*6.1.2. FIX and thrombosis*

FIX is a key component of intrinsic/contact pathway. Levels of FIX are important to regulate the hemostasis [3]. Lower levels of FIX antigen leads to hemophilia and recently two studies showed that higher levels of FIX lead to thrombosis [109]. Saenko et al. demonstrated that risk of thrombosis increases by 2.3–2.8 fold in the subjects with plasma FIX activity >150 IU/dl and van HylckamaVlieg et al. demonstrated that risk of thrombosis increases by 2.8 fold with plasma FIX levels >129 U/dl [110, 111]. Age, increase in blood lipids and use of oral contraceptive pills are some of the reasons for elevated plasma FIX levels [110, 111]. Some of the mutations in FIX gene lead to increase in FIX activity thereby, increase in the risk of thrombosis. FIX Padua variant is a one among the FIX mutants to show enhanced risk of thrombosis. FIX Padua is a single amino acid substitution variant where arginine 388 is mutated to leucine [112].
