Compute INR from PT and ISI for anticoagulation monitoring.
The International Normalized Ratio standardizes prothrombin time across laboratories. INR = (Patient PT / Mean Normal PT)^ISI. The therapeutic range for most indications (e.g., atrial fibrillation, DVT) is 2.0–3.0. Mechanical heart valves may require 2.5–3.5. An ISI of 1.0 means the reagent is equivalent to the WHO reference.