Laboratory proficiency testing of aflatoxins in corn and peanuts--a cooperative project between Thailand and the United States

The objective of this project was to conduct an aflatoxin proficiency test program in government, academia, and industry laboratories in Thailand. Aflatoxin-free corn and peanuts and corn and peanuts naturally contaminated with aflatoxins diluted to approximately 25 micrograms/kg were analyzed. Homogeneity of prepared, naturally contaminated test samples was checked on multiple replicates. The test was conducted according to the ISO/IUPAC/AOAC INTERNATIONAL Harmonized Protocol with z scores indicating laboratory performance. The participants used 3 methods: enzyme-linked immunosorbent assay, thin-layer chromatography, and the minicolumn. Of 19 laboratories that reported results for aflatoxins in naturally contaminated corn, 13 (68%) performed satisfactorily, on the basis of the mean obtained by an expert laboratory, a calculated target value for standard deviation, and the z score. Of 21 laboratories that reported results for aflatoxins in naturally contaminated peanuts, 10 (48%) performed satisfactorily. For aflatoxin-free corn, 6 laboratories reported finding aflatoxins at > or = 10 ng/g, chiefly by the minicolumn method; for aflatoxin-free peanuts, 1 laboratory reported finding aflatoxins at > 10 ng/g. Subsequently, a workshop of lectures and laboratory sessions was conducted to improve performance. A new and simple successive outlier removal procedure applied to the same data removed the same laboratories as did the use of z scores.