Protein C and Protein S Tests
The Protein C and Protein S tests are blood tests used to measure the levels of two proteins in the blood that play a crucial role in the regulation of blood clotting.
What are Protein C and Protein S?
Protein C and Protein S are vitamin K-dependent proteins produced by the liver. They work together as anticoagulants to prevent excessive blood clotting. Protein C is an enzyme that helps to break down blood clots, while Protein S serves as a cofactor for Protein C, enhancing its activity.
Why are these tests performed?
The Protein C and Protein S tests are typically ordered in the following situations:
- To diagnose inherited or acquired disorders that affect blood clotting, such as thrombophilia
- To investigate the cause of recurrent or unexplained blood clots (thrombosis)
- To monitor patients with a history of blood clots who are at risk of developing another clot
- To evaluate the effectiveness of anticoagulant therapy in patients with blood clotting disorders
How are the tests performed?
The Protein C and Protein S tests involve taking a blood sample from a vein, usually in the arm. The blood is then sent to a laboratory for analysis.
What do the test results mean?
The results of the Protein C and Protein S tests are typically reported as a percentage of normal activity or concentration. Abnormal results may indicate:
- Deficiency: Low levels of Protein C or Protein S, which can increase the risk of blood clots
- Dysfunction: Abnormal function of Protein C or Protein S, which can also increase the risk of blood clots
- Normal: Normal levels and function of Protein C and Protein S, which suggests that other factors may be contributing to blood clotting disorders
Reference ranges
The reference ranges for Protein C and Protein S tests vary depending on the laboratory and the specific test used. Generally:
- Protein C: 70-140% of normal activity
- Protein S: 60-150% of normal concentration (free and total)
Limitations and potential interferences
Certain factors can affect the accuracy of the Protein C and Protein S tests, including:
- Vitamin K deficiency or excess
- Warfarin therapy (affects vitamin K-dependent proteins)
- Liver disease or dysfunction
- Pregnancy or estrogen therapy (can increase Protein S levels)
It is essential to consult with a healthcare professional for proper interpretation of test results and to discuss any concerns or questions.
Article last updated on: 15th June 2025.
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