Microbiological bioassay of erythromycin thiocyanate: optimisation and validation

The validation of an analytical method for the quantitative determination of erythromycin thiocyanate formulated in an antibiotic preparation for veterinary use was carried out. This method is based on the microbiological method described in the European Pharmacopoeia to analyze erythromycin thiocyanate as a raw material. This erythromycin thiocyanate preparation is presented as a powder for oral administration after mixing with feed. For that reason, it was planned to validate the method for the quantitative determination of erythromycin thiocyanate incorporated both in the medicated premix and the mixture with feed. The microbiological method followed a linear model and was not proportional. The number of replicates needed to obtain a valid result was less than four in all cases. The small difference in concentration, expressed in natural logarithm detected by the method, was 0.1.