International Normalized Ratio (INR) – Proficiency Tests by ÖQUASTA for the Prothrombin Time

SummaryAlthough the INR (International Normalized Ratio)/ISI (International Sensitivity Index) scheme was introduced by the WHO (13) in 1983 to standardize the PT (prothrombin time) expression, the use of the non-comparable percent of a normal fresh plasma, seconds or PT-ratio (patient plasma/normal plasma) is still common in the coagulation laboratories. The availability of the INR/ISI scheme to monitor quality control of the laboratories in a reagent and method independent manner was examined by the comparison of 13 PT proficiency tests carried out by the ÖQUASTA (Austrian Society of Quality Assurance and Standardization of Diagnostic Medical Investigations).In each proficiency test approximately 250 laboratories had to determine the PT of two to three lyophilized plasma samples with their routinely used reagents and methods. The INR mean values of the AK-plasmas (plasmapools from patients under anticoagulant therapy) were between 2 and 5. The determined data and the calculated INR-values were returned to the ÖQUASTA.According to the INR/ISI scheme, all data should be considered as belonging to the same collective (TC = total collective). To prove this demand, additionally each reagent and method was evaluated separately (SC = single collective). It could be shown that the INR mean values obtained from all data and using TC or SC evaluation are nearly equivalent indicating that the TC evaluation is suitable for use in proficiency tests.The aim of a better comparability of the PT values can not only be reached by the laboratories through the use of the INR/ISI scheme. Additionally, the manufacturer are asked to standardize their ISI and 100% value determination. The manufacturer took this into account by establishing a candidate reference plasma (5).It could be shown that the introduction of the INR was not only an important step forward in terms of standardization and comparability of different thromboplastin reagents, but also in the quality control of the laboratories checked in proficiency tests in Austria.