Protein S

What is being tested?
Proteins C and S are normally present in the blood and help prevent excessive blood clotting. Protein C, together with protein S, regulates the rate at which clots form so that they form only at sites where there has been an injury. When a blood vessel or tissue is injured, the body starts the coagulation cascade – a step-by-step process in which up to 20 protein factors are activated – which results in the formation of a stable clot. This clot prevents further blood loss and protects the wound until it heals. When it is no longer needed, other factors break down the clot, which is then removed.

Thrombin is a coagulation factor that can speed up or slow down clot formation by promoting or inhibiting its own activation. It exerts a feedback action that uses protein C and protein S to slow down the coagulation cascade. Thrombin first binds to a protein called thrombomodulin and then activates protein C. This activated protein C (APC) then binds to protein S (its cofactor), and together they break down coagulation factors VIIIa and Va (these activated factors are needed to produce thrombin). This mechanism slows the formation of new thrombin and inhibits further clotting. If there is not enough protein C or protein S, or if either does not work normally, thrombin formation proceeds unchecked. This can lead to excessive or inappropriate clotting, which can block blood flow in the veins (VTE – venous thromboembolism) and, more rarely, in the arteries.

Protein C and protein S disorders can be congenital or acquired. There are two types of protein C deficiency: type 1 relates to quantity and type 2 to function. Protein S exists in two forms, free and bound, but only the free form can bind to protein C. There are three types of protein S deficiency: type 1 is due to an insufficient amount, type 2 to impaired function and type 3 to a shift from the free form to the bound (inactive) form.

Decreased protein C and protein S may be due to insufficient production or increased consumption. Since both proteins are produced by the liver and are vitamin K-dependent, liver disease, vitamin K deficiency or anticoagulant therapy that reduces vitamin K can lead to lower protein C and protein S levels. Various conditions, such as disseminated intravascular coagulation (DIC), which causes clotting and bleeding in various parts of the body, use up coagulation factors including protein C and protein S at an increased rate and so lower their concentration in the blood.

Although inherited mutations in the genes that produce proteins C and S are relatively rare, they can lead to:

• Reduced production of proteins C and S,
• Abnormal proteins that cannot bind to their cofactor (C or S)
• Abnormal proteins that, when bound to their cofactor, do not break down factors VIIIa and Va normally.

When these mutations are present, they are independent of each other, and the mutation usually affects one or the other (C or S). The genetic mutation may be heterozygous (one mutated copy of the C or S gene) or homozygous (both copies mutated). A heterozygous mutation increases the risk of developing VTE, but a homozygous mutation in either of these genes can cause severe clotting: it can lead to purpura fulminans or DIC in newborns and requires close lifelong vigilance against frequent thrombotic episodes.

How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.

How is the test result used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is the test result used?
Protein C and S tests can measure their function (activity) or their quantity. They are normally used to help find the cause of a venous thromboembolism (VTE), especially if the thromboembolic event occurs in young people (under 50) or the clot has formed in an unusual place, such as the veins leading to the liver or kidneys or the cerebral veins.
While the immediate treatment of VTE does not depend on the test result, once the situation has stabilised your doctor will want to find out its cause and how often thrombotic events may recur.

Functional tests for protein C and protein S are usually requested together with other tests for hypercoagulability, to screen factor function. Depending on these results, protein C and protein S concentrations are often requested to detect reduced production due to an inherited or acquired condition and to classify the type of deficiency. If the deficiency is due to an inherited genetic mutation, the amounts of protein C and protein S available and their degree of activity can be used to determine whether the patient is heterozygous or homozygous.

A test showing reduced protein S or protein C activity or quantity should be repeated before a diagnosis is made, because several conditions can cause temporary changes in the levels and function of proteins S and C. If an acquired deficiency is identified, proteins S and C may be monitored from time to time as the disease progresses (as can happen with liver disease) or resolves (as can happen with vitamin K deficiency). An inherited deficiency is not usually monitored, but your doctor will take it into account when you are exposed to situations that increase thrombotic risk, such as surgery, chemotherapy or the use of oral contraceptives.

