Modifications to the official method for testing chlortetracycline HCl in animal feeds.

Results are compared for the microbiological analysis of chlortetracycline using the AOAC method and a modified method applicable to potencies above 50 g/ton. Two modifications are presented: substitution of a pH range of 4.0-4.5 instead of the specified pH of 4.5 for the plating solution, and substitution of extraction by shaking instead of the blending procedure. There were no significant differences in results between the AOAC method and the modified method.