The international normalized ratio (INR) is the preferred parameter for monitoring patients taking vitamin K antagonists. This variable is also used to assess the risk of bleeding and to evaluate the coagulation status of these patients. Individuals on oral anticoagulants must monitor their INR values to adjust doses of vitamin K antagonists. The INR is derived from the prothrombin time (PT) and calculated as the ratio between the patient's PT and 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)ISI, where ISI is the international sensitivity index.
PT, measured in seconds, represents the time required for plasma to form a clot in the presence of a sufficient concentration of calcium and tissue thromboplastin, which activates coagulation through the extrinsic pathway. The reference values for INR account for variations in PT measurement due to device-related differences, the type of reagents used, and sensitivity differences in the tissue factor activator. INR is dimensionless, and values for individuals without health issues are typically 1.1 or lower.
Optimizing a patient's INR and ensuring it remains within the therapeutic range can be challenging, as vitamin K antagonists have a narrow therapeutic window. Factors such as the patient's characteristics, comorbid conditions, diet, and intake of potentially interfering medications can affect the INR. Patients are typically monitored every 3 to 4 weeks or more frequently at thrombosis centers, point-of-care (POC) clinics, or in-home settings.
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