Selection of Zone Size Interpretive Criteria for Disk Diffusion Susceptibility Tests of Three Antibiotics against Streptococcus pneumoniae, Using the New Guidelines of the National Committee for Clinical Laboratory Standards

Disk diffusion and broth microdilution susceptibility tests were performed with cefotaxime, ceftriaxone, telithromycin, and erythromycin (control) against 407 selected isolates of Streptococcus pneumoniae. Scattergrams were prepared from the results of these tests, and the current NCCLS guidelines for setting disk diffusion test interpretive criteria were applied. Erythromycin zone diameter breakpoints were confirmed. Telithromycin interpretive criteria for the disk test could be easily set with acceptable discrepancy rates. For cefotaxime and ceftriaxone, the minor discrepancy rates for MICs in the intermediate category ± 1 dilution were far in excess of the acceptable 40% limit, i.e., 52 and 71%, respectively. We conclude that the 30-μg disk of these two drugs cannot be reliably used to test pneumococci.

本研究采用头孢噻肟（cefotaxime）、头孢曲松（ceftriaxone）、泰利霉素（telithromycin）及红霉素（erythromycin，作为对照），对407株筛选获得的肺炎链球菌（Streptococcus pneumoniae）临床分离株开展纸片扩散法（disk diffusion）与肉汤微量稀释法（broth microdilution）药敏试验。基于上述试验结果绘制散点图，并采用当前美国国家临床实验室标准化委员会（NCCLS）制定的纸片扩散法药敏试验判读标准进行分析。结果证实了红霉素抑菌圈直径折点的合理性；泰利霉素的纸片扩散法药敏试验判读标准可便捷设定，且差异率符合可接受范围。针对头孢噻肟与头孢曲松，中介度类别下最低抑菌浓度（MIC）值±1倍稀释度的次要差异率分别达52%与71%，远高于可接受的40%限值。本研究结论认为，采用30μg剂量的上述两种药物纸片，无法可靠地对肺炎链球菌进行药敏检测。