False-Positive Outcome and Drug Residue in Milk Samples Over Withdrawal Times

This study was conducted to identify false-positive outcomes and drug residues in milk samples over withdrawal times and to determine whether the positive results were caused by drug residues or natural inhibitors. A total of 73 milk samples over withdrawal times after the last intramammary infusion were collected from each treated quarter of cows and tested using the Delvotest SP assay. Reading time was 150, 165, and 180 min, and results of samples were recorded according to the color of the well containing the control milk sample. There were 24, 20, and 12 positive samples at the reading times of 150, 165, and 180 min, respectively. All 24 positive milk samples were heated at 82°C for 5 min and retested to verify that the positive results were caused by drug residues or natural inhibitors. Twenty-one samples that exhibited positive results were negative after heat treatment, and drug residues were not identified by LacTek and Charm tests. However, 3 samples that exhibited positive results from heat treatment of 82°C were positive for drugs. In our study, most positive results (89%) in the milk samples over withdrawal times were false-positive results by natural inhibitors. Moreover, the heat treatment is a fast, simple, and inexpensive method to remove false-positive results and has no effect on positive samples containing drugs. We suggest that heat treatment before screening tests is an effective way to reduce false-positive results in the milk samples.