How is functional activity of protein S tested?
Many labs perform functional clotting assays to test free PS activity. These will detect PS deficiency, but will not discriminate between Types I, II, and III. Clotting tests utilize an excess of protein S deficient plasma, thereby keeping the amount of prothrombin essentially constant. APTT and PT methods commonly used. In a FXa based method, coagulation is triggered by FXa in the presence of calcium ions and phospholipids. This method is not currently used in routine settings, however.