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.

Formula:

INR = (PT_patient/PT_normal)^ISI

Where,

PT_patient = Patients Measure PT (seconds)

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

ISI = International Sensitivity Index

Example

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.