We have evaluated a newly introduced immunoturbidimetric transferrin receptor assay (IdeA TfR-IT, Orion Diagnostica, Finland) in healthy subjects and in a study population consisting of patients with rheumatoid arthritis and juvenile chronic arthritis. The IdeA TfR-IT assay was found to provide reproducible results which were in good agreement with the ELISA assays from Orion Diagnostica (IDeA-ELISA, correlation R2=0.8, n=102) and R&D systems (Quantikine TfR ELISA assay, correlation R2=0.95, n=39). The analysis of the patient samples suggested that, on the basis of serum transferrin receptor and ferritin concentrations, in approximately one third of patients with rheumatoid arthritis anemia is due to the depletion of iron stores. Apparently, in all patients with rheumatoid arthritis iron deficiency must be considered as a potential cause of the anemia. Now, that assays which are suitable for automated analyzers have become available for the measurement of serum transferrin receptor, this analyte has the potential to become a part of the routine evaluation of iron status.
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.
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.
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