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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: Protein S Ag

Test Code(s): PS-AG / XPSAG

CPT Code(s): 85305

Test Description: Test Component:

Protein S Antigen, Factor X Antigen and Protein S:Factor X Ratio

Methodology: ELISA

Clinical Significance: A vitamin K dependent protein. Reported as a ratio to Factor X antigen to aid in interpretation of patients receiving warfarin.

Request Form: UWHC Special Coagulation Outreach Request Form (UWH SCLR-O)

Days Performed: Wed.

Turnaround Time: Stat: Not available stat.
Routine: 7 - 10 working days.

Specimen Requirements

Specimen: Blood

Patient Preparation: Patient must be off warfarin therapy for at least one week or on a stable dose of warfarin for at least one week.

Collection Instructions: Do NOT draw through a line containing heparin. The venipuncture MUST be atraumatic to avoid hemolysis and contamination with tissue factors and platelets. Tube must be allowed to fill completely.  

Indicate if patient is receiving anticoagulant (warfarin/Heparin/Vitamin K antagonist), direct thrombin inhibitor or fibrinolytic agent therapy.

Collection Container: Preferred: Light blue top (3.2% NaCitrate)

Collection Volume: Preferred: Full 3.5 mL tube
Pediatrics: Pediatric tube filled to fill mark

Sample Analyzed: Plasma

Volume Required: Preferred: 1.5 mL
Pediatrics: 1 mL

Specimen Processing: Centrifuge specimen to yield platelet poor plasma (platelet count should be less than 10 K/uL). Separate plasma and transfer to plastic tube or vial. Freeze plasma within 4 hours of collection at -20C or below.  

Specimen Transport: Transport whole blood specimen at room temperature to UWHC Core Laboratory (B4/220) within 4 hours of collection. See Specimen Processing for instructions if transport criteria are not met. Transport frozen plasma aliquot on dry ice to UWHC Core Laboratory (B4/220).

Outreach Specimen Transport: Centrifuge specimen to yield platelet poor plasma (platelet count should be 10 K/µL or less). Separate plasma and transfer to labeled plastic tube and spin again. Transfer plasma from second spin to clean plastic labeled tube or vial. Indicate on tube "spun twice". Freeze plasma within 4 hours of collection at -20°C or below.

Unacceptable Criteria: Hemolyzed specimens, clotted specimens, partially filled tubes and whole blood specimens greater than 4 hours old when received by the laboratory are not acceptable. Specimens from patients who have not been off warfarin therapy for at least one week or on a stable dose of warfarin for at least one week are not acceptable.

Stability: Ambient: 4 hours on spun tube
Refrigerated: Not acceptable
Frozen: 2 weeks at -20C; 6 months at -70C


Reference Interval:
Adults age 12 months and up:
Protein S Antigen:


Factor X Antigen:                 70-200 %
Protein S/Factor X Ratio:                 0.6-3.0
Protein S Antigen  
0-4 days:  20-60%
5 days-2 months: 35-85 %
3-11 months: 45-100 %

Test Limitations:

Results may be affected if patient has developed Human Anti-Murine Antibodies (HAMA) due to treatment or tumor imaging utilizing murine based products.

Additional Information:

The performance characteristics of the Factor X Antigen were validated by UWHC Clinical Laboratories. The U.S. Food and Drug Administration (FDA) has not approved or cleared this test; however, FDA approval or clearance is currently not required for clinical use of this test. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions. The UWHC Clinical Laboratories is authorized under Clinical Laboratory Improvement Amendments (CLIA) to perform high-complexity testing.


An ELISA test for the free portion of the Protein S antigen is also available (see Protein S Ag,Free).  A clotting test for Protein S (functional portion) is also available (see Protein S Activity).  Note: This is NOT the test for PSA (Prostate Specific Antigen).

Total Protein S

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