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Incidence of False-Positive Results for Assays Used To Detect Antibiotics in Milk

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The incidence of false-positive results from milk assays for antimicrobial agents was determined for composite milk samples collected from 407 lactating dairy cows with a history of no antibiotic treatment for a minimum of 30 days. Milk samples were also cultured for bacteria and analyzed for somatic cell count. Mean herd prevalence of intramammary infections (±SEM) caused by Streptococcus agalactiae and Staphylococcus aureus was 3.3 ± 2.8 and 20.2 ± 9.5% of lactating cows, respectively.

All 407 milk samples were assayed for antibiotics with three commercial tests; a fourth test was used to assay 391 samples. Samples were assayed twice with each test, and if the results from these repetitions did not agree, a third assay was performed and the result obtained in two of the three repetitions was reported. Because samples were only collected from cows with no antimicrobial treatment for a minimum of 30 days, positive assays were considered to be false-positive results. Three test kits did not yield any false-positive results, one test kit had 5 false-positive results of 407 samples collected (specificity, 98.8%). Although there was a trend for false-positive samples to have a higher somatic cell count than negative samples, the low incidence of false-positive results did not allow a meaningful comparison.

We conclude that the incidence of false-positive results is very low when testing milk from cows that have a history of no clinical mastitis or antimicrobial treatment.


Document Type: Research Article

Affiliations: 1: Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824, USA 2: Michigan Milk Producers Association, Novi, Michigan 48376-8002, USA

Publication date: August 1, 1996

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