Nitroblue tetrazolium (NBT) reduction by bacteria employed for rapid determination of antibiotic concentrations in serum.

The ability of bacteria to reduce nitroblue tetrazolium (NBT) to formazan, and the inhibition of this process by antibiotics, have been utilized in a method to determine antibiotic concentrations in serum. Escherichia coli, growing in the presence of increasing but subinhibitory concentrations of antibiotics, produces diminished amounts of formazan. This is photometrically quantified and the optical density obtained with the sample is compared with those obtained with know concentrations. Levels of aminoglycosides below 1 microgram/ml can be measured in the presence of other antibiotics also, and the assay can be carried out in three hours. Good correlation was obtained when 49 serum samples containing varying concentrations of sisomicin were tested simultaneously by the NBT assay and a disc diffusion method.