What Causes Pruritus While Using Caumadin?

My father has been on coumadin for several years now. He has had puritis for quite some time now and we have not been able to find the source. He has seen a dermatologist as well as an allergist and we still haven't found a source after multiple test. He was off coumadin for 4 days now in preparation for a heart cath and has not had any itching. Could he have developed an allergy to coumadin after several year?