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.

Certain medications can interfere with this test. Your doctor may ask you to discontinue these medications for a time prior to the test. Be sure to tell you doctor all medications and supplements you are taking prior to taking this test.

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

• Infant test or procedure preparation (birth to 1 year)
• Toddler test or procedure preparation (1 to 3 years)
• Preschooler test or procedure preparation (3 to 6 years)
• Schoolage test or procedure preparation (6 to 12 years)
• Adolescent test or procedure preparation (12 to 18 years)

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, the test is used to investigate repeated fetal loss.

Deficient protein C can result in excessive clotting. These clots tend to form in veins, not arteries.

Protein C deficiency can be either inherited or acquired in association with other conditions. Liver disease, disseminated intravascular coagulation (DIC), warfarin, and chemotherapy can result in decreased protein C levels

• 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

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