The International Normalized Ratio (INR) standardizes prothrombin time results so that a clotting-time measurement from one lab means the same thing as a measurement from another lab using a different reagent. It is the key number used to monitor warfarin and other vitamin K antagonist therapy, and this calculator computes it directly from patient PT, control PT, and the reagent's ISI.
How the INR formula works
The formula is INR = (Patient PT / Control PT) ^ ISI. Prothrombin time on its own varies between labs because different thromboplastin reagents and coagulation instruments respond differently to the same blood sample. The World Health Organization addressed this by assigning each reagent/instrument combination an International Sensitivity Index (ISI) through calibration against an international reference thromboplastin, first formalized in WHO Technical Report Series 889 (1999).
Raising the PT ratio (patient PT divided by the lab's control PT) to the power of the ISI corrects for the reagent's responsiveness, producing a value — the INR — that means approximately the same thing regardless of which lab or reagent generated the underlying PT.
Inputs and what they mean
Patient PT is the measured prothrombin time for the patient's blood sample, in seconds. Control PT is the lab's mean normal PT — established from a panel of healthy donors on the same reagent and instrument — also in seconds. ISI is the reagent-specific sensitivity index, unitless, found on the thromboplastin package insert or the lab's own quality documentation, typically between about 0.9 and 1.7.
Because ISI is an exponent, small differences in its value can noticeably shift the resulting INR, especially at higher PT ratios — this is exactly why standardization via ISI matters more as the PT ratio moves further from 1.0.
Limits and edge cases
This calculator computes the INR value only — it does not provide warfarin dosing guidance. The appropriate target INR range varies by clinical indication (for example, 2.0-3.0 for atrial fibrillation or venous thromboembolism versus a higher range such as 2.5-3.5 for some mechanical heart valves), and dosing decisions depend on bleeding risk, drug interactions, diet, and other patient-specific factors that only a treating clinician can weigh. A single INR reading is a snapshot, not a trend, and unusually high or low values should prompt confirmation and clinical correlation rather than an immediate dosing change. Point-of-care INR meters can also read differently from laboratory-based testing in some patients; when results are discordant with the clinical picture, a laboratory-confirmed PT/INR is preferred.