INR, Standardized Prothrombin Time Calculation
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International Normalized Ratio(INR) Calculator
This online calculator is used to figure out the prothrombin time or INR ratio.
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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 = (PTpatient/PTnormal)ISI
Where,
PTpatient = Patients Measure PT (seconds)
PTnormal = 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.
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