Diagnostic Tests

Protein S Antigen


Protein S Antigen

Also known as Protein S Free Blood
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What is this test?

Protein S Free Blood Test measures the levels of free protein S in the blood. This test helps to determine the protein S deficiency types and evaluate inappropriate clot formation in the blood.

What is Protein S?

Protein S is one among two proteins in the blood that regulates the clot formation in the blood. Normally, in case of an injury in order to stop the bleeding small cell fragments called platelets and clotting factors aggregate to form a plug at the injury site. This clot which is formed will prevent the blood loss and remains in place until the injury has healed. It is then broken down once the injury has healed.

Protein S helps to control the blood clot formation by inactivating specific coagulation factors (factors V and VIII) that are necessary to form blood clots. If there are not enough Protein S in the blood, it may lead to excessive clotting. This protein is found in two forms, free and bound. The free protein S combines with protein C, another protein in the blood and regulates the clot formation in the blood.

Inadequate levels of Protein S can be due to many disease conditions such as liver disease, kidney disease, cancer or severe infections. Some individual may have an inherited protein S deficiencies. There are three types of inherited protein S deficiencies:

  1. Type 1 - This type occurs due to not sufficient quantity of Protein S.
  2. Type 2 - This type occurs due to abnormal function (Reduced Protein S activity).
  3. Type 3 - This type occurs due to low free protein S levels even in presence of normal total protein S levels.

Assessing the levels of free protein S in the blood helps to analyze whether the blood clot formation is normal or not. It also helps to differentiate the types of inherited protein S deficiencies.

Why this test is performed?

This test is performed to determine whether you may have a protein S deficiency or inappropriate blood clotting. Your doctor may ask to perform this test in case if you have any clotting disorders or had an unexplained blood clot. This test is one among many other tests to differentiate the types of inherited protein S deficiencies. You may be advised to perform this test if you have a family history of protein S deficiency disorders.

How often this test is performed?

If you have a family history of inherited protein S deficiencies or clotting disorders your doctor may ask to perform this test in a 6 monthly or a yearly basis. If you are diagnosed with inherited protein S deficiencies or clotting disorders, then you may have to perform this test on a regular basis as instructed by the doctor.

Also known as Protein S Antigen Blood, Protein S Antigen Free Blood, Protein S Antigen Free, Protein S Free.

Test Preparation

Inform your doctor if you are on any medications, have any allergies or underlying medical conditions before your Protein S Antigen. Your doctor will give specific instructions depending on your condition on how to prepare for Protein S Antigen.

You may have to wait at least 10 days after an inappropriate blood clotting episode and stop taking oral warfarin anticoagulant therapy for two weeks prior to the test. However, follow all the instructions given by your healthcare provider.

Understanding your test results

If the test results show low levels of free protein S than the normal range in the blood it may indicate that you may have inappropriate blood clotting. Individuals with Type 1 and 2 inherited protein S deficiencies may also have low levels of free protein S in the blood. However, your doctor may ask to perform a few more additional tests to confirm the diagnosis.

Based on the test results, your doctor may advise appropriate medical treatments or further diagnostic tests.

GenderAge groupsValue
UNISEXAll age groupsNormal value for Free Protein S is 40% of Total Protein S
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