In blood testing, INR stands for international normalized ratio, states WebMD. A prothrombin-time blood test gauges the blood's ability to clot by looking at five factors, while the INR system ensures consistency in all PT test results, regardless of the locations of their testing labs or their methods.
The INR and the PT form part of a group of tests known as the coagulation study, says Jennifer Whitlock of About.com. Doctors perform these tests before surgery to check for the possibility of bleeding or clotting complications during and after surgery. Normal INR values are 0.08 to 1.1, while the normal PT results are 11 to 13 seconds, states WebMD.