A bridging study for oxytetracycline in the edible fillet of rainbow trout: Analysis by a liquid chromatographic method and the official microbial inhibition assay

Oxytetracycline (OTC) is a drug approved by the U.S. Food and Drug Administ ration (FDA) to control certain diseases in salmonids and catfish. OTC is a lso a likely control agent for diseases of other fish species and for other diseases of salmonids and catfish not currently on the label. One requirem ent for FDA to extend and expand the approval of this antibacterial agent t o other fish species is residue depletion studies. The current regulatory m ethod for OTC in fish tissue, based on microbial inhibition, lacks sensitiv ity and specificity. To conduct residue depletion studies for OTC in fish w ith a liquid chromatographic method, a bridging study was required to deter mine its relationship with the official microbial inhibition assay. Triplic ate samples of rainbow trout fillet tissue fortified with OTC at 0.3, 0.6, 1.2, 2.4, 4.8, and 9.6 ppm and fillet tissue with incurred OTC at approxima tely 0.75, 1.5, and 3.75 ppm were analyzed by high-performance liquid chrom atography (HPLC) and the microbial inhibition assay. The results indicated that the 2 methods are essentially identical in the tested range, with mean coefficients of variation of 1.05% for the HPLC method and 3.94% for the m icrobial inhibition assay.