The anticoagulation clinics at the University of Virginia Health Sciences Center and the University of California at Davis Medical Center are nurse-practitioner-operated, are affiliated with the general medicine clinic, and rely on portable prothrombin time (PT) monitors that use whole blood and provide timely as well as accurate results reported in PT seconds or as the international normalized ratio (INR). On-site PT/INR testing at these clinics simplifies anticoagulation, mandates direct patient contact, and facilitates primary as well as comprehensive care for patients requiring multispecialty services in large tertiary care centers. Encounters are relatively brief, averaging 19 minutes; 72% of the encounter time involves anticoagulation care and 28% involves primary care. Anticoagulation results using portable PT/INR monitors are safe and accurate based on comparisons with results from clinics relying on standard instruments.