International Normalised Ratio (INR) | Coagulation - MedSchool
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Coagulation
 
 
Coagulation
The prothrombin time (PT) and international normalised ratio (INR) are measures of the extrinsic coagulation pathway. The prothrombin time is measured by adding tissue factor to a plasma sample. 
 

International Normalised Ratio (INR)

 
 

Overview

  • 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.
Last updated on January 19th, 2018
 
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