\[ \text{INR} = \left( \frac{\text{Patient's PT}}{\text{Mean Normal PT}} \right)^{\text{ISI}} \]
where PT stands for prothrombin time and ISI is the International Sensitivity Index. The prothrombin time is a measure of how quickly blood begins to clot, and the ISI is a correction factor that accounts for variations between different testing reagents and equipment.
