Broadening the factor V Leiden paradox: pulmonary embolism and deep-vein thrombosis as 2 sides of the spectrum

Risk factors for deep-vein thrombosis have been shown not to be always the same as for pulmonary embolism. A well-known example is the factor V Leiden (FVL) paradox: the FVL mutation poses a clearly higher risk for deep-vein thrombosis (DVT) than for pulmonary embolism. We aimed to expand this paradox and therefore present risk estimates for several established risk factors for DVT and pulmonary embolism separately. When such separate risk estimates could not be retrieved from the literature, we calculated these risks in our own data, a large population-based case-control study on venous thrombosis (the MEGA study). Our results showed that the FVL paradox can be broadened (ie, the risk factors oral contraceptive use, pregnancy, puerperium, minor leg injuries, and obesity have an effect comparable with FVL). Furthermore, we found that pulmonary conditions, such as chronic obstructive pulmonary disease, pneumonia, and sickle cell disease, were risk factors with an opposite effect: a higher risk of pulmonary embolism, but little or no effect on DVT. These findings suggest that pulmonary embolism and DVT may not always have the same etiology, and encourage unraveling this phenomenon in further studies.