The present invention provides a test medium comprising a test microorganism, an indicator and a metal in ionized form, wherein the valence of said metal is at least 2 and the concentration of said metal is between 0.001 M and 1 M. Furthermore, there is provided a method for the determination of the presence of an antibiotic characterized in that a metal salt is added to said test medium and/or to said sample of fluid. Finally, there is provided a kit suitable for the determination of an antibiotic in a fluid.