Defining acceptable limits for the metrological traceability of specific measurands

Although manufacturers are compelled by the European IVD Directive, 98/79/EC, to have traceability of the values assigned to their calibrators if suitable higher order reference materials and/or procedures are available, there is still no equivalence of results for many measurands determined in clinical laboratories. The adoption of assays with metrological traceable results will have a significant impact on laboratory medicine in that results will be equivalent across different laboratories and different analytical platforms. The IFCC WG on Allowable Errors for Traceable Results has been formed to define acceptable limits for metrological traceability chains for specific measurands in order to promote the equivalence of patient results. These limits are being developed based on biological variation for the specific measurands. Preliminary investigations have shown that for some measurands, it is possible for manufacturers to assign values to assay calibrators with a measurement uncertainty that allows the laboratory enough combined uncertainty for their routine measurements. However, for other measurands, e.g., plasma sodium, current assays are too imprecise to fulfil limits based on biological variation. Although an alternative approach based on probability theory is being investigated, the most desirable approach would be for industry to improve measurement methods so that they meet clinical requirements.