苯唑西林和头孢西丁纸片扩散法现行耐药折点判断凝固酶阴性葡萄球菌甲氧西林耐药性的评价

Evaluation of the breakpoints of oxacillin and cefoxitin discs for judging methicillin resistance in coagulase-negative staphylococci

目的评价苯唑西林和头孢西丁纸片扩散法现行耐药折点在判断凝固酶阴性葡萄球菌(CNS)甲氧西林耐药性中的准确性。方法苯唑西林(1μg)和头孢西丁(30μg)纸片扩散法测定149株临床分离的CNS对甲氧西林的耐药性,量取两种纸片的抑菌环直径。对苯唑西林组分别以≤14mm、≤15mm、≤16mm、≤17mm(CLSI/NCCLS2004标准)为耐药折点,对头孢西丁组分别以≤21mm、≤22mm、≤23mm、≤24mm(CLSI/NCCLS2004年标准)为耐药折点,与mecA基因检测结果比较,评价两种纸片扩散法在各折点对耐甲氧西林CNS判断的敏感性和特异性。结果与mecA基因检测结果比较,苯唑西林纸片扩散法在不同的耐药折点判断耐甲氧西林表皮葡萄球菌的敏感性分别为52.3%、75.0%、90.9%、100%,特异性都为100%,判断耐甲氧西林非表皮葡萄球菌CNS的敏感性分别为66.7%、79.0%、90.1%、97.5%,特异性分别为100%、100%、100%、94.4%;头孢西丁纸片扩散法在不同耐药折点判断耐甲氧西林表皮葡萄球菌的敏感性分别为45.5%、70.1%、90.9%、100%,特异性都为100%,判断耐甲氧西林非表皮葡萄球菌CNS的敏感性分别为83.6%、91.8%、97.3%、100%,特异性都为100%。结论苯唑西林和头孢西丁纸片扩散法检测耐甲氧西林CNS的现行耐药折点标准在判断表皮葡萄球菌和非表皮葡萄球菌CNS的甲氧西林耐药性中都有很高的敏感性和特异性,适用于CNS对甲氧西林耐药性的判断。对于非表皮葡萄球菌CNS,头孢西丁纸片扩散法较苯唑西林纸片扩散法结果更可靠。

Objective To evaluate the accuracy of the existing oxacillin and cefoxitin disc diffusion method for the resistant breakpoints judgement of methicillin resistance in coagulase-negative staphylococci （CNS）. Methods One hundred and forty-nine strains of clinically isolated CNS were tested. The oxacillin （1μg）, cefoxitin （30μg） disc diffusion methods and PCR amplification of the mecA gene were performed simultaneously. The diameters of the inhibition zone were measured. Taking ≤ 14mm, ≤ 15mm, ≤ 16mm and ≤ 17mm （CLSI/NCCLS 2004 standard） as the resistant breakpoints to oxacillin group; taking ≤ 21mm, ≤ 22mm, ≤ 23mm and ≤ 24mm （CLSI/NCCLS 2004 standard） as the resistant breakpoints to cefoxitin group, respectively. The sensitivity and specificity of the above two groups in the judgement of methicillin-resistant CNS were evaluated by comparing with the results of mecA PCR. Results Compared with the results of mecA gene amplification, the sensitivity of oxacillin disc diffusion against S. epidermidis at different breakpoints were 52.3%, 75.0%, 90.9% and 100%, respectively, and the specificity of all were 100% in the judgement of methicillin-resistant S. epidermidis, while the sensitivity of oxacillin against methicillin-resistant non- S. epidermidis CNS were 66. 7%, 79.0%, 90. 1%, 97.5% and 100%, 100%, 100%, 94.4%, respectively. Those of cefoxitin disc diffusion against S. epidermidis at different breakpoints were 45.5%, 70. 1%, 90. 9% 100% and 100% , respectively, while the sensitivity of cefoxitin against non-S. epidermidis CNS were 83.6%, 91.8%, 97.3%, 100%, respectively and the specificity of all break points was 100% in non-S, epidermidis CNS. Conclusions The exsiting resistant breakpoints standards of oxacillin and cefoxitin disc diffusion methods for the judgement of methicillin-resistant CNS are of higher sensitivity and specificity to methicillin-resistant S. epidermidis and non- S. epidermidis, and can be used to judge methicillin resistance of CNS. Nevertheless the results of cefoxitin disc is more reliable than oxacillin disc for non-S, epidermidis CNS.