What does PT/INR stand for?

Prothrombin Time / International Normalized Ratio

The INR is calculated using the following formula and Is a ratio of Prothrombin Times that doctors or pharmacists use to adjust your Coumadin® dosage.


Prothrombin Time or Protime-Measures the amount of time, in seconds, that it takes for your blood to clot.

PT Patient = Patient’s measured PT (seconds)
PT Normal = Laboratory’s geometric mean value PT for normal patients not on Coumadin (seconds)
ISI = International Sensitivity Index (1.0)
PT= Prothrombin Time (number of seconds it takes blood to clot)

Start Testing Your INR at Home
Start Testing Your INR at Home