INR

INR stands for International Normalized Ratio. It is a standardized measure of how long it takes blood to clot, derived from the prothrombin time (PT) test. The INR adjusts the PT result to account for variations in laboratory reagents and instruments, using the formula INR = (PT_patient / PT_normal)^ISI, where ISI is the International Sensitivity Index of the thromboplastin reagent.

In clinical practice, INR is primarily used to monitor anticoagulant therapy with warfarin and to guide dosing to keep the patient within a specified therapeutic range. Target ranges vary by indication and patient risk, but commonly fall around 2.0 to 3.0 for many conditions; alternative targets may be used for certain mechanical heart valves or other situations.

Interpretation of INR values follows the principle that higher values indicate slower blood clotting and increased bleeding risk, while lower values indicate faster clotting and higher thrombosis risk. Regular monitoring helps maintain the balance between preventing clots and avoiding excessive bleeding. Monitoring intervals depend on stability and indication but may range from every few days after initiating therapy to every few weeks once stable.

Factors that can affect INR include changes in warfarin dose, interactions with medications and herbal supplements, dietary vitamin K intake, liver disease, acute illness, kidney disease, and adherence. Lab variability and sample handling can also influence results.

Limitations: INR is specific to vitamin K antagonist anticoagulants such as warfarin and is not routinely used for direct oral anticoagulants. It does not directly measure bleeding risk and is one component of overall clinical assessment. History and development of INR emerged to standardize PT results across laboratories and reagents.