59-year-old male with polycythemia vera & elevated PSA. He needs biopsy. Is it safe to discontinue his Coumadin (warfarin) for one week?

Perhaps. This depends on the reason for his warfarin and also his history. If he has a mechanical mitral valve then one may not want to stop his warfarin or you would need to bridge him. PCV also increases his risk for thrombosis. If it is for atrial fibrillation and he has no other significant risk factors or mechanical valves then it may be reasonable to discontinue for a brief period of time.