Evaluation of milk antibiotic residue screening tests in cattle with naturally occurring clinical mastitis

Milk from 172 commercial cows with mild to moderate clinical mastitis was tested with five antibiotic residue detection assay systems. One hundred cows were treated with one of two intramammary beta-lactam antibiotics, and the remaining 72 cows were treated with intramuscular oxytocin. Milk samples were collected pretreatment, twice after therapy, and again 21 d following the initiation of treatment. Presumptive false-positive assay results were tabulated from all pretreatment and 21-d milk samples and from samples collected following oxytocin therapy. The percentage of false-positive results was 43.6, 37.7, 81.7, 2.6, and 18.8% for the CITE probe (beta-lactam), Delvotest-P, Charm Farm, LacTek (beta-lactam), and Bacillus stearothermophilus var. calidolactis disk assay, respectively. In four of the assay systems, average SCC were significantly higher in samples yielding false-positive results than in those with negative results. Specificity and sensitivity were estimated for each assay system, and, based on these estimates, positive and negative predictive value curves were graphed as the prevalence of milk samples containing detectable concentrations of exogenous antibiotic residues in the sample population was varied from 0 to 100%.