The success of anticoagulation depends on ensuring the safety and effectiveness of the drug-induced coagulation defect. Too little anticoagulation is ineffective, whereas too much causes dangerous bleeding.
Safety is assessed by prothrombin time (PT) blood tests performed on each patient at frequent intervals, and the results are given as international normalized ratios (INRs).
INRs are based on a system of PT standardization pioneered in the early 1960s in Manchester, which was eventually recognized as the World Health Organisation primary biological standard for thromboplastin.
Professor Leon Poller, who leads the European Action on Anticoagulation, has reported the breakthrough in Clinical Chemistry, explaining:
‘The aim of the WHO scheme has been to make the treatment safe and effective on a world scale by providing a reliable international system for blood testing and reporting of results as INRs. In recent years, the WHO Scheme has proved increasingly difficult to apply not only because of its heavy demands but because the manual PT testing procedure on which it is based has increasingly been replaced by automated systems of measurement.
‘These automated PT systems, as well as differing from manual PT testing, may vary in INR results on the same patient’s blood specimen and coagulometers of the same manufacture and even may give different INR results on the same patient’s blood. As a result of the difficulties, the INR system has become increasingly difficult to apply at the local level and has been falling into disfavour.’
The new method provides a simplified method of local INR derivation while preserving the safety and effectiveness of the INR system.
Results of the large international study at 28 experienced centres co-ordinated over five years from The University of Manchester showed that the simplified method of INR derivation using the new PT/INR Line gives reliable INR. Furthermore, the PT/INR Line does not demand the now almost entirely discarded manual PT testing for traditional WHO ISI calibration and INR interpretation.