When is it requested?
Protein S and C tests are requested when a thrombotic episode has occurred, especially in young people (under 50) and/or people with no other obvious reason to develop excessive clotting. However, protein C and S tests should not be requested for at least 10 days after the thrombotic episode, nor while you are taking oral anticoagulants. This means that the doctor will treat the venous thromboembolism by removing the immediate danger of clot formation and putting you on oral anticoagulant therapy for a limited period (usually 3 to 6 months). During this period the doctor may request other tests to look for underlying conditions, such as liver disease, vitamin K deficiency or cancer, that can cause inappropriate clotting.

Once the situation is stable, the doctor will request protein C and S function levels, together with other tests associated with hypercoagulability, to help determine the cause of the clot and assess the risk of recurrence. Protein C and S concentrations may be measured together with their activity to assess whether enough is produced for normal function and, if not, how severe the deficiency is. When an acquired condition is identified, protein S and C levels may be monitored to assess whether the condition is progressing or resolving (whether protein levels have fallen further or returned to near-normal values).

Although protein S and C tests are not recommended as routine tests, they may be requested for close relatives of a person with an inherited protein S or C deficiency, especially if the affected person has a severe form or had their first thrombotic episode at a young age.

What does the test result mean?
High levels of protein C and protein S are not usually associated with medical problems, although protein C has been found to increase with the use of oral contraceptives or during pregnancy. If both the activity and the concentration of proteins C and S are normal, clotting is well regulated. Low levels of protein C or protein S can lead to the formation of clots. If the protein is not functional (normal protein levels but abnormal activity), the clotting process will likewise not be adequately regulated. Both situations increase the risk of developing a clot that obstructs blood flow in the veins (venous thromboembolism, VTE), but how high the risk is depends on how dysfunctional or deficient the protein is.

Decreased protein C and protein S levels may be found in vitamin K deficiency, liver disease, severe infections (inflammatory processes), kidney disease, cancer, disseminated intravascular coagulation (DIC), HIV, pregnancy, immediately after a thrombotic episode, or with warfarin or heparin anticoagulant therapy. These conditions reflect decreased production or increased consumption of protein C and/or protein S. They may be mild and temporary (as in pregnancy) or of variable severity, and acute, chronic or progressive.

Is there anything else I should know?
If there is also a deficiency of other factors such as antithrombin, or inherited conditions such as factor V Leiden or the prothrombin 20210 mutation, the effect of a coexisting protein C or S deficiency can be aggravated. Fresh frozen plasma contains protein C and protein S and can be used as short-term prevention when the patient is undergoing surgery. A protein C concentrate is also available for use as temporary protection.

1. Is there anything I can do to raise my protein S and protein C levels?
2. Should relatives of a person with an inherited protein C or protein S deficiency be tested?

1. Is there anything I can do to raise my protein S and protein C levels?
Not directly. If your deficiency is due to a temporary condition (pregnancy or an infection), levels will return to normal on their own. If it is due to an underlying cause, such as liver disease, that condition needs to be investigated. If you have an inherited protein C or S deficiency, your doctor will advise you to reduce other thrombotic risk factors, for example by lowering homocysteine levels, not smoking and avoiding oral contraceptives. If necessary, fresh frozen plasma, which contains proteins S and C, may be given as short-term prevention (for example before surgery), but this treatment cannot be used routinely.

2. Should relatives of a person with an inherited protein C or protein S deficiency be tested?
Not necessarily. Routine screening is not recommended because the penetrance of the genes is low (this means that even if you have the genetic mutation, you may or may not ever have clotting problems). Your doctor may, however, request this test if you have a strong family history of severe protein S or C deficiency, or a family history of developing thrombosis at a young age.