PERFORMANCE OF VARIOUS TESTS USED TO SCREEN ANTIBIOTIC RESIDUES IN MILK SAMPLES FROM INDIVIDUAL ANIMALS

The 10-point Milk and Dairy Beef Quality Assurance Program was develop ed collaboratively by the National Milk Producers Federation and the A merican Veterinary Medical Association and is designed to promote and document the responsible use of antibiotics in the dairy industry. One area of emphasis in this program is testing of individual animals for antibiotic residues after a specified post-treatment withdrawal time. We examined the performance of various assay systems on milk samples from individual cows. These assays are used at present on bulk tank mi lk samples by regulatory agencies, processing plants, producers, and v eterinarians to detect the presence of beta-lactam antibiotics. A high proportion of false-positive results was obtained for both the pretre atment milk samples from cows with clinical mastitis and the milk samp les obtained 21 days after initial therapy (nonantibiotic and antibiot ic) for the treatment of mastitis. A high proportion of false-positive outcomes was obtained from the milk of clinically normal cows that ha d not received any medication for at least 30 days prior to evaluation . The results indicate a serious problem in the use of some assays tha t were designed to evaluate residues bulk tank milk samples to analyze samples from individual cows. This error in assay specificity results in the unjustifiable discarding of milk that meets regulatory standar ds and may be misused to accuse the producer or veterinarian of not ad hering to regulatory guidelines. Maintaining a safe, high-quality milk supply is a constant goal of the dairy industry, which must be provid ed the appropriate tools and techniques to meet this challenge.