International normalized ratio (INR) is the preferred test of choice for patients taking vitamin K antagonists (VKA). It can also be used to assess the risk of bleeding or the coagulation status of the patients. Patients taking oral anticoagulants are required to monitor INR to adjust the VKA doses because these vary between patients. The INR is derived from prothrombin time (PT) which is calculated as a ratio of the patient’s PT to a control PT standardized for the potency of the thromboplastin reagent developed by the World Health Organization (WHO) using the following formula:
- INR = Patient PT ÷ Control PT
PT, the time in seconds, is measured in plasma to form a clot in the presence of sufficient concentration of calcium and tissue thromboplastin by activating coagulation via the extrinsic pathway. The reference values for INR take into account in PT measurement in device related variations, type of reagents used, and sensitivity differences in the TF activator. INR value is dimensionless and ranges from a score of 2.0 to 3.0. Optimizing the patient’s INR therapeutic range can be challenging as narrow therapeutic range had been seen in VKAs and can be affected by patient's characteristics, co-morbid conditions, diet, and other drug interactions. Patients are monitored every 3–4 weeks or less at the thrombosis centers (TC), point-of-care (POC) clinics, or in the home setting.