The problem of testing horse kidneys for the presence of antibiotics at meat inspection: how to avoid a false positive reaction.

When 33 horse kidneys were tested for the presence of inhibitory substances by the Bacillus subtilis BGA method at pH 8 and the Micrococcus luteus ATCC 9341 method, 24 were positive and 9 negative. The pH of the seeded M. luteus test medium changed from pH 6.6 before incubation to 8.7 after 24 hours incubation at 30 degrees C. When the same 33 kidneys were tested by the B. subtilis BGA method, medium pH 6, and 15 of them also by the M. luteus method using a medium buffered to pH 6, all were negative. The cadmium concentration of the 33 horse kidneys was found to be 70.17 +/- 81.28 mg/kg wet weight (m +/- s, range 10.40-355.67). The authors attributed the positive results to the presence of cadmium. The diffusion of cadmium from the horse kidney samples to the test media was found to differ at pH 6 and pH 8. It is recommended that the testing of horse kidney samples for the presence of antibiotics and chemoterapeutic substances be done at substrate pH 6 to avoid false positive reactions.