The Italian National Reference Laboratory (NRL) for pesticides in Food of Animal Origin and Commodities with High Fat Content yearly organizes proficiency tests (PTs) on olive oil, addressed to: Mediterranean laboratories of International Olive Council, European laboratories (NRLs, official control laboratories and private laboratories), involved in the national and European monitoring programs. The main aim of these PTs was to compare the performances of the laboratories in Mediterranean and European countries in order to promote mutual acceptance of pesticide residue data regarding the analytical controls of olive oil. Since 2011, the protocol of the last three PTs has not been changed and also the list of participants has been steady, in the range of 40–44 laboratories. The test material consisted of commercial olive oil spiked with six different pesticides chosen in each exercise from a possible list of 25–26 compounds. In particular, the pesticide residues selected in our PTs are mainly those considered in the official control plans, with spiked mass fraction levels around their reference values set in the European Regulations. Pertinent and detailed information regarding multi-residue methods has been demanded to all participants, and an overall evaluation by means of a combined index of performance, such as the sum of the weighted z -scores, SWZ , and the sum of squared z -scores, SZ2 , has been performed. This type of exercise provides the possibility for the laboratories to control and improve the performance of their analytical methods employed during the official control on more parameters over the years. In this paper, a comparison of the overall performance of the laboratories was shown by using the SWZ or SZ2 indexes, giving significance to the Italian participants as required by the NRL aim. During the three investigated COIPTs, 91 % of the laboratories obtained satisfactory results with SWZ or SZ2 not exceeding 3. Focusing on the results of Italian laboratories, the performance improved during the selected PTs, respectively, from 70 % to 100 %. Moreover, the effect of the analytical methods on the dispersion of the results was evaluated in order to review the long-term performance of the participants.
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