Proficiency testing in food control: an evaluation of more than 13 years

According to article 32 and 33 of Regulation (EC) No 882/2004, the European Union and National Reference Laboratory for pharmacologically active substances (EURL/NRL) at the Federal Office of Consumer Protection and Food Safety (BVL) is responsible, among other tasks, to support official control laboratories and as competent authorities on a national and EU level. The EURL has used in-house reference materials with residues of β-agonists, coccidiostats, nitroimidazoles, anthelmintics and NSAIDs from animal studies for proficiency testings (PT) for more than 13 years. The presentation provides a detailed picture of how to evaluate PTs for the determination of veterinary drugs. Proficiency testings are evaluated according to the prescriptions of ISO/IEC 17043, ISO 13528 and DIN 38402-45, using the statistical software package “ProLab Plus 2014” (QuoData GmbH in Dresden, Germany). The presentation illustrates the statistical parameters for: (a) Single analyte concentration (robust estimation of assigned value (HAMPEL), reproducibility standard deviation (Q-method) and scoring of laboratory results, (b) Graphical methods for combining performance scores (z-scores overview, combination scores and Mandels-h statistics), (c) Graphical methods for monitoring performance over years (control charts for z-scores and combination scores). In view of false negative results, it was regarded as not sufficient to include only the z-scores and combination scores for the assessment of the laboratories proficiency. Therefore a modified combination score was established that accounts for the overall evaluation of the examination results as well as for the false negative (n. a.—not analysed and n. f.—not found) examination results.