Ritonavir has been predicted to raise
warfarin levels and INRs. This was confirmed by one case report, whereas two others found entirely the opposite. A case report suggests that indinavir might cause a moderate reduction in anticoagulation. There is also a report of a marked reduction in anticoagulation in a patient taking acenocoumarol with
ritonavir or nelfinavir. An isolated report describes a gradual INR rise in an elderly patient taking
warfarin with saquinavir.
It would seem prudent to monitor any anticoagulant/protease inhibitor combin- ation for an alteration in anticoagulant effect, although current evidence tends to suggest that a clinically significant interaction is rare.