How is the INR calculated?

The International Normalized Ratio (INR) is calculated to standardize prothrombin time (PT) results across different laboratories and testing methods. The INR is specifically derived from the ratio of the patient’s PT to a control PT (which is a standard value that reflects normal coagulation).

When determining the INR, the patient’s PT is divided by the control PT. This ratio is then adjusted using a specific mathematical formula that includes the International Sensitivity Index (ISI) of the test, which helps to account for variances in the thromboplastin used for the PT. This standardized approach allows healthcare providers to effectively monitor patients who are on anticoagulant therapy, ensuring consistent and comparable results regardless of the testing laboratory.

The other options do not represent the correct methodology for calculating the INR. For example, dividing the control PT by the patient PT would yield a value that is the inverse of what is needed, and the other options involving PTT (partial thromboplastin time) are irrelevant, as INR specifically pertains to PT measurements.