CHARACTERISTICS OF THE LACTEK TEST AS APPLIED TO TISSUE SAMPLES - ASSESSMENT OF PERFORMANCE USING INCURRED FIELD SAMPLES

The Lactek test, marketed for antimicrobial residue detection in milk, was validated for the detection of antimicrobial residues in tissues. A previous study found that the LacTek test could confidently identif y tissue samples spiked with antimicrobial residues. However, the test could not reliably distinguish violative from nonviolative spiked sam ples relative to Canadian maximum residue limits (MRLs). The objective s of this study were to assess and compare the performance of the LacT ek tests for beta-lactams, tetracyclines, gentamicin, and sulfamethazi ne on samples containing naturally incurred residues by running the te st in parallel with the standard microbial inhibition test (MIT) prese ntly used for the routine testing of tissues at our facility and to as sess the agreement with high pressure liquid chromatographic (HPLC) de terminative methods. Parallel testing with the official MIT found that the Lactek tests could be confidently used for testing tissue samples containing incurred residues. Among 1,008 MIT-positive samples, the L acTek test found that 90% contained beta-lactams and/or tetracyclines. A further 7.3% of violative residues could not be identified to an an timicrobial class. In addition, 9% of samples testing negative on the MIT were found to contain an antimicrobial residue by the LacTek tests . Comparative testing with HPLC methods found that there was very good agreement between the two tests and that most violations were due to penicillin G and oxytetracycline. Although the LacTek test cannot be u sed to distinguish violative from nonviolative residue levels, it does offer several advantages over the present MIT. These include speed, e ase of use, the ability to identify residues to a specific class, and an improved sensitivity at the MRL level for the most commonly found a ntimicrobials in tissue.