homozygous carriers of protein C or protein S deficit are susceptible for severe clinical diseases, such as porpora fulminans, whereas heterozygous carriers can suffer DvT episodes before 45 years of age even at uncommon sites such as upper limb veins.
Other congenital risk factors include mutation of the Leiden factor, which is responsible for protein C-activated resistance, and mutation of prothrombin G20210A, which determines a 30% increase in circulating prothrombin.
Age, neoplasms, anti-phospholipid antibodies, and previous episodes of DvT represent permanent, acquired risk factors.
Moreover, in some clinical cases the thromboembolic risk is hardly detectable, such as in cases of hyperhomocysteinemia, hyperfibrinogenemia, and an increase in factor VIII, IX, and XI levels.
Among the most easily detectable transient risk factors are surgery (mainly orthopaedic surgery or neurosurgery), multiple trauma, prolonged immobilization, pregnancy, and oral contraceptive use .
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