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International Normalised Ratio (INR)



  • The prothrombin time (PT) and international normalised ratio (INR) are measures of the extrinsic coagulation pathway. 
      • Normal Range

      • Prothrombin time - 11 - 15 sec
      • INR - 0.9 - 1.3
    • Important Lab Points

    • Collected in a sodium citrate tube to prevent coagulation prior to testing.
    • Performed by adding calcium and thromboplastin to a plasma sample and measuring the time to clot.
    • Tests the extrinsic and common pathways - VII, V, X, II, fibrinogen
    • Prothrombin time (PT) is the raw clotting time, while international normalised ratio (INR) is a standardised measure to account for variation of reagents between laboratories.
    • Practical Points

    • PT/INR can be used to monitor patients on warfarin
    • PT/INR does not correspond to the coagulative status of patients on novel anticoagulants such as dabigatran, rivaroxaban and apixaban
    • INR does not correspond to the coagulative status of patients with liver disease, and most often these patients are also coagulopathic
    • Heparin can also prolong the INR, and so heparin neutralisers are used when performing the test - excess heparin in the sample can overcome this and prolong the PT/INR

Prolonged INR

    • Causes of Prolong PT/INR

    • Isolated Prolonged PT/INR

    • Warfarin (low dose)
    • Mild vitamin K deficiency
    • Factor VII deficiency
    • Prolonged PT/INR and APTT

    • Artefactual - high haematocrit
    • Liver disease
    • Disseminated intravascular coagulation (DIC)
    • Common pathway deficiency - X, V, II, fibrinogen
    • Severe vitamin K deficiency
    • High dose heparin
    • Warfarin
    • Factor IIa inhibitors (dabigatran)
    • Factor Xa inhibitors (rivaroxaban, apixaban)
    • Approach

    • Determine whether the patient is on warfarin or any other anticoagulants.
    • Check the patient’s haematocrit, which if elevated will falsely elevate PT/INR and APTT
    • Look for evidence of chronic liver disease, either in the patient’s past medical history or current clinical state.
    • Assess the patient’s risk of DIC given their clinical state.
    • If unsure about the cause of prolonged PT/INR, mixing studies can be performed.

PT/INR Mixing Studies

  • The patient's plasma is mixed 50:50 with control plasma, and then the PT/INR is measured.
    • Significance

    • PT/INR mixing studies are used to determine the cause of prolonged PT/INR.
    • Interpretation

    • PT/INR corrects - factor deficiencyWarfarin, vitamin K deficiency, congenital factor deficiency, DIC, liver failure
    • PT/INR does not correct - inhibitor presentFactor IIa inhibitors, excess heparin

INR in Warfarin Monitoring

  • The INR is used to monitor coagulative state of patients on warfarin, to ensure that they are receiving a therapeutic dose.
      • Common INR Targets

      • Atrial fibrillation, DVT/PE: 2.0 - 3.0
      • Mechanical heart valve: 2.5 - 3.5
    • Practical Points

    • Consult local guidelines when commencing warfarin.
    • Changes in warfarin dosing will take several days to have an effect on the INR, so dose titration should rely on the trend rather then being immediately reactive.
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 Barras M. Anti-Xa assays. Australian Prescriber. 2013 Jun 1;36(3). Besser MW, MacDonald SG. Acquired hypofibrinogenemia: current perspectives. Journal of blood medicine. 2016;7:217. Chitolie A, Mackie IJ, Grant D, Hamilton JL, Machin SM. Inaccuracy of the'derived'fibrinogen measurement. Blood coagulation & fibrinolysis. 1994 Dec 1;5(6):955-7.
Chng WJ, Sum C, Kuperan P. Causes of isolated prolonged activated partial thromboplastin time in an acute care general hospital. Singapore medical journal. 2005 Sep;46(9):450.
 Chopra N, Doddamreddy P, Grewal H, Kumar PC. An elevated D-dimer value: a burden on our patients and hospitals. International journal of general medicine. 2012;5:87. Eisenberg JM, Clarke JR, Sussman SA. Prothrombin and Partial Thromboplastin Times as Preoperative Screening Tests. Arch Surg. 1982; 117(1): 48-51.  Eisenberg JM, Clarke JR, Sussman SA. Prothrombin and Partial Thromboplastin Times as Preoperative Screening Tests. Arch Surg. 1982; 117(1): 48-51. Giannitsis E, Mair J, Christersson C, Siegbahn A, Huber K, Jaffe AS, Peacock WF, Plebani M, Thygesen K, Möckel M, Mueller C. How to use D-dimer in acute cardiovascular care. European Heart Journal: Acute Cardiovascular Care. 2017 Feb;6(1):69-80. Kamal AH, Tefferi A, Pruthi RK. How to interpret and pursue an abnormal prothrombin time, activated partial thromboplastin time, and bleeding time in adults. InMayo Clinic Proceedings 2007 Jul 31 (Vol. 82, No. 7, pp. 864-873). Elsevier. Katayev A, Balciza C, Seccombe DW. Establishing Reference Intervals for Clinical Laboratory Test Results: Is There a Better Way?. Am J Clin Pathol. 2010; 133(2): 179. Levi M, Opal SM. Coagulation abnormalities in critically ill patients. Critical care. 2006 Jul 19;10(4):222. Lowe GD, Rumley A, Mackie IJ. Plasma fibrinogen. Ann Clin Biochem. 2004; 41: 430. Mani H. Interpretation of coagulation test results under direct oral anticoagulants. International journal of laboratory hematology. 2014 Jun 1;36(3):261-8. Oliver Speer O, Schmugge M, Metzger C, Albisetti M. Reference Ranges of Coagulation Tests. Methods in molecular biology. 2013; 992: 85. Palta S, Saroa R, Palta A. Overview of the coagulation system. Indian journal of anaesthesia. 2014 Sep;58(5):515. Peyvandi F, Garagiola I, Baronciani L. Role of von Willebrand factor in the haemostasis. Blood Transfusion. 2011 May;9(Suppl 2):s3. Wakai A, Gleeson A, Winter D. Role of fibrin D-dimer testing in emergency medicine. Emergency Medicine Journal. 2003 Jul 1;20(4):319-25.