MIC determination, disc sensitivity testing, and analysis of regression in cefazedone and cefazolin

Minimum inhibitory concentrations (MIC) of (6R,7R)-7-(2-[3,5-dichloro-4-oxo-1(4H)-pyridyl-a1-acetamido)-3-([(5-methyl-1,3,4-thiadiazol-2-yl)-thio]methyl)-8-oxo-5-thia-1-azabicyclo[4,2,0]oct-2-ene-2-carboxylic acid (cefazedone, Refosporen)), a new semisynthetic cephalosporin derivative were determined by a broth dilution technique. In comparison with cefazolin 130 gram-positive and gram-negative bacteria, recent clinical isolates, were tested. In parallel, inhibition zones for the same organisms were determined by a standardized disc technique using 30-micrograms discs. According to the calculated regression lines a good correlation was found for cefazedone and cefazolin between the MIC values and the diameters of the inhibition zones (correlation coefficients r = --0.90 and --0.92, respectively). Taking into account the dosages recommended for cefazedone and the mean serum concentrations to which they give rise, appropriate categories of sensitivity (break points) for susceptibility testing are recommended.