The INR is a method of standardizing the Prothrombin time across different labs.
Since different labs use different machines and reagents to calculate the Prothrombin time, the INR allows these values to be normalized so that they can be compared.
It also depends on the specific reagent of chemical used for the test. The INR is derived from the mean Prothrombin Time value of a cohort of patient samples, using a factor called the ISI (provided by the equipment manufacturer)