Abstract
The objective of this study was to identify factors affecting the accuracy of four commercial tests for ceftiofur drug residue in milk samples from bulk tank waste milk (WM). WM samples were collected from 12 California dairy farms which were initially tested using liquid chromatography (LC-MS/MS) to confirm their negative status for drug residues above the FDA established tolerance/safe levels. The milk samples were also tested for fat, protein, lactose, solids non-fat (SNF), somatic cell count (SCC), coliform count, and standard plate count (SPC). Each WM sample was divided into two aliquots, one labeled as negative for drug residues (WMN) and the second spiked with ceftiofur as positive for ceftiofur residues (WMPos). Both types of WM samples were tested to evaluate the performance of 4 commercially available tests: Penzyme® Milk Test, SNAP® β-lactam, BetaStar® Plus and Delvo SP-NT®. Three assays in triplicates for the WMN and WMPos were conducted for each WM sample. Test were evaluated using sensitivity, specificity, positive predictive value, negative predictive value and positive likelihood ratio. Kruskal-Wallis method was used to evaluate the effect of milk quality parameters on true positive (TP) and false negative (FN) test results. All WMPos samples were identified as positive by all four tests, rendering 100% sensitivity for each test. The specificity for Penzyme, BetaStar, Delvo, and SNAP tests were 59.2, 55.5, 44.4, and 29.6, respectively. Overall, all tests correctly identified samples with ceftiofur residues (WMPos), as shown by 100% sensitivity. Greater variability was observed regarding identification of samples free of any drug residue, with Penzyme and BetaStar having the highest risk for correctly identifying TN samples. Our findings indicate that when selecting commercial tests to detect drug residues in WM, milk quality parameters must be considered if the aim is to reduce FP test results.
Highlights
Antimicrobial resistance is a great concern for human and animal health
The objective of this study was to identify factors affecting the accuracy of four commercial tests for ceftiofur drug residue in milk samples from bulk tank waste milk (WM)
Mean value for WM quality parameters for samples used in the study (n = 9) were: 4.7% (3.4–5.9) fat, 3.7% (3.4–3.9) protein, 4.3% (4.1–4.5) lactose, 8.7% (8.5–8.9) solids non-fat (SNF), 2,378 104cells/ml (1,457–3,299) somatic cell count (SCC), 293 CFU/ml (81–667) coliform count, and 92,000 CFU/ml (39,382–144,617) standard plate count (SPC) (S3 Table)
Summary
Antimicrobial resistance is a great concern for human and animal health. Ceftiofur is a thirdgeneration cephalosporin, a drug class of critical importance to both human and veterinary medicine; extending bacterial susceptibility to ceftiofur is a priority [1, 2]. Transition milk from recently calved dairy cows and milk from hospital cows that contains drug residues during treatment regimens are not fit for human consumption. Such milk, commonly identified as waste milk (WM), is harvested separately from saleable milk and utilized as a feed source for newborn calves by one-third of US dairy farms [3] and by 75.2% of CA dairies in a recent survey [4]. Feeding calves WM containing ceftiofur residues at the concentrations observed on dairies has been shown to result in selection of E. coli resistant to ceftiofur and ceftriaxone, as well as other medically important drugs [7]
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