Chemical tests commonly used to screen for antibiotics in the milk of commercial dairy cows often falsely indicate the presence of antibiotic residues, according to a recent study by a veterinary researcher at UC Davis. The tests are frequently unable to differentiate between disease-fighting agents produced naturally by the cow's own immune system and antibiotics administered to the cow for medical treatment, says Dr. James S. Cullor, associate professor of pathology at the UC Davis School of Veterinary Medicine. Such false positive tests could unduly alarm the public, result in unwarranted discard of milk, prove costly to the dairy producer and cast a shadow of suspicion on the actions of the producer, veterinarian and antibiotic manufacturer, he says. Cullor presented his findings this month during the annual meeting of the National Mastitis Council. Residues of antibiotics, which are used to treat a variety of diseases in dairy cattle, are undesirable in commercial milk from a food safety standpoint and for processing reasons. After running more than 4,000 tests on approximately 600 cows at commercial dairies in Northern California, Cullor and his colleagues determined that the tests, which are efficient when used to monitor bulk tanks of milk, are not specific enough to use when testing the milk of an individual cow. "Technology simply is not ready to do what the regulations require," says Cullor.