Turbidimetric microbiological determination of oxytetracycline hydrochloride in finished feeds and a feed premix: collaborative study.

New automated and manual turbidimetric microbiological assay methods for oxytetracycline (OTC) in finished feeds and a feed premix were submitted to 17 collaborating laboratories for comparison with the official AOAC method. Six finished feeds of 50 and 100 g/ton levels and one premix containing 20 g OTC/lb were sent to the collaborators. Results were received from 12 laboratories. Six laboratories provided results derived from the Autoturb System and 7 laboratories poerformed the turbidimetric analyses manually. All 12 laboratories assayed the samples by the AOAC cylinder-plate method. Ranges of recoveraies expressed as percent were 89.5-102.0, 91.2-97.0, and 91.4-96.0 for the AOAC, Autoturb turbidimetric, and manual turbidimetric methods, respectively. Average recoveries were 96.3, 93.4, and 93.6%, respectively. The mean relative standard deviation values were 5.7, 6.3, and 7.2, respectively. There was little significant difference between the turbidimetric methods and the AOAC method; however, the turbidimetric methods were faster.