Protein S

This test measures the amount of protein S in the blood. Deficiencies may lead to the formation of Blood clots in arteries or veins.

How the test is performed 

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test 

Special preparation is usually not necessary.

For infants and children:
The preparation you can provide for this test depends on your child’s age and previous experience. For general information regarding how you can prepare your child, see the following topics:

How the test will feel 
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed 

Protein C and protein S are involved in regulation of blood clotting. Deficiencies may lead to the formation of Blood clots in arteries or veins.

Sometimes this test is used to investigate repeated fetal loss.

Normal Values
Normal Values
are 60-150% inhibition.

What abnormal results mean

Deficient protein S can result in excessive clotting tendencies. These are usually vein clots, such as deep vein thrombosis (DVT), but occasionally this disorder can be associated with arterial clots.

A protein S deficiency may be inherited. It can also be acquired due to pregnancy or certain diseases, including disseminated intravascular coagulation and HIV infection.

What the risks are

  • Excessive bleeding  
  • Fainting or feeling light-headed  
  • Hematoma (blood accumulating under the skin)  
  • Infection (a slight risk any time the skin is broken)  
  • Multiple punctures to locate veins

Special considerations

Protein C and protein S are decreased by some anticoagulants (Coumadin or warfarin). Measurements of protein C or S may be difficult to interpret in patients taking anticoagulants.

During a clot-related event like a Pulmonary embolism, proteins C and S are reduced anyway, and their measurements may be misleading until the episode is resolved.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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