In Sweden, milk delivered to dairies is assayed with either the Arla microtest or the Delvo test P for the presence of inhibitory substances. A positive result has to be verified with the Delvo test P to be considered positive. In the present investigation, Charm test II was used for confirmation analysis of approximately 300 milk samples positive in the routine assays. The samples were analyzed for the presence of beta-lactams, tetracyclines, and aminoglycosides. In addition, the samples were analyzed with the Valio T 101 test. Approximately 90% of the samples, which were positive in the verification with Delvo test P, were positive for beta-lactams in the Charm test II. Of the samples, which were found positive by the Arla microtest, 37% were negative in the verification with Delvo test P. The majority of these samples were found to be positive for tetracyclines in Charm test II. The T 101 test showed a different pattern of sensitivity compared to the Delvo test P and Arla microtest. Of the samples which were positive in the verification with Delvo test P, only 68% were positive in the T 101 test, while 49% of the samples which were positive in the Arla microtest but negative in the verification with Delvo test P were positive in the T 101 test. Most of the latter samples, i.e., 95%, were positive for tetracyclines in the Charm test II. It can be concluded that the microbial assays used have quite different patterns of sensitivity toward inhibitory substances in milk and that confirmation with Charm test II gives a high degree of clarification of antibiotic substances present.

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