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 h undred cows were treated with one of two intramammary beta-lactam anti biotics, and the remaining 72 cows were treated with intramuscular oxy tocin. Milk samples were collected pretreatment, twice after therapy, and again 21 d following the initiation of treatment. Presumptive fals e-positive assay results were tabulated from all pretreatment and 21-d milk samples and from samples collected following oxytocin therapy. T he percentage of false-positive results was 43.6, 37.7, 81.7, 2.6, and 18.8% for the CITE(R) probe (beta-lactam), Delvotest-P(R), Charm Farm (R), LacTek(R) (beta-lactam), and Bacillus stearothermophilus var. cal idolactis disk assay, respectively. In four of the assay systems, aver age SCC were significantly higher in samples yielding false-positive r esults than in those with negative results. Specificity and sensitivit y were estimated for each assay system, and, based on these estimates, positive and negative predictive value curves were graphed as the pre valence of milk samples containing detectable concentrations of exogen ous antibiotic residues in the sample population was varied from 0 to 100%.