A CROSS-CANADA SURVEY OF PROTHROMBIN TIME TESTING - DOES THE ESTABLISHMENT OF LOCAL ISI VALUES IMPROVE THE ACCURACY OF INTERNATIONAL NORMALIZED RATIO REPORTING

The international normalized ratio (INR) was established as a means of standardizing the prothrombin time regardless of the thromboplastin u sed in the individual laboratories. The INR is the prothrombin time ra tio of the sample raised to the power of the International Sensitivity Index (ISI). Traditionally, the ISI is determined by using a manual c lotting technique by comparing the test thromboplastin with a World He alth Organization international reference thromboplastin with results from 60 patient samples standardized on warfarin, and 20 samples from normal volunteers. Most laboratories no longer perform prothrombin tim e testing by a manual technique but instead automate the procedure by using a variety of coagulation instruments. Thromboplastin ISI values have the potential to be modified considerably by instrumentation and as a consequence they may result in INRs that are unreliable when comp ared to those obtained by the traditional manual method It has been pr oposed that the use of calibrant plasmas tested with the laboratory's thromboplastin and instrument may overcome this problem and a local IS I value could be established Our study objective was to determine whet her a local ISI calibration with the plasma calibrants could reduce th e variation of the INR over a wide range of thromboplastin/instrument combinations. A total of 58 laboratories from across Canada, including community hospitals, private laboratories and large tertiary care uni ts, participated. The findings indicate that the use of calibrant plas mas to calculate the local ISI does improve the accuracy of INR report ing in the majority of thromboplastin/instrument combinations.