Elevated factor VIII levels and risk of venous thrombosis

Modern thrombophilia testing fails to identify any underlying prothrombotic tendency in a significant number of patients presenting with objectively confirmed venous thromboemboembolism (VTE). This observation has led to a search for other novel inherited or acquired human thrombophilias. Although a number of putative mechanisms have been described, the evidence behind many of these candidates remains weak. In contrast, an increasing body of work supports the hypothesis that increased plasma factor VIII (FVIII) levels may be important in this context. An association between elevated plasma FVIII levels and VTE was first described in the Leiden Thrombophilia Study (LETS). Subsequently, these conclusions have been supported by an increasing number of independent casecontrol studies. Cumulatively, these studies have clearly demonstrated that high FVIII levels constitute a prevalent, dose-dependent risk factor for VTE. Furthermore, more recent studies have shown that the risk of recurrent venous thrombosis is also significantly increased in patients with high FVIII levels. In this review, we present the evidence supporting the hypothesis that elevated FVIII levels constitute a clinically important thrombophilia. In addition, we examine the biological mechanisms that may underlie persistently elevated FVIII levels, and the pathways through which high FVIII may serve to increase thrombotic risk.