Traceable assigned values in external quality assessment schemes compared to those obtained by alternative procedure: a case study for Cu, Se and Zn in serum

International standards for the recognition of the competence of testing laboratories require that measurement results should be traceable to a conventionally agreed reference. This should be achieved by appropriate calibration of equipment and method validation involving analysis of certified reference materials (CRM). However, these are costly and for many analytical procedures, few are available. Participation in external quality assessment schemes (EQAS) may provide a mean to support the laboratory traceability statement, if the values assigned to test samples are traceable to a stated reference. Values may be assigned to EQAS test samples by a variety of techniques but there has been no direct comparison of results obtained when these procedures are applied to the same samples. In this study, traceable values for Cu, Se and Zn concentrations were assigned to three batches of EQAS serum samples, by analysis by expert laboratories together with CRMs, and compared with those obtained by three other of the approaches described in ISO 13528; analysis by a definitive method (ID-ICP-MS); determination of robust consensus mean from the results of expert laboratories; robust consensus mean of results from EQAS participants. The assigned values (mu mol L-1) +/- expanded uncertainty (%) for the low, medium and high pools obtained by ID-ICP-MS were: Cu 13.37 +/- 1.2, 21.03 +/- 1.8, 28.73 +/- 1.2; Se 0.74 +/- 3.5, 1.51 +/- 3.4, 3.11 +/- 3.6; Zn 9.69 +/- 4.9, 22.52 +/- 1.5, 30.85 +/- 3.8. Concentrations determined using the three other approaches were similar but the uncertainties increased as the methodologies became increasingly less rigorous.