PT/INR pt Values question

core0 · Sep 11, 2010

Hello!
I have a question about PT/INR values. I understand the concept but here is the question.
I have an example coagulation report that says [ PT 12.3 10.0-13.5 ] [ INR 1.06 L 2.0-3.5 ]
I get that the INR is low and not be therapeutic at that dose but how does the PT interact with that.... My book says normal PT is 11.0-12.5 seconds and the therapeutic level is 1.5-2 times normal... when I do the math the PT in seconds is 16.5-25. if this is correct then what is the 10.0-13.5 printed on the form? does this number differ because of differences in the lab and my text book or am I very confused?
Thanks for help!

The PT is listed as the normal range. Ie if someone is healthy the PT should be between 10 and 13.5 seconds. The normal for INR is listed assuming that someone is on Coumadin. Ie the therapeutic range for Coumadin in an INR between 2 and 3.5. The normal range for an INR for someone who is healthy and not on Coumadin is 0.8 to 1.2. The reason that the therapeutic range is not listed for PT is to prevent people from trying to use it to dose Coumadin as explained below.

The reason that the INR is used is that PT varies according to which test and device is used. In order to compare PT between various labs the International Normalized Ratio was created. The formula is:

INR = (PT test/PT normal) power ISI.

The ISI is the international sensitivity index that is assigned by the manufacturer usually between 1 and 2 (essentially a comparison to a norm).

For example if the test result is 14 and the normal for the lab is 12 and the ISI is 1 then the INR would be 14/12 or 1.17 to the first power or 1.17. If the ISI is 2 then it would be 1.17 squared or 1.36. The higher the PT test is from the norm the greater the difference the ISI will make. For example if the PT test is 24 and the PT norm is 12 then then 24/12= 2 to the power of one = INR 2. For an ISI of 2 24/12 = 2 squared = INR 4.

Probably more than you wanted to know about INR and PT.

David Carpenter, PA-C

soren1813 · Sep 11, 2010

Sweet! That helps me.. Thanks.

I do have one other question, Where does the 1.5-2 times the normal therapeutic level come in to play. do i need to multiply the normal level to establish and new upper and lower therapeutic level of PT? I just dont get why I need to look at the INR then figure out the PT at 1.5-2 times normal levels on the PT?

grandmawrinkle · Sep 11, 2010

I don't think you are quite getting it yet. There is no widely accepted "therapeutic level" for the PT -- it's too bad your book says that, it's confusing. What is used is the INR, which is "normalized" to give a standard level for patients from different labs. The actual PT is not used for dosing medications because of the testing differences described by the other poster. The range reported behind the PT are normal, non-anticoagulated levels in your sample lab report.

I'm not quite sure what your question is asking. If your INR is elevated, your PT will be elevated as well since that is what it is based on. They go together.

I personally think your lab report is confusing and have not seen one like it in the past -- don't know if they are usually that way for the INR. I can see how as a student this is screwing you up. Your patient's results are both in the normal range and make sense with each other(the PT 12.3/INR 1.06.) An INR of 2.0-3.5 is actually elevated and technically abnormal, although to read your lab report, you wouldn't know that (esp. if the patient wasn't on warfarin, etc. to make it that way.) Usually what I have seen as a reference range is the .8-1.2 that previous poster described, which would put your patient's INR in the normal range, not "L" as reported.