Reliability of three bulk-tank antimicrobial residue detection assays usedto test individual milk samples from cows with mild clinical mastitis

Objective To determine the likelihood of false-positive results when testin g milk samples from individual cows by use of 3 commercially available assa ys (Penzyme Milk Test and the SNAP beta -lactam and Delvo-SP assays) labele d for use with commingled milk. Sample Population-Milk samples from 111 cows with mild clinical mastitis. Procedure-Cows were randomly assigned to the control (no antimicrobials) or intramammary treatment group, Posttreatment milk samples were collected at the first milking after the labeled withholding period or an equivalent ti me for controls, randomly ordered, and tested twice by use of each assay an d once by use of high-performance liquid chromatography. Sensitivity, speci ficity, and positive and negative predictive values were determined for eac h assay. Concordance of results for the same sample was assessed for each a ssay by calculating kappa. Results-Sensitivities of the Delvo-SP and SNAP beta -lactam assays were > 9 0%, whereas the sensitivity of the Penzyme Milk Test was 60%. Positive pred ictive values (range, 39.29 to 73.68%) were poor for all 3 assays. Concorda nce of test results was excellent for the SNAP beta -lactam and Delvo-SP as says (kappa = 0.846 and 0.813, respectively) but was less for the Penzyme M ilk Test (kappa = 0.545). Conclusions and Clinical Relevance Because of the low positive predictive v alues, these 3 assays may not be useful for detecting violative antimicrobi al residues in individual milk samples from cows treated for mild clinical mastitis. However, repeatability of each assay was considered good to excel lent.