Editing Prothrombin time (section)

===International normalized ratio===
The result (in seconds) for a prothrombin time performed on a normal individual will vary according to the type of analytical system employed. This is due to the variations between different types and batches of manufacturer's tissue factor used in the reagent to perform the test. The INR was devised to standardize the results. Each manufacturer assigns an ISI value (International Sensitivity Index) for any tissue factor they manufacture. The ISI value indicates how a particular batch of tissue factor compares to an international reference tissue factor. The ISI is usually between 0.94 and 1.4 for more sensitive and 2.0–3.0 for less sensitive thromboplastins.<ref>{{cite journal|pmid=14995988|year=2004|last1=Houdijk|first1=W. P.|title=International multicenter international sensitivity index (ISI) calibration of a new human tissue factor thromboplastin reagent derived from cultured human cells|journal=Journal of Thrombosis and Haemostasis|volume=2|issue=2|pages=266–70|last2=Van Den Besselaar|first2=A. M.|doi=10.1111/j.1538-7836.2004.00434.x|s2cid=20151897}}</ref><ref>{{cite journal|pmid=15917425|pmc=1770687|year=2005|last1=Poller|first1=L|title=European Concerted Action on Anticoagulation. A multicentre calibration study of WHO international reference preparations for thromboplastin, rabbit (RBT/90) and human (rTF/95)|journal=Journal of Clinical Pathology|volume=58|issue=6|pages=667–9|last2=Keown|first2=M|last3=Chauhan|first3=N|last4=Van Den Besselaar|first4=A. M.|last5=Tripodi|first5=A|last6=Shiach|first6=C|last7=Jespersen|first7=J|doi=10.1136/jcp.2004.019810}}</ref><ref name=mayolabs>{{Cite web|url=https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9236|title=Test ID: PT Prothrombin Time, Plasma}}</ref>

The INR is the ratio of a patient's prothrombin time to a normal (control) sample, raised to the power of the ISI value for the analytical system being used.

:<math>
\text{INR}= \left(\frac{\text{PT}_\text{test}}{\text{PT}_\text{normal}}\right)^\text{ISI}
</math>

PT<sub>normal</sub> is established as the geometric mean of the prothrombin times (PT) of a reference sample group.<ref name="pmid9365404">{{cite journal| author=D'Angelo A, Galli L, Lang H| title=Comparison of mean normal prothrombin time (PT) with PT of fresh normal pooled plasma or of a lyophilized control plasma (R82A) as denominator to express PT results: collaborative study of the International Federation of Clinical Chemistry. IFCC Working Group Standardization of Coagulation Tests. | journal=Clin Chem | year= 1997 | volume= 43 | issue= 11 | pages= 2169–74 | pmid=9365404 | doi= | pmc= | url=https://pubmed.ncbi.nlm.nih.gov/9365404  }} </ref>