Clinical Topics
Lab Notes
Prothrombin Time with International Normalized Ratio (INR) for Monitoring Anticoagulated Patients
The World Health Organization along with the International Committee on Thrombosis and Haemostasis recommend that reporting of PT results for patients on oral anticoagulant therapy include the use of INR (International Normalized Ratio) values. Reported INR results are independent of the reagents and methods used and are specifically intended for assessing patients stabilized on long-term oral anticoagulant therapy. INR is the result of the patient PT/mean normal PT, exponentially equalized by the ISI. ISI is the International Sensitivity Index of the reagent/instrument combination.
Utilizing the INR provides for a reliable monitoring value irrespective of the performing laboratory's reagents or instruments.
For normal patients (patients not on anticoagulant therapy) with present patient PT/control ratio equal to 1:0, the INR value is also going to equal 1.0 regardless of the ISI or the particular performing laboratory. Properly anticoagulated patients INR values will range between 2.0-3.0. However, if the patient is being treated for recurrent embolism or have mechanical prosthetic heart valves a range of 2.5-3.5 is desirable. INR levels above 4.9 are considered Critical values and are immediately phoned to the Physician.