Protein C and protein S test (PC/PS)
What protein C and protein S are
Protein C and protein S are proteins in the blood that are made by the liver. Normally they work together and help the body hold blood clotting back, so that it does not go too far. The protein C and protein S tests measure how much of these proteins there is in the blood and how well they work.
MedlinePlus states that these tests are used to find the cause of an unexplained blood clot and to assess the risk that more clots may form in the future, to check whether there is a protein C or protein S deficiency, and to establish which type of deficiency it is. A deficiency can be congenital — caused by a gene change inherited from one or both parents — or acquired because of other illnesses, conditions or medicines being taken. The test is also used to monitor protein levels in people whose deficiency-causing conditions have already been diagnosed. The blood sample is taken from a vein in the arm and the procedure is short; if the test is done after a blood clot, you have to wait until you have fully recovered, and the doctor may ask you to stop certain medicines temporarily.
Reference range
For this test, the numeric limits depend on the laboratory's method, and public sources (such as MedlinePlus) do not give a common range — so no number is given here. You will find the range on your own result sheet, and the result is assessed by a doctor.
Possible causes of a deficiency
- Congenital (inherited) protein C or protein S deficiency — because of a gene change passed on by one or both parents
- Liver or kidney disease
- Severe infection or disseminated intravascular coagulation (DIC)
- Vitamin K deficiency
- Cancer or chemotherapy
- Pregnancy, hormonal contraceptive pills or the use of warfarin
| Type of deficiency | What it is caused by | Why the distinction matters |
|---|---|---|
| Congenital (inherited) | A gene change passed on by one or both parents | Whether the deficiency is congenital or acquired is what decides if close family members are worth testing too |
| Acquired | Other illnesses, conditions or medicines — for example liver or kidney disease, severe infection or DIC, vitamin K deficiency, cancer or chemotherapy, pregnancy, hormonal contraceptive pills or warfarin | Which conditions or medicines (for example liver disease, warfarin) may have affected the result is assessed by the doctor |
When the test is done
- When there is a blood clot that appeared before the age of 50 without a clear cause (MedlinePlus)
- When a blood clot appears in an unusual place — for example in the veins of the kidney, liver or brain (MedlinePlus)
- When there have been blood clots several times (MedlinePlus)
- When treating a clotting disorder, for example an already diagnosed protein C and/or protein S deficiency (MedlinePlus)
- When there are cases of inherited deficiency or of early blood clots in the family, or a newborn has been diagnosed with a clotting disorder (MedlinePlus)
- When there are conditions or medicines that could cause a deficiency — for example liver or kidney disease, severe infections (MedlinePlus)
How to prepare
- If the test is done after a blood clot, you have to wait until you have fully recovered — if the test is done too soon, the results will not be accurate (MedlinePlus)
- The doctor may ask you to stop blood-thinning medicines temporarily before the test — but no prescribed medicine may be stopped on your own, only after talking to the doctor (MedlinePlus)
A low result: what it means
A reduced amount, or poorly working protein C or protein S, means a higher risk that a blood clot will form. According to MedlinePlus, a deficiency can be congenital — because of an inherited gene change — or acquired because of other illnesses, conditions or medicines. The result does not establish a diagnosis on its own; the cause and the next steps are assessed by a doctor.
Possible signs
- A warning sign of a blood clot in the leg: sudden swelling or pain in a leg — see a doctor without delay
- Chest pain — can be a sign of a clot in the lungs, which is life-threatening: call 112
- Sudden difficulty breathing — can be a sign of a clot in the lungs: call 112
What to discuss with the doctor
- Whether the deficiency is congenital or acquired — this decides whether it is worth testing close family members too
- Which conditions or medicines (for example liver disease, warfarin) could have affected the result
- How to act in risky situations — when travelling, after an operation, during pregnancy
- Which warning symptoms require urgent help
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Frequently asked questions
›What are the protein C and protein S tests?
They are blood tests that measure how much of two proteins made by the liver there is and how well they work. According to MedlinePlus, proteins C and S together help hold blood clotting back, so that it does not go too far.
›When does a doctor order the protein C and protein S tests?
Most often when looking for the cause of an unexplained blood clot — when the clot appeared at a young age, in an unusual place or has recurred — or when checking for an inherited or acquired deficiency (MedlinePlus).
›What does a low protein C or protein S level mean?
MedlinePlus states that a lower-than-normal amount of protein C or protein S, or poorly working proteins, means a higher risk of a blood clot forming. The cause and the next steps are assessed by a doctor.
›Is a raised protein C or protein S level dangerous?
No — MedlinePlus states that a higher-than-usual amount of protein C or protein S is not known to cause health problems.
›What is the difference between a congenital and an acquired protein C or S deficiency?
A congenital deficiency is caused by a gene change inherited from the parents. An acquired deficiency develops because of other illnesses or conditions — for example liver disease, a severe infection, vitamin K deficiency, pregnancy or certain medicines (MedlinePlus).
›How should I prepare for the protein C and protein S test?
MedlinePlus states that if the test is done after a blood clot, you have to wait until you have fully recovered, and the doctor may ask you to stop certain medicines temporarily.
›Are there any risks in this blood test?
MedlinePlus states that the risk is very small — there may be short-lived pain or a bruise at the puncture site, which pass quickly.
Sources
- MedlinePlus: Protein C and Protein S Tests — 2026-09-07
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.