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inr

INR

introduction

  • the International Normalised Ratio or INR test measures the overall activity of the extrinsic coagulation pathway.
  • performed by adding thromboplastin to an aliquot of the patient's plasma
  • length of time taken for the mixture to clot is called the prothrombin time.
  • the same test is performed using a normal control plasma and the prothrombin ratio is the derived by dividing the patient clotting time by the control clotting time.
  • finally, the INR is calculated using the 'international sensitivity index' of the thromboplastin reagent so that the ratio obtained is independent of the reagent brand or the testing laboratory.
  • normal range for INR is 1.0-1.3
  • usual target therapeutic INR for warfarin Rx are:

causes of a prolonged INR

  • deficiency of one or more coagulation factors in the extrinsic coagulation pathway
    • this may be caused by either:
      • malnutrition
      • impaired liver function
      • other causes
  • presence of a lupus type coagulation inhibitor - an in vitro phenomenon giving falsely prolonged results in some patients.
  • presence of heparin (but not LMW heparin)
inr.txt · Last modified: 2009/09/01 15:03 (external edit)