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Updated: Jan 24, 2026
Biosensor for Detection of Antibiotic Resistant Staphylococcus Bacteria
Published on: May 8, 2013
Incidence of False-Positive Results for Assays Used To Detect Antibiotics in Milk
Lisa W Halbert1, Ronald J Erskine1, Paul C Bartlett1
1Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824.
Abstract:
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.
Insights
False-positive results in milk antibiotic assays are rare in cows with no recent treatment. Three of four commercial test kits showed zero false positives, indicating high accuracy for antimicrobial residue testing.
Area of Science:
- Veterinary Medicine
- Food Safety
- Dairy Science
Background:
- Accurate detection of antimicrobial residues in milk is crucial for food safety and preventing antimicrobial resistance.
- Milk antibiotic testing is essential for ensuring milk quality and compliance with regulations.
- False-positive results can lead to unnecessary milk withdrawal, impacting dairy farm economics.
Purpose of the Study:
- To determine the incidence of false-positive results from commercial milk assays for antimicrobial agents.
- To evaluate the specificity of commonly used milk antibiotic test kits.
- To assess the reliability of antimicrobial residue testing in dairy cows with a documented history of no antibiotic treatment.
Main Methods:
- Composite milk samples were collected from 407 lactating dairy cows with no antibiotic treatment for at least 30 days.
- Samples were analyzed using four different commercial milk antibiotic assay kits, with repeated testing to ensure result validity.
- Milk bacterial cultures and somatic cell counts were performed to assess intramammary infection status.
Main Results:
- Three of the four commercial test kits yielded no false-positive results.
- One test kit produced 5 false-positive results out of 407 samples, indicating a specificity of 98.8%.
- The overall incidence of false-positive results was very low in milk samples from cows with no recent antimicrobial exposure.
Conclusions:
- Commercial milk assays for antimicrobial agents demonstrate high specificity and low false-positive rates when used on milk from cows with no recent antibiotic treatment.
- The findings support the reliability of these tests for detecting antimicrobial residues in dairy herds adhering to withdrawal periods.
- Minimizing false positives is key to maintaining milk supply integrity and farm profitability.
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