Turbidimetric assay for tetracyclines in feeds using a microtiter plate system

The microtiter plate system for turbidimetric assay of chlortetracycline (CTC) and oxytetracycline (OTC) levels in feeds uses a 96 well microtiter plate, a multichannel pipette, and an ELISA reader to measure turbidity. Feeds are extracted for both tetracyclines using AOAC extraction systems. For CTC, the range of the standard curve is 0.001-0.005 microgram CTC/mL; for OTC, the range is 0.004-0.016 microgram OTC/mL. Repeatability of CTC assays, as shown by the coefficient of variation (CV), ranged from 0.54 to 5.65% for same-day assays and from 2.01 to 9.39% for assays on different days. For OTC, CVs ranged from 2.69 to 10.01% for same-day assays and 3.24 to 9.08% for different-day assays. Average recoveries for CTC were 108.7% for same-day assays and 106.8% for different-day assays; for OTC, average recoveries were 112.4% and 106.5% for same-day and different-day assays, respectively.