Preparation of Ormetoprim–Sulfadimethoxine‐Medicated Discs for Disc Diffusion Assay

Romet (a blend of ormetoprim and sulfadimethoxine) is a type A medicated article for the manufacture of medicated feed in the catfish industry. Recently, the commercial manufacture of ormetoprim–sulfadimethoxine susceptibility discs was discontinued. Ormetoprim–sulfadimethoxine discs were prepared at the Mississippi State University College of Veterinary Medicine Aquatic Diagnostic Laboratory according to Clinical and Laboratory Standards Institute guidelines. The performance of the laboratory‐prepared discs was compared with that of commercially prepared discs in disc diffusion assays with 23 field isolates of Edwardsiella ictaluri. The a priori limits of agreement for the laboratory‐prepared ormetoprim–sulfadimethoxine discs were ±5 mm of the zones for the commercial discs. The resulting zones of inhibition ranged from 33 to 50 mm and from 35 to 52 mm for the laboratory‐prepared and commercially prepared discs, respectively. The data comparing the two sets of discs were analyzed to determine the level of agreement. The analysis demonstrated that the laboratory‐prepared discs produced a slightly smaller zone of inhibition than the commercially prepared ones. The observed limits of agreement were within the a priori established limits of agreement, indicating that the laboratory‐prepared ormetoprim–sulfadimethoxine discs were acceptable replacements for the commercially prepared discs. Received April 21, 2010; accepted June 29, 2010