Kristi Williams, a participant in George's online Michigan State University (go Sparties!) graduate Hemostasis course, iasked this excellent question:
I suspect practitioners in our area don’t necessarily always know or understand when the protein C and S activity vs. antigen tests should be ordered. It’s clear that protein C and S activity level assays are not valid if the patient is on various anticoagulants, have a LA , or have experienced a thrombotic event in the past 10–14 days. For the antigen assays, they are then performed when there is a low activity level and acquired causes of thrombosis are ruled out. Is there ever a valid instance where a protein S or C antigen test should be ordered in absence of the corresponding activity test?. For example, if the patient had a recent thrombotic event and the doc wants to evaluate protein C and S and the patient has never had this evaluated before, should the doc wait until the patient is out of that 10–14 day anticoagulant or thrombotic event window and then order the protein C activity test?
George answered that he has found that Coumadin reduces both the protein C and protein S antigen immunoassay and activity results. This doesn't make a lot of sense when you know the mechanism, however it is a consistent finding. George concludes that the antigen as well as the activity assays should be postponed until after 10–14 days after treatment is discontinued.
Please comment and confirm or refute George's observation.