title: Correction factor to improve agreement between point-of-care and laboratory International Normalized Ratio values.
abstract: OBJECTIVE
  Results of a research project to quantify and improve the accuracy of point-of-care (POC) International Normalized Ratio (INR) values are reported.
  
  
  METHODS
  The accuracy of POC INR values relative to laboratory-measured INR values was retrospectively assessed in a cohort of patients with same-day INR determinations by both methods. Univariate linear regression was performed to derive a correction factor for POC INR values of >3; this correction factor was validated in a second cohort.
  
  
  RESULTS
  In the derivation cohort (259 patients and 344 paired INR results), agreement of POC values with corresponding laboratory INR values at two specified thresholds (±15% and ±25%) was 51.2% and 66.6%, respectively; for POC INR values of >3 (n = 205), agreement was lower (24.9% and 44.9%, respectively). Univariate linear regression yielded a coefficient of 0.77 (95% confidence interval, 0.76-0.79; p < 0.001). Applying a correction factor of 0.8 to POC INR values in a validation cohort (169 patients and 209 paired INR values) significantly improved the accuracy of POC INR values of >3 relative to laboratory values (from 7% to 71.1% at the lower threshold and from 23.5% to 88.8% at the higher threshold, p < 0.0001 for both comparisons).
  
  
  CONCLUSIONS
  Agreement between POC and laboratory INR results in one institution was poor, especially when POC INR values exceeded 3. Application of an institution-specific correction factor to POC INR values of >3 improved agreement with laboratory INR results but would not have significantly reduced differences in protocol-based warfarin dosage adjustments.
authors.affiliation: University of Utah Health Care Thrombosis Service, Salt Lake City, UT stacy.a.johnson@hsc.utah.edu.|Department of Internal Medicine, University of Utah School of Medicine, Salt Lake City, UT. stacy.a.johnson@hsc.utah.edu.
authors.forename: Stacy A
authors.initials: SA
authors.lastname: Johnson
authors.affiliation: University of Utah Health Care Thrombosis Center, Salt Lake City, UT.
authors.forename: Sara R
authors.initials: SR
authors.lastname: Vazquez
authors.affiliation: University of Utah Health Care Thrombosis Center, Salt Lake City, UT.
authors.forename: Ryan
authors.initials: R
authors.lastname: Fleming
authors.affiliation: Division of Pulmonary and Critical Care Medicine, University of Utah and Intermountain Medical Center, Murray, UT.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lanspa
mesh_terms: D000368:Aged; D000925:Anticoagulants / Q000009:adverse effects*; D001777:Blood Coagulation / Q000187:drug effects; D015331:Cohort Studies; D016903:Drug Monitoring / Q000379:methods; D005260:Female; D006470:Hemorrhage / Q000097:blood / Q000139:chemically induced / Q000517:prevention & control; D006801:Humans; D019934:International Normalized Ratio / Q000379:methods*; D008297:Male; D008875:Middle Aged; D019095:Point-of-Care Systems; D058996:Quality Improvement; D012189:Retrospective Studies; D013923:Thromboembolism / Q000097:blood / Q000139:chemically induced / Q000517:prevention & control; D014859:Warfarin / Q000009:adverse effects*
chemical_list: D000925:Anticoagulants; D014859:Warfarin
keywords: International Normalized Ratio; correlation study; point-of-care systems; warfarin