# Computing the INR

Here is a question from Joseph A. De Vito

In reading an article, Quote: The INR is a standardized number and is calculated from the patient PR, the normal range for PT, and the sensitivity of the reagent used.   Please explain to me the following:
(1) The patient PR
(2) Normal range for PT
(3) Reagent used
I will await your reply. Thanking you. I remain, respectfully, Joseph A. De Vito

Joseph, thank you for your question. the INR is the international normalized ratio, computed from the results of the prothrombin time (PT, “protime”) assay, a standard laboratory assay used to monitor warfarin (Coumadin) therapy. The INR is computed as follows:

INR = (PT of patient/mean of the PT normal range) ^ISI

Where…

PT of patient is the prothrombin time result for an individual patient in seconds.
Mean PT of the normal range (mean normal PT) is the average PT of at least 20 normal subjects computed by the local laboratory. Normal ranges vary by laboratory and reagent formulation, but typically run around 12.5 to 14.5 seconds with a mean of 13.5.
ISI is an expression of prothrombin time test reagent sensitivity and is used as an exponent (^ISI).
The prothrombin time reagent is called thromboplastin. In the lab we add it to patient plasma and record the time interval to clotting. This is the prothrombin time. The prothrombin time ratio (PTR, PR) is the ratio of patient PT to the mean normal PT. The ISI is computed by the thromboplastin manufacturer by comparison to a standard thromboplastin maintained by the World Health Organization.

By using the INR computation, patients taking warfarin may theoretically have their PT performed at any laboratory in the world and receive a generalizable, normalized result. Before the advent of the INR, warfarin therapy was based on the PTR and, because thromboplastin sensitivity varies,  patients had to return to the same laboratory each time for their PT testing. Geo