PROS1 and deep vein thrombosis: Is this a correct evaluation by his primary care physician?<h4>Conclusion</h4>This case illustrates that Coumadin, a vitamin K agonist, was exerting a therapeutically acceptable negative influence on plasma activity levels of vitamin K-dependent protein C and protein S. Relying on the outcome of a thrombophilia work-up for a decision to maintain or cease Coumadin treatment of patients at risk for recurrent deep vein thrombosis has pitfalls that can be avoided.