@article{feb42cde-0757-45fe-b435-371509a2cafc,
  abstract     = {{<p>Background All hospital laboratories in Sweden use the Owren prothrombin time (PT) method primarily to monitor anticoagulant treatment. Direct international normalized ratio (INR) calibration was introduced in 1999 under the supervision of Equalis, the national organization for external quality assessment (EQA) in laboratory medicine. Objectives We present 25 years of combined experience in developing a national calibration procedure and conducting INR EQA. Methods Since the introduction of direct INR calibration, 14 different Equalis INR calibrator kits have been produced. Variability between laboratories and across different reagent and calibrator lots was determined (&gt;51 000 results and 96 EQA materials). The stability of the calibrators and the calibration procedure were evaluated through remeasurements of the calibrators, weekly warfarin dosing in patient records, and a comparison study with an international reference thromboplastin (RBT/05). Results Interlaboratory variability was significantly improved after the introduction of direct INR calibration. Results were consistent over 25 years, with a mean coefficient of variation &lt; 6% in the therapeutic range. The INR calibrators showed reproducible measurements, and the calibration procedure was stable over time. Furthermore, anticoagulated patients had a stable mean weekly warfarin dose relative to the mean INR. The Owren's PT agreed with the Quick PT in a direct comparison with RBT/05. Conclusion There is good agreement among PT measurements across Swedish laboratories, with no significant differences in reagents. The user-friendly, unique calibration procedure, including the preparation of certified plasmas, improved the performance of national PT measurements and has been shown to remain stable over decades.</p>}},
  author       = {{Hillarp, Andreas and Nordin, Gunnar and Boija, Elisabet Eriksson and Strandberg, Karin and Lindahl, Tomas L.}},
  issn         = {{1538-7933}},
  keywords     = {{calibration; international normalized ratio; laboratory proficiency testing; prothrombin time; warfarin}},
  language     = {{eng}},
  publisher    = {{Elsevier}},
  series       = {{Journal of Thrombosis and Haemostasis}},
  title        = {{A 25-year evaluation of direct local international normalized ratio calibration of the Owren type prothrombin time method in Sweden}},
  url          = {{http://dx.doi.org/10.1016/j.jtha.2025.12.030}},
  doi          = {{10.1016/j.jtha.2025.12.030}},
  year         = {{2026}},
}