Protein C is a protein present in the blood that helps to regulate the formation of clot. The protein C test is performed to evaluate and measure the presence of Protein C in the blood and to check if the protein is properly functioning or not. Your doctor might ask you to get the test done if you had a thrombotic episode, or if you developed a blood clot for no obvious reason. Your doctor may also perform the test to evaluate and diagnose underlying diseases or conditions such as liver disease or cancer.