Comparability of method results and performance in a national external quality assessment scheme between 1993 and 2003 using thyroid associated antibodies as examples.

Four thyroid antibodies (antibodies to microsomes [MAb], thyroid peroxidase [anti-TPO], thyroglobulin [anti-Tg] and TSH-receptor [TRAB, THYBIA]) have been followed up over a 10-year period in a national external quality assessment scheme (EQAS) organised by the Institute for Standardisation and Documentation in the Medical Laboratory (INSTAND e.V.). The following points were observed: I. The introduction of samples with properties similar to patient serum (filtered, recalcified defibrinated plasma without stripping) improved performance and inter-method comparability for thyroid antibodies. II. Regular statistical analysis of EQAS data allows adjustment of target ranges to be made when necessary. III. There are large inter-method variations in reporting, both on qualitative and quantitative results. IV. The samples often gave rise to different constellations of antibodies, which were kit-dependent. V. Despite use of international reference preparations, there was no numerical comparability between quantitative methods for the same analyte. In general, the performance in EQAS for thyroid antibodies has improved over the past decade. There is still a real need for standardisation in the field of thyroid antibody analysis.