Q: Combination of Argatroban and Warfarin can give falsely very high INR. What would be the best method to know that INR would be in therapeutic range if Argatroban is discontinued? 

Answer: Measurement of chromogenic factor X level

The combination of argatroban and warfarin may raise the INR falsely to value higher than even beyond 5.0. Measuring chromogenic factor X level may help in smooth transition.  Once level of chromogenic factor X is below 40-45%, it typically indicates that the INR will be therapeutic (2-3) when the argatroban is discontinued.

Reference:

Hursting MJ, Lewis BE, Macfarlane DE. (2005). "Transitioning from argatroban to warfarin therapy in patients with heparin-induced thrombocytopenia.". Clin Appl Thromb Hemost 11 (3): 279–87.