Summary
Although 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.
Keywords
INR - ISI - proficiency test
