A Practical Cylinder Plate Assay Method for Determination of Chlortetracycline in Tissues

The regular bioassay method for determination of chlortetracycline (CTC) residue in tissues1, 2, 3, 4) presents a difficulty when dealt with many samples for the analysis. Experiences have revealed that such procedures are very tedious as washing and drying of many Petri dishes, the preparation of assay medium in each dish and the standard curve for each set of determination. Attempt has been made, therefore, to simplify the method without sacrificing its accuracy. It was ascertained that routine procedures could be greatly simplified by substituting the Petri dish with glass plate as was reported by GRADY and WILLIAMS1a). It was found that the slope of inhibitory zone size on logarithm of CTC concentration remained constant irrespective of difference in plate and date of the determination. Therefore, the preparation of the standard curve at each determination can be omitted by employing the linear regression line instead when it is calibrated for the difference in zone size for 0.01mcg/cc between the regression line and a sample test plate.