An external quality assessment (EQA) survey on 14 fresh-frozen, single-donation sera assigned with reference measurement procedure (RMP) values revealed a mean bias of + 5.2% and + 3.7% for the cholesterol oxidase and the photometric glucose oxidase procedure groups, respectively. Conversely, on lyophilized sera, the same procedure groups showed almost bias-free results, the differences from the RMP values being only -0.8% for cholesterol and + 0.7% for glucose. These data, which are in fairly good agreement with the literature, suggest the existence of artificial matrix effects in processed materials. Therefore they indicate that, currently, assessment of trueness is hampered in many European EQA schemes, as most of them use lyophilized sera. This approach may give a false impression about the trueness of laboratory results as well as carrying the risk that laboratories calibrated on the RMP values of the survey samples could make errors in patient testing. Consequently, if European EQA is willing to fulfil a post-market vigilance function of the performance of in vitro diagnostic medical devices, then the time has come to tackle the problem of the quality of the survey samples. EQA organizers urgently need to make an effort to seek out materials that analytically behave like authentic clinical specimens. In the meantime, alternative approaches should be used. Although not ideal, the special survey described in this article is one of the possibilities. Naturally, it implies logistic problems and increased costs for the individual EQA schemes. However, both can be overcome with the cooperation of the predominantly nationally organized schemes.
Permissible limits for internal and external quality assurance are either based on biological variation or on the state of the art (technical feasibility). The former approach has a scientific basis, but, in some cases, leads to limits which are either not achievable under the present technology, or which are not stringent enough. If proficiency testing is mandatory, stringent limits which cannot be fulfilled by the majority of laboratories could lead to juristic consequences. Therefore, most national guidelines were based on the state of the art, however, without providing the underlying reasoning. A simple algorithm for permissible limits in external quality assessment schemes (EQAS) is proposed based on biological variation, technical feasibility and correlated to the rate of false positive results. The proposed limits are compared with some limits from several EQAS (RiliBÄK, SEKK, RCPA, CLIA, PROLARIT). The suggested limits are slightly more stringent than the German RiliBÄK, less stringent than the Australasian guidelines and agreed best with the Czech SEKK and the Italian PROLARIT scheme. The graphical presentation of permissible limits strictly derived of biological variation with the proposed limits led to straight lines with different slopes and a cross-over at the limits for quantities with a medium biological variation (e.g., trijodthyronine). The greatest discordance between the various recommendations was observed for calcium, chloride, hemoglobin A(1c) and sodium.
The state of the art is that 1 of 20 laboratories does not meet the IFCC criterion, but there are substantial differences between country and between manufacturer groups. Efforts to further improve quality should focus on reducing between-laboratory variation. With some limitations, fresh whole blood and well-defined lyophilized specimens are suitable for purpose.
The Czech External Quality Assessment Scheme organized a survey using 14 fresh-frozen sera targeted for cholesterol and glucose by reference measurement procedures. The objective was to investigate whether it could fulfil a post-market vigilance function for in vitro diagnostic medical devices and assess trueness of participants' results. It revealed a mean bias of +5.1% for cholesterol and +3.7% for glucose (n approximately 150). However, the bias source (manufacturer or laboratory) could not be identified unequivocally because of the lack of homogeneous groups. This was due to the fact that laboratories mainly used reagents from manufacturers that do not market instruments or combined calibrators and reagents from different sources. Consequently, these habits did not allow the survey to fulfil the vigilance function. On the other hand, we were able to show the individual participants results for patient samples deviating from the true value (deviations >10% in approximately 20% of the laboratories). However, again, the survey failed in problem-solving via peer-group evaluation, even for participants that applied homogeneous tests. If other European schemes confirm this outcome, cooperation and/or participation of manufacturers may be the solution. The survey pointed out to the other participants, interchanging instrument, reagent and calibrator, that they are themselves responsible for the problems shown and hence also for problem-solving.
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