Evaluation of the Delvo-X-Press assay for detecting antibiotic residues inmilk samples from individual cows

Performance of the Delvo-X-Press beta-lactam antibiotic assay was examined using bulk-tank milk samples and milk samples from individual cows. Bulk-ta nk milk samples fortified with bovine lactoferrin at a concentration of 1 m g/ml or more consistently tested positive. False-positive results were also obtained from bulk-tank milk samples fortified with bovine plasma at conce ntrations of 20 and 40%. The assay yielded positive results for milk with a ntibiotic concentrations as low as 2 ppb. Individual milli samples were col lected from 144 healthy lactating cows and from 34 cows with chronic Staphy lococcus aureus mastitis. Specificity estimates for samples from healthy an d mastitic cows were 0.88 (95% confidence interval [CI], 0.82, 0.93) and 0. 94 (95% CI, 0.86, 1.00), respectively. Individual milk samples were collect ed from three cows with experimentally induced mastitis for 21 consecutive days. False-positive results occurred as late as 12 days postchallenge. A m oderate but significant (P < 0.01) positive linear correlation (r = 0.61) w as observed between test result and somatic cell count (SCC) values in milk samples with SCCs of >10(6)/ml.