Nurses General Nursing


Can anyone PLEASE PLEASE tell me exactly WHAT the INR measures? I know it's part of the PT, but everything I have read about it simply confuses me. Is it simply the PT, but reported in a different way or am I way off? I am VERY confused.

aimeee, BSN, RN

932 Posts

"the pt evaluates the extrinsic pathway factors by measuring the ability of a patient's recalcified plasma to clot when mixed with a crude mixture of a tissue factor activator and phospholipid, known as thromboplastin. common sources of thromboplastins are rabbit brain or lung extracts. the patient's pt is usually reported as the clotting time, in seconds, and is compared with the pt of a normal control or in some laboratories to a normal range."

note that different sources are used for those thromboplastins used for the tests. clotting times may vary somewhat depending on which thromboplastin your lab uses. and the variations from lab to lab, region to region, were sometimes significant. so the who came up with the inr.

there is a mathematical factor determined for each thromboplastin and lab method. by using this formula they are able to make the lab values from anywhere comparable. they also normalize the values, so that an inr of 1 is equivalent to the normal clotting time for the average person who is not receiving anticoagulants.

so, in answer to your question, yes, it is just the pt, but its been run through a formula to standardize it. so all you have to know is the range which the physician wants to keep the patient within. usually this is 2.0-3.0 for a person on coumadin.

aimeee, BSN, RN

932 Posts

Whoops. I forgot to add this caveat:

"Often the clinical reason for requesting the PT is not specified and therefore some laboratories convert all PTs to an INR. It should be remembered that the INR only has meaning for patients on a stable does of chronic anticoagulants. The INR should not be used to evaluate the coagulation status of patients who have not been anticoagulated for at least one week or in those with an abnormal PT for other reasons, e.g., liver disease."

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Learned something new here Aimee. Great answer!


Guest LauraRN0501

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Aimee, thanks so much. That really helped clear it up! :)


427 Posts

I believe it stands for International normalization ratio


6,011 Posts



I really think that waiting a WEEK to run an INR just might be a little TOO long. I've seen them go sky high after just two doses of coumadin.

Specializes in Trauma acute surgery, surgical ICU, PACU.

At our hospital, it seems that when a patient is recieving heparin, the PTT is considered most important, and when on coumadin, they look at the INR.... I never really understood that, since both heparin and coumadin work on coagulation.


427 Posts

Aimeee where did yoiu get that info from, an INR will change 24 hrs afterstarting on coumadin.

Pebbles, ptt's are used for heparin anticoags, and pt/inr are used to reflect coumadin, oddly lovenox doesnt' really change either one.

aimeee, BSN, RN

932 Posts

Kewl, I found that explanation of the INR here, and I used it because I thought it explained it well.

I did not interpret that as meaning that the INR wouldn't be affected before a week. Don't they continue to use the PTT in addition to the PT while they are in the process of loading the coumadin?

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