Results of a European interlaboratory study for the determination of oxytetracycline in pig muscle by HPLC

An interlaboratory study was organised in 1996 in order to determine labora tory testing performance for measuring oxytetracycline (OTC) and 4-epioxyte tracycline (4-epiOTC) residues in pig muscle. The organisation was designed according to the 'Intemational Harmonised Protocol for Proficiency Testing of (Chemical) Analytical Laboratories' produced by the Association of Offi cial Analytical Chemists (AOAC international) and the international Union o f Pure and Applied Chemistry (IUPAC). Fourteen National Reference Laborator ies (NRLs) out of the 15 asked to participate agreed to analyse the samples . These laboratories were from 13 different European Union (EU) Member Stat es and each participant was allowed to use the extraction method of their o wn choice but had to use liquid chromatography as the analytical technique. Most of the methods used were based on UV detection (simple wavelength or diode-array detection) but some involved fluorimetric detection. The produc tion of incurred samples was prepared on-site. The OTC and 4-epiOTC concent rations present in the samples were determined by taking the mean of the re sults (excluding outliers) and the deviation of each result from the assign ed value was measured. The performance of the laboratories was evaluated by calculating the 'z-scores', The results were globally satisfactory and sho wed that all 14 laboratories were capable of determining OTC and 4-epiOTC i n pig muscle with satisfactory accuracy. Only two laboratories obtained a q uestionable result in terms of repeatability.