Validation of receptor-binding assays to detect antibiotics in goat's milk

The suitability of different receptor-binding assays to detect antibiotics in raw goat's milk was investigated. Detection capability of most β-lactams and tetracyclines assessed applying the Betastar Combo, the SNAP Betalactam, the SNAP Tetracycline, and the Twinsensor tests was at or below maximum residue limits established by European legislation. Regarding test specificity, cross-reactions with antibiotics other than β-lactams and tetracyclines were not found, and no false-positive results were obtained for the Betastar Combo and the SNAP tests when bulk samples of goat's milk were analyzed. For the Twinsensor test, the false-positive rate was 1%. The performance of the Betastar Combo and the SNAP tests was practically unaffected by the milk quality parameters using individual samples of goat's milk collected at points throughout the entire lactation period (false-positive rate, ≤5%). However, a larger number of positive results were obtained by the Twinsensor test in this type of milk sample (>10%), especially in the last weeks of lactation. Interferences related to the use of the preservative azidiol were not observed in any case. Neither were any significant differences found in relation to the interpretation method (visual versus instrumental) applied. In general, the response of the Betastar Combo, SNAP, and Twinsensor tests was optimal for the analysis of bulk caprine milk; thus, they may be used to monitor milk for the presence of β-lactam and tetracycline residues in quality control programs.