Heparin is a fast acting anticoagulant where you can achieve anticoagulation with in 24 hours. Whereas coumadin will take at least 3-5 days before achieving a therapeutic level. During hospitalization, depending upon the reason for anticoagulation, most clinician prefers to initiate coumadin and discharge pt once INR is therapeutic. During the days when INR still subtherapeutic, Heparin is used to bridge the gap. Of note, Heparin does not affect PT/INR but PTT.