Evaluation of the cefonicid disk test criteria, including disk quality control guidelines

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Abstract
Cefonicid (SKF 75073) is a 2nd-generation cephalosporin which has a spectrum of antibacterial activity similar to that of cefamandole; cefoxitin (a cephamycin) and cephalothin have uniquely different spectra of activity. The 2nd-generation cephalosporins tested displayed comparable susceptibility to .beta.-lactamases and inhibited type I .beta.-lactamases. Although cefonicid has a longer serum half-life (3 to 4 h) than the currently used drugs, the same minimal inhibitory concentration breakpoints separating susceptible and resistant categories were applied to tests with cefonicid, cefamandole and cephalothin. Regression analysis of the disc diffusion test results confirmed the use of identical zone size breakpoints for 30 .mu.g cefonicid, cefamandole and cephalothin disc; all produced similar parabolic regression lines. Further analysis of disc test data confirmed that cefonicid and cefamandole discs might be used interchangeably. For routine tests, cefonicid discs might be preferred in order to minimize very major (false-susceptible) interpretive errors. Suggested cefonicid 30 .mu.g disc interpretive criteria are: susceptible, .gtoreq. 18 mm (.ltoreq. 8.0 .mu.g/ml), and resistant, .ltoreq. 14 mm (> 16 .mu.g/ml). Quality control zone diameter limits were calculated from data obtained in a multilaboratory collaborative study.