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Protein S is one of the many vital proteins in the human body. It plays a large role in controlling your blood clotting process. Your blood’s ability to clot is very important. It prevents excessive blood loss when an injury occurs. However, a blood clot in an artery or a vein (called thrombosis) can be extremely dangerous.
Your body contains coagulants and anticoagulants. Coagulants encourage clotting, while anticoagulants help prevent it. Protein S is an anticoagulant. If there is not enough of it, a harmful type of blood clot may form. The correct amount of protein S is needed to ensure your blood clotting process functions properly.
If you develop a blood clot, your doctor will often order a full evaluation of your clotting factors. Low levels of protein S is one of the many potential problems in the clotting system.
One of the most common reasons your doctor may want you to do a protein S test is that you have developed a blood clot in your leg or lung. Having several unexplained miscarriages may also prompt your doctor to check your body’s clotting factors.
Certain medical conditions can cause protein S levels to drop, including:
In some instances, a protein S deficiency is inherited. Some people are simply born with a shortage of this particular anticoagulant. Your doctor may order testing if you have one or more close family members with a history of dangerous blood clots or if someone in your family has a known protein S deficiency.
For most people with a protein S deficiency, a potentially dangerous blood clot is often the first sign that something is wrong. The clot most often appears in the leg or lung, and there are usually no symptoms leading up to the event.
If you develop a blood clot (thrombosis) in a vein or artery, your doctor will often test your protein S level. This can help them determine the cause of the thrombosis. Clots associated with a lack of protein S tend to form in veins.
A protein S deficiency does not always mean you will develop thrombosis. If you have this deficiency, it is possible to go through your entire life without a problem.
Your doctor will evaluate your medical history and medication use before the test to decide when it should be done and if you need to do anything to prepare.
The test should not be done during an active clotting event because having a blood clot will naturally lower protein S levels, making the test results inaccurate.
You will also need to stop taking anticoagulants for a minimum of two weeks before testing, to ensure accurate results. Never stop taking anticoagulation drugs without your doctor’s approval.
You will need to provide a blood sample for your protein S measurement. Your doctor will insert a needle into one of your veins and collect a sample of your blood in a vial. You may experience some minor pain as the needle is being inserted and some soreness afterward. Serious complications are rare.
Your doctor will interpret your results and discuss any abnormalities with you, as well as the diagnosis if there is one. Results are usually presented in terms of percent inhibition. These percentage values should usually fall between 60 and 150.
There might be slight differences among testing facilities. High levels of protein S are not typically cause for concern, whereas low levels may increase your risk of blood clots. Follow-up testing is often recommended to confirm the diagnosis.
If a protein S deficiency does exist, follow-up steps will depend on the cause. Sometimes there is another condition causing protein S levels to be lower than they should be. In these cases, addressing the underlying condition is the logical next step.
For those with an inherited deficiency, the focus will usually be on reducing or eliminating risk factors for clots. Lifestyle changes, such as quitting smoking, exercising often, maintaining a healthy weight, and avoiding estrogen-containing medications, are some ways to lessen the chances that a lower-than-optimal amount of protein S will lead to a dangerous clot.
Written by: Krista O'Connell
Medically reviewed on: Jan 05, 2017: Judith Marcin, MD
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