title: The international normalized ratio (INR) for monitoring warfarin therapy: reliability and relation to other monitoring methods.
abstract: OBJECTIVE
  To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.
  
  
  METHODS
  Prospective cohort study.
  
  
  METHODS
  A university hospital.
  
  
  METHODS
  79 patients attending an anticoagulation clinic.
  
  
  METHODS
  International normalized ratios obtained with a portable capillary monitor (Coumatrak) and the following from a simultaneous plasma sample: INRs from prothrombin times done with six thromboplastins, prothrombin-proconvertin (P&P) test activity, specific prothrombin activity, and native prothrombin antigen.
  
  
  RESULTS
  Converting to INRs failed to standardize prothrombin time results obtained with high- and low-sensitivity thromboplastins. Coumatrak INRs correlated best with INRs obtained with high-sensitivity thromboplastins. The INR range of 2.0 to 3.0 corresponded to a P&P range of 30% to 13%, a native plasma prothrombin antigen range of 56 to 24 micrograms/mL, and a specific prothrombin activity range of 43% to 21%.
  
  
  CONCLUSIONS
  Low-sensitivity thromboplastins may give erroneously high INRs in the upper therapeutic range. Plasma prothrombin times should be done with a high-sensitivity thromboplastin, particularly in patients maintained at the upper limit of the therapeutic range. An INR so obtained correlated well with an INR obtained with a portable capillary blood monitor.
authors.affiliation: University of California, San Diego, La Jolla.
authors.forename: D T
authors.initials: DT
authors.lastname: Le
authors.forename: R T
authors.initials: RT
authors.lastname: Weibert
authors.forename: B K
authors.initials: BK
authors.lastname: Sevilla
authors.forename: K J
authors.initials: KJ
authors.lastname: Donnelly
authors.forename: S I
authors.initials: SI
authors.lastname: Rapaport
mesh_terms: D001324:Autoantigens / Q000097:blood; D002196:Capillaries; D016903:Drug Monitoring / Q000379:methods / Q000592:standards*; D005167:Factor VII / Q000378:metabolism; D006801:Humans; D011446:Prospective Studies; D011516:Prothrombin / Q000276:immunology; D011517:Prothrombin Time; D012044:Regression Analysis; D013925:Thromboplastin / Q000378:metabolism; D014859:Warfarin / Q000627:therapeutic use*
chemical_list: D001324:Autoantigens; D014859:Warfarin; D005167:Factor VII; D011516:Prothrombin; D013925:Thromboplastin