Protein C and Protein S Tests

Protein C and Protein S Tests

Protein C and Protein S tests are medical lab tests that measure the levels of these proteins in the blood. Protein C and Protein S are naturally occurring proteins in the body that help prevent excessive bleeding. The tests measure these proteins to determine if someone has a deficiency. Such a deficiency can lead to a higher risk of abnormal blood clotting.

Preparation for Tests:

  • No specific preparation is required for this test.
  • Your doctor may ask you to stop taking medications that alter blood clotting, such as anticoagulants and some types of pain relievers, for a few days before the test.
  • Make sure to tell your doctor about all medications you are taking beforehand.


During a Protein C and Protein S test, a sample of your blood will be taken from a vein in your arm or in the bend of your elbow. The sample will then be sent to the laboratory for testing.


Two types of Protein C and Protein S tests are available:

  • Activated protein C resistance (APCR): This tests for any defects that may cause an excess in the clotting of blood.
  • Functional protein C: This test is to measure the amount of functional protein C present in the blood.


Drawing blood carries very minor risks, such as lightheadedness, bruising at the injection site, or fainting. More serious risks are rare.

Why Take Protein C and Protein S Tests?

Your doctor may order these tests to help diagnose the cause of an abnormal blood clotting problem, such as deep vein thrombosis (DVT), pulmonary embolism, or excessive bleeding. These tests may also be used if you have a family history of an inherited blood disorder or if you have a clot-related disorder.

When to Take Protein C and Protein S Tests?

The decision to have these tests will depend on your history and symptoms. Your doctor might also consider family history and other risk factors. If you think your doctor might want to order these tests, speak to them for a more detailed discussion.