Increased levels of factor VIII and fibrinogen in patients with venous thrombosis are not caused by acute phase reactions

Factor VIII activity (factor VIII:C) levels greater than or equal to 150 IU/dl are associated with a 5- to 6-fold increased risk of venous thrombosis compared to levels <100 IU/dl, and fibrinogen levels greater than or equal to 5.0 g/l increase the thrombosis risk 4-fold. These high levels are present in 25%, resp. 3% of the patients with a first episode of venous thrombosis. These findings were based on measurements after the thrombotic event, so the factor VU[I and fibrinogen levels in thrombosis patients may have been influenced by acute phase reactions or ongoing inflammatory responses. In the present study we measured plasma C-reactive protein (CRP) as a sensitive marker of an acute phase reaction in 474 thrombosis patients and 474 age- and sex-matched healthy controls, that were part of the Leiden Thrombophilia Study (LETS). Mean and median CRP levels were higher in thrombosis patients than in the controls, suggesting inflammation in some patients. CRP affected both factor Vm and fibrinogen levels, in patients and controls alike. After adjustment for the effect of CRP, high factor WI:C levels still increased the thrombosis risk 6-fold and high fibrinogen levels Lf-fold, which is for both very similar to the risk before correction for CRP levels. These results show that although systemic inflammation may be present in some of the patients, elevated levels of factor VIII:C and fibrinogen were in general not caused by acute phase reactions. This further supports a causal relationship between both high factor VIII:C and fibrinogen levels and venous thrombosis.