Protein S Antigen:Factor VII Antigen Ratio

Plasma, frozen

2 mL

1 mL

Blue-top (sodium citrate) tube

Do not draw from an arm with a heparin lock or heparinized catheter.

This assay is used to obtain a presumptive diagnosis of protein S deficiency while a patient is receiving antivitamin K therapy. Use of this assay is discouraged.

This assay may not detect rare qualitative types of protein S deficiency.

This procedure may be considered by Medicare and other carriers as investigational and, therefore, may not be payable as a covered benefit for patients.

Enzyme-linked immunosorbent assay (ELISA)

Because it is vitamin K dependent, the protein S level begins to decrease 60 hours after oral anticoagulant warfarin therapy is begun.⁶ When oral anticoagulant is stopped, its effects on protein S may be seen for up to two weeks or longer. For several days after surgery or a thrombotic event, the protein S levels may be diminished even in the absence of oral anticoagulants; during this period, assays cannot be used to establish a congenital deficiency. The total protein S antigen concentration assay:factor VII antigen concentration assay ratio may be used to determine a presumptive protein S deficiency in patients on oral anticoagulant therapy if clinical indications prevent suspension of therapy. A ratio ≤0.5 suggests protein S deficiency, however, deficiency cannot be ruled out when the ratio is ≥0.6, as a functional protein S deficiency will not be detected using this method.