Effect of fat and protein content of milk from individual cows on the specificity rates of antibiotic residue screening tests

The effect of high concentrations of milk protein and milk fat on the speci ficity rates of several antibiotic residue screening tests was evaluated in this study. Milk was sampled from 60 Jersey and 30 Holstein cows at one mi lking and analyzed for beta -lactam residues using four antibiotic residue screening tests. Cows selected were not treated with an antibiotic for at l east 30 d prior to sample collection, and milk was visibly normal. Before m ilk collection, quarter foremilk was aseptically sampled for mastitis patho gen analysis. Milk subsamples were analyzed for fat and protein contents, a nd somatic cell counts (SCC). Ten Jersey and four Holstein cows were infect ed with one or more mastitis pathogens. Concentrations of milk fat (4.78 vs . 3.39%) and protein (3.81 vs. 3.00%) were greater for Jersey cows compared with Holstein cows. Milk SCC averaged 148,000/ml and did not differ by bre ed. The specificity rates were greater than 0.9 for three of the four scree ning tests. Across breeds, there was an increased probability of a false-po sitive outcome for the Penzyme test (Coulter Food Science, Milwaukee, WI)), with increased milk protein content and decreased SCC. Increased milk fat content was associated with an increased probability of false-positive outc omes for the CITE Snap test (IDEXX Laboratories, Inc., Westbrook, ME). High concentrations of milk protein and milk fat can adversely affect antibioti c residue test performance, but the degree of the effect is dependent upon the analytical method of the screening test.