We enrolled 386 subjects in a multicenter study of a point-of-care (POC) prothrombin time (PT) testing device. POC tests were performed by health care professionals using venous and finger-stick specimens and by patients using finger-stick specimens. Venous blood also was analyzed in the local hospital laboratory and a national reference laboratory. Accurate POC results were obtained by professionals using both types of specimens. Patients' results were equivalent to those of professionals. The identification of the patient's therapeutic status based on the International Normalized Ratio (INR) was equivalent for POC and local hospital laboratory PT results; 75% of local laboratory results and 77% of POC results were within 0.4 INR of reference laboratory results, while 93% of either system (POC or local laboratory) were within 0.7 INR. Patients overwhelmingly reported satisfaction with the self-test, including the finger stick and device operation. The INR from the POC device is clinically equivalent to the laboratory INR for assessment of anticoagulation status and management decisions in professional and self-testing environments. Patients can learn to perform accurate PT testing, and POC PT testing is feasible in patients' homes.