Interlaboratory proficiency tests for the detection of staphylococcal enterotoxin type A in food.

Staphylococcal enterotoxins (SEs) are among the leading causes of food intoxications, affecting consumer health even in nanogram (ng) amounts. In the European Union, certain food safety criteria are specified, including the absence of SEs in cheeses, milk powder, and whey powder. Until 2019, the analytical reference method used was the European Screening Method, which was replaced by EN ISO 19020. For the official laboratories involved in food control, the German Reference Laboratory for coagulase-positive staphylococci including Staphylococcus aureus organized three interlaboratory proficiency tests (ILPTs) to detect SE type A in food during the years 2013-2018. The selected food products (cream cheese, vanilla pudding) were successfully tested beforehand with regard to easy handling, homogeneity, and stability of the added toxin. In 2013, ILPT participants overall were not competent in detecting SE type A in food. Identified factors to improve performance comprised: i) concentration of sample extract using dialysis, ii) selection of a sensitive detection kit, and iii) proper sample handling. By taking these factors into account and instructing and training the laboratories, their competence greatly improved. In 2018, all performance criteria (specificity, sensitivity, accuracy) were >90%, even at very low concentrations of SE type A of approximately 0.01 ng g-1 food.