Some people are born with an increased tendency to form blood clots, which increases their risk for developing blood clots in the legs (deep vein thrombosis) and in the lungs (pulmonary embolism). This tendency is due to inherited blood-clotting irregularities, which are generally related to:

• Mutated genes (such as factor V Leiden, factor II).
• Decreased amounts of certain proteins (protein C, protein S, and antithrombin III).
• Increased levels of other substances (antiphospholipid or lupus anticoagulant).

Many of these blood-clotting irregularities can be identified with special tests. If your doctor suspects that you may have an inherited blood-clotting irregularity, discuss whether testing is needed.