Terbium-sensitised luminescence screening method for fluoroquinolones in beef serum

Enrofloxacin and danofloxacin are the only fluoroquinolone antibiotics approved for use in cattle in the United States. Microbial screening methods commonly used for monitoring veterinary drug residues are not sensitive or selective for fluoroquinolones. In this work, a luminescence-based screening assay was developed to detect fluoroquinolones in beef serum. This approach takes advantage of the DNA-enhanced luminescence signal of a fluoroquinolone-Tb⁺³ complex. In this method, serum samples were extracted with acidified acetonitrile in the presence of magnesium sulfate. After centrifugation, evaporation of the supernatant was followed by dissolution of the residue in buffer and filtration. Addition of Tb⁺³ and DNA then allowed a reading of the luminescence signal. The technique was illustrated using enrofloxacin, and provided good recoveries (73-88%) at 25, 50 and 100 ng ml⁻¹, with reasonable RSDs averaging at 11%. The LOD was 2.5 ng ml⁻¹ based on the variability of response of control serum samples from 18 different steers. The method provided no false-positive or false-negative results while screening blind samples for enrofloxacin and was demonstrated to be quantitative over a range of 0-100 ng ml⁻¹.