Near-patient testing devices (monitors) capable of measuring prothrombin time on an unmeasured drop of blood would be suitable alternatives to centralized laboratory monitoring of patients receiving oral anticoagulants. The essential prerequisite for the use of these monitors is their conformity to the international sensitivity index (ISI) calibration model recommended by the World Health Organization. We report on the ISI calibration of the ProTime monitor (International Technidyne, Edison, NJ) designed and approved for patient self-testing. According to our results, this monitor can be calibrated by adopting the model already used for other monitors. The apparent ISI was close to unity. Overall, the international normalized ratio values displayed by the monitor agreed with those measured with the international reference preparation for thromboplastin. Confirmation of these results in a large multicenter study is warranted.