Rapid multi-residue screening for veterinary antibiotics in animal feed: validation of existing lateral flow assays and a microbiological inhibition test to assess chicken feed

Undeclared antibiotics in poultry feed contribute to exposure to antimicrobial drug residues, particularly in low- and middle-income countries where routine surveillance is limited by cost and laboratory constraints. Conventional analytical screening requires specialised equipment, making rapid, accessible alternatives valuable. This study evaluated the feasibility of re-purposing qualitative antibiotic screening techniques, originally developed and validated for milk, in rapid, multi-residue antibiotic detection in broiler feed after specialised sample pre-treatment. A microbial inhibition assay and three lateral-flow tests were validated through CCβ determination, stability assessment, and false-positive evaluation. Blank feed samples (pellets, mash, and crumbles) from three Belgian suppliers were spiked using milk detection limits, and 124 Kenyan commercial broiler feed samples (November - December 2022) were screened. CCβ values were established for twelve antibiotic classes. The inhibition assay offered broad detection but higher CCβ values (200 to > 100,000 µg/kg) than rapid tests (3-800 µg/kg). Rapid tests produced no false positives; the inhibition assay had one false positive in 30 samples (3.3%). Extracted samples remained stable for 24 h at 4 ± 2 °C, and frozen controls performed consistently. LC-MS/MS confirmed tylosin, quinolones, and five sulphonamides in 12 of 124 field samples. These methods, currently validated for use in milk, reliably detected multiple antibiotic classes in broiler feed, showing promise as practical monitoring tools for antibiotic contamination or carryover in feed in resource-limited settings.