Proficiency testing for the evaluation of the ability of European Union-National Reference laboratories to determine aflatoxin M-1 in milk at levels corresponding to the new European Union legislation

In 1992, the Europe an Union set up a network of National Reference Laborat ories and charged the Community Reference Laboratory with the responsibilit y to design a proficiency testing scheme for assessing the analytical abili ty of laboratories involved in the official control of aflatoxin M-1 in mil k. 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 c ontents. The trials were conducted according to ISO Guide 43, in particular for the homogeneity testing of sample batches and for the calculation of l aboratory z-scores. The National Reference Laboratories officially designat ed by their governments participated in this programme. Samples were natura lly-contaminated milk obtained by feeding cows with aflatoxin B-1-contamina ted feed. The levels of aflatoxin M-1 in the samples ranged from 0.2 to 0.7 mug/kg in milk powder and from 0.05 to 0.07 mug/l in liquid milk. These le vels were chosen as being close to the European Union-regulated limit of 0. 05 mug of aflatoxin M-1 per litre. The results produced by laboratories wer e compiled and statistically analysed to detect any outlying results and to calculate the individual z-scores. Except for one laboratory in each exerc ise, all laboratories exhibit ed acceptable or questionable z-scores. The i nterlaboratory relative standard deviation for reproducibility (RSDR) obtai ned 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 aflato xin M-1 in milk.