Proficiency testing for the evaluation of the ability of European Union-National Reference laboratories to determine aflatoxin M1 in milk at levels corresponding to the new European Union legislation

In 1992, the European Union set up a network of National Reference Laboratories and charged the Community Reference Laboratory with the responsibility to design a proficiency testing scheme for assessing the analytical ability of laboratories involved in the official control of aflatoxin M 1 in milk. Since 1996, two exercises of proficiency testing have been performed on samples of milk powder and liquid milk at various levels of aflatoxin M 1 contents. The trials were conducted according to ISO Guide 43, in particular for the homogeneity testing of sample batches and for the calculation of laboratory z-scores. The National Reference Laboratories officially designated by their governments participated in this programme. Samples were naturally-contaminated milk obtained by feeding cows with aflatoxin B 1 -contaminated feed. The levels of aflatoxin M 1 in the samples ranged from 0.2 to 0.7 μg/kg in milk powder and from 0.05 to 0.07 μg/I in liquid milk. These levels were chosen as being close to the European Union-regulated limit of 0.05 μg of aflatoxin M 1 per litre. The results produced by laboratories were compiled and statistically analysed to detect any outlying results and to calculate the individual z-scores. Except for one laboratory in each exercise, all laboratories exhibited acceptable or questionable z-scores. The interlaboratory relative standard deviation for reproducibility (RSD R ) obtained for both 1996 and 1998 exercises were in the range 15.7-30.3%. Compared with other published studies, this indicates a very good precision for the performance of this laboratory network in the analysis of traces of aflatoxin M 1 in milk.