What is the difference between PT, INR, and PTT?

1 Answer
Aug 28, 2016

Here are the differences.


Coagulation factors

Several proteins called coagulation factors are involved in the formation of a blood clot.

Not enough coagulation factors can lead to excessive bleeding; too much may lead to excessive clotting.


The prothrombin time (PT) is the time it takes blood to clot after the addition of tissue factor.

The normal range is 11 s to 13.5 s.

The PT measures some of the blood clotting factors,

It is used in the management of clotting disorders.


The international normalized ratio (INR) is a calculation based on results of a PT.

The PT varies depending on differences between different types and batches of manufacturer's tissue factor.

Each manufacturer assigns an ISI value (International Sensitivity Index) to each of their tissue factors.

It indicates how a particular batch compares to an international standard.

The INR is the ratio of a patient's PT to a "normal" PT, raised to the power of the ISI value.

#"INR" = ("PT"_ "patient"/"PT"_"normal")^"ISI"#

The normal INR range for a healthy person not using warfarin is 0.8 – 1.2.

For most patients on warfarin therapy, the INR is usually between 2.0 and 3.0.

Thus, if PT = 23 s and a normal PT = 12 s, using a tissue factor with ISI = 1.2,

#"INR" = ((23 color(red)(cancel(color(black)("s"))))/(12 color(red)(cancel(color(black)("s")))))^1.2" = 2.18#


The partial thromboplastin time (PTT) evaluates different coagulation factors than PT does.

The typical PTT range is between 30 s and 50 s.

It is often used in the management of bleeding disorders.

By evaluating the results of the PT, INR, and PTT together, a health practitioner can gain clues as to what bleeding or clotting disorder may be present.