What are protein C and protein S tests?
These tests measure the levels of protein C and protein S in your blood. Protein C and protein S tests are two separate tests that are often done at the same time.
Protein C and protein S work together to prevent your blood from clotting too much. Normally, your body makes blood clots to stop bleeding after a cut or other injury. If you don't have enough protein C (protein C deficiency) or enough protein S (protein S deficiency), your blood can clot more than you need it to. If this happens, you may get a clot that partly or completely blocks blood flow in a vein or artery. These clots can form in the arms and legs and travel to your lungs. When a blood clot forms in the lungs it's called a pulmonary embolism. This condition is life-threatening.
Protein C and protein S deficiencies can be mild or severe. Some people with mild deficiencies never have a dangerous blood clot. But certain factors can increase the risk. These include surgery, pregnancy, certain infections, and extended periods of inactivity, such as being on a long airline flight.
Protein C and protein S deficiencies are sometimes inherited (passed down from your parents), or can be acquired later in life. Testing may help find ways to prevent the formation of clots, regardless of how you got the deficiency.
Other names:  protein C antigen, protein S antigen
