INR, Standardized Prothrombin Time Calculation

 International Normalized Ratio(INR) Calculator

This online calculator is used to figure out the prothrombin time or INR ratio.


International Normalized Ratio (INR) Calculation Definition, Formulae

The INR is a standardized prothrombin time designed to account for differences in thromboplastin.

Anyone may need this test if you take medicines, such as warfarin, that change the way your blood clots.


INR = (PTpatient/PTnormal)ISI


PTpatient = Patients Measure PT (seconds)

PTnormal = Laboratorys Geometric Mean Value for Normal Patients (seconds)

ISI = International Sensitivity Index


The PT of a patient receiving oral anticoagulant is 64 seconds (= 18% Quick).

The prothrombin time of a normal plasma is 22 seconds (= 100% Quick).

The ISI of the thromboplastin used is 0.93. Substituting this value in the formula above gives the following INR:

INR = (64)/(22)0.93

  = 2.7 INR

This signifies a coagulation time that is 2.7 times longer than the standard. The longer the patients coagulation time, the higher the INR

INR (although this value has no units, it is a ratio, it is usual to express INR results with INR as the unit)

PT/INR monitoring is commonly done at intervals of three to four weeks.