Argatroban, a direct thrombin inhibitor used for thromboprophylaxis or treatment in heparin-induced thrombocytopenia (HIT), is routinely monitored using the activated partial thromboplastin time (aPTT) yet also prolongs the international normalized ratio (INR). Peritransitional INRs, aPTTs, anticoagulant dosing patterns, and outcomes were evaluated in 165 HIT patients who were transitioned, without guidelines, from argatroban to warfarin therapy. Argatroban (median doses: 1.5-2.0 mcg/kg/min) and warfarin (median dose: 5 mg initially with 3.8 mg/day thereafter) overlapped a median 4 days. Median (5-95th percentile) aPTTs (in seconds) and INRs, respectively, were 59.8 (38.8-82.9) and 3.2 (1.7-7.0) during argatroban monotherapy, 68.6 (44.5-104) and 5.3 (2.4-16) maximally during cotherapy, 59.9 (38.7-92.2) and 4.0 (2.2-11.6) immediately before argatroban cessation during cotherapy, and 36.0 (25.6-60.2) and 2.3 (1.3-7.3) within a median 10-12 hours after argatroban cessation. Major bleeding occurred in 1 (0.6%) patient pretransitionally and no patient during or after cotherapy. Eighteen (10.9%) patients experienced 19 peritransitional adverse outcomes (one death, two amputations, 16 new thromboses); these patients had more severe HIT than event-free patients (median baseline platelet count, 39 vs. 83 x 10(9)/L). Of 108 patients with post-transitional INR data, 43 achieved a therapeutic INR (prospectively defined as 1.9-3.5), 34 were subtherapeutic, and 31 were supratherapeutic, with no across-group trend in new thrombosis. Hence in the absence of guidelines, physicians transfer patients from argatroban to warfarin therapy with acceptably low complication rates in HIT, without systematically over- or under-dosing warfarin. Furthermore, INRs greater than 5 commonly occur in HIT patients during argatroban monotherapy and argatroban/warfarin cotherapy, without major bleeding.