Evaluation of Selected Antibiotic Residue Screening Tests for Milk from Individual Cows and Examination of Factors That Affect the Probability of False-Positive Outcomes

Abstract

Total composite milk samples from 131 cows in one herd were analyzed. Eight β-lactam residue screening tests were evaluated for performance using milk from individual cows and factors that affect the rate of false-positive outcomes were determined. Cows were not treated with an antibiotic for at least 30 d prior to sampling. Tests evaluated were Delvotest® P, Charm Cowside, Charm Farm, Penzyme®, Valio T101, LacTek™, CITE Probe®, and Charm Bacillus stearothermophilus disk assay. Cows averaged 155 d of lactation. Milk production at the time of sampling ranged from 3.6 to 26.3 kg per milking per cow. The somatic cell count of milk averaged 243 × 103/ml and ranged from 8.5 × 103/ml to 3437 × 103/ml. Total viable bacteria counts averaged 197.8 × 103/ml. Total coliform counts ranged from 0 to 205/ml. Selectivity rates (rate of truly negative samples that were found to be negative by the assay) were greater than 90% for all tests except the CITE Probe® test. Use of logistic regression showed that an increase in colony-forming units was associated with a decrease in the probability of a false-positive outcome for the CITE Probe® test. Milk production, coliform counts, and parity each affected the probabilities of positive outcomes for different tests. Except for one test, selectivity rates of the β-lactam residue screening tests for milk from individual cows was greater than 0.9.