Abstract
Protein S (PS) is a Vitamin K-dependent protein that functions as a cofactor for the regulation of the coagulation system. PS works in conjunction with Activated Protein C to inactivate factors V and VIII. PS circulates in plasma either complexed to the complement protein, C4b Binding Protein or unbound. The unbound (or free) component is the functional form for the regulation of the coagulation system. PS can be measured in plasma by functional activity, the free (or unbound form) or both free and bound fractions (Total PS). The test most widely used for clinical evaluations is the Free PS Antigen assay (which is the surrogate of PS anticoagulant activity) and represents the protocol described in this chapter. The Free PS Antigen assay is an immunologic assay which specifically measures the unbound fraction of PS in test plasma. Other methods for assessing PS are also available, including PS activity and total PS Antigen assays, but protocols for these assays are not provided.
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