Provided is a test system, a test method and a test kit for the determination of the presence of an antibiotic in a fluid based on a test medium comprising a test organism, at least one substance that provides a solid state, nutrients and an indicator, wherein the end-pH of said test medium at or after the time required for said determination is equal to or higher than 7.2 Said pH value can be realized by adding oligosaccharides, preferably a disaccharide or trisaccharide (eg. Lactose, gentobiose, maltotriose). Most preferably, said oligosaccharide is a non-reducing oligosaccharide (eg. Raffinose, sucrose or trehalose).