title: The clinical significance of differences between point-of-care and laboratory INR methods in over-anticoagulated patients.
abstract: BACKGROUND
  Patients receiving warfarin are at increased risk of bleeding when their International Normalised Ratio (INR) >4.5. Although not standardised above 4.5 the INR is measured in over-anticoagulated patients, consequently we have examined the reliability of INR results ≥4.5. We assessed: the relationship between different prothrombin time systems for INRs >4.5; the relationships between the INR and levels of vitamin K-dependent coagulation factors (VKD-CF) and thrombin generation test (TGT) parameters; and the impact that variation in results would have on warfarin dosing.
  
  
  METHODS
  INRs were performed using a CoaguChek XS Plus point-of-care (POC) device (measuring range 0.6-8.0). For POC INRs ≥4.5, laboratory INRs were also measured using a recombinant tissue factor (rTF) and a rabbit brain (RBT) thromboplastin.
  
  
  RESULTS
  There was good correlation between POC (INR ≥4.5, <8.0) and Lab INRs (rTF n=154, rs=0.87, p<0.0001; RBT n=102, rs=0.76, p<0.0001); and significant correlations between each of the VKD-CF and the INR, the strongest being with FVII (POC INR rs=-0.53 p<0.0001; Lab rTF-INR rs=-0.70 p<0.0001). TGT peak thrombin and ETP also showed good correlations with INR values (R(2)>0.71). Using POC and Lab rTF-INR, 109/154 (71%), or POC and Lab RBT-INR 75/102 (74%) results exhibited dosage concordance and/or were within 0.5 INR units. In the remaining patients variation in warfarin dosing was generally slight.
  
  
  CONCLUSIONS
  Our data suggest that CoaguChek XS Plus INRs >4.5 and <8.0 are comparable to laboratory INRs (both methods) and it is probably unnecessary to perform laboratory INRs for clinical management of patients with INRs >4.5 including those >8.0.
authors.affiliation: Haemostasis Research Unit, Department of Haematology, University College London, London, UK. andrew.lawrie@ucl.ac.uk
authors.forename: A S
authors.initials: AS
authors.lastname: Lawrie
authors.forename: J
authors.initials: J
authors.lastname: Hills
authors.forename: I
authors.initials: I
authors.lastname: Longair
authors.forename: L
authors.initials: L
authors.lastname: Green
authors.forename: C
authors.initials: C
authors.lastname: Gardiner
authors.forename: S J
authors.initials: SJ
authors.lastname: Machin
authors.forename: H
authors.initials: H
authors.lastname: Cohen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D000925:Anticoagulants / Q000008:administration & dosage*; D001779:Blood Coagulation Factors / Q000378:metabolism; D005260:Female; D006801:Humans; D019934:International Normalized Ratio / Q000379:methods*; D008297:Male; D008875:Middle Aged; D019095:Point-of-Care Systems; D011517:Prothrombin Time / Q000379:methods*; D011817:Rabbits; D013917:Thrombin / Q000378:metabolism; D014859:Warfarin / Q000008:administration & dosage*; D055815:Young Adult
chemical_list: D000925:Anticoagulants; D001779:Blood Coagulation Factors; D014859:Warfarin; D013917:Thrombin