What is your approach to persistently low INR despite escalating doses of warfarin in a patient with bioprosthetic mitral valve replacement who is unable to be on DOACs?

The assumption here is that the mitral valve is mechanical, or there are presumably other reasons that a DOAC cannot be used. If this is, in fact, a mechanical mitral valve, it would make sense to start with low molecular weight heparin injections as soon as feasible while awaiting a therapeutic INR. In fact, I would start much sooner than 4 weeks, no longer than 1, given the higher risks of mechanical mitral valve thrombosis.

VKOR testing would only have been useful if done in advance, given th...

Depends on whether it is mechanical vs. bio prosthetic.