Usually, these anti-clotting proteins are strong enough to stop clotting in the veins when it shouldn't happen. However, certain situations or risk factors can make it easier for your blood to clot too much. Some people are born with a tendency to develop clots. Most of the time, increased clotting happens because protein C in the blood isn't doing its job properly. A few people don't have enough antithrombin III, or have antithrombin III that doesn't work.
Some people have an extra protein in their blood that causes too much clotting.
Hypercoagulation has a few other causes, but those causes are rare. If your doctor suspects you have hypercoagulation, tests can measure APC resistance, protein C, protein S and antithrombin III in your blood and can check for the lupus anticoagulant protein. The tests will also show if your proteins are working the right way. Several medicines can thin your blood and make it less likely to clot. Other people need to take medicine for the rest of their lives.