Lee Jaewoong, Park Yeon Joon, Park Dong Jin, Park Kang Gyun, Lee Hae Kyung
Department of Laboratory Medicine, College of Medicine, Seoul St. Mary's Hospital, Catholic University of Korea, Seoul, Korea.
Department of Laboratory Medicine, College of Medicine, Uijeongbu St. Mary's Hospital, Catholic University of Korea, Uijeongbu, Korea.
Ann Lab Med. 2017 Jan;37(1):39-44. doi: 10.3343/alm.2017.37.1.39.
We evaluated the performance of the BD MAX StaphSR Assay (SR assay; BD, USA) for direct detection of Staphylococcus aureus and methicillin resistance not only in S. aureus but also in coagulase-negative Staphylococci (CNS) from positive blood cultures.
From 228 blood culture bottles, 103 S. aureus [45 methicillin-resistant S. aureus (MRSA), 55 methicillin-susceptible S. aureus (MSSA), 3 mixed infections (1 MRSA+Enterococcus faecalis, 1 MSSA+MRCNS, 1 MSSA+MSCNS)], and 125 CNS (102 MRCNS, 23 MSCNS) were identified by Vitek 2. For further analysis, we obtained the cycle threshold (Ct) values from the BD MAX system software to determine an appropriate cutoff value. For discrepancy analysis, conventional mecA/mecC PCR and oxacillin minimum inhibitory concentrations (MICs) were determined.
Compared to Vitek 2, the SR assay identified all 103 S. aureus isolates correctly but failed to detect methicillin resistance in three MRSA isolates. All 55 MSSA isolates were correctly identified by the SR assay. In the concordant cases, the highest Ct values for nuc, mecA, and mec right-extremity junction (MREJ) were 25.6, 22, and 22.2, respectively. Therefore, we selected Ct values from 0-27 as a range of positivity, and applying this cutoff, the sensitivity/specificity of the SR assay were 100%/100% for detecting S. aureus, and 97.9%/98.1% and 99.0%/95.8% for detecting methicillin resistance in S. aureus and CNS, respectively.
We propose a Ct cutoff value for nuc/mec assay without considering MREJ because mixed cultures of MSSA and MRCNS were very rare (0.4%) in the positive blood cultures.
我们评估了BD MAX葡萄球菌SR检测法（SR检测法；美国BD公司）用于直接检测金黄色葡萄球菌以及不仅在金黄色葡萄球菌中而且在来自阳性血培养物的凝固酶阴性葡萄球菌（CNS）中的耐甲氧西林情况的性能。
通过Vitek 2从228瓶血培养物中鉴定出103株金黄色葡萄球菌[45株耐甲氧西林金黄色葡萄球菌（MRSA）、55株甲氧西林敏感金黄色葡萄球菌（MSSA）、3例混合感染（1例MRSA + 粪肠球菌、1例MSSA + MRCNS、1例MSSA + MSCNS）]和125株CNS（102株MRCNS、23株MSCNS）。为了进一步分析，我们从BD MAX系统软件中获取循环阈值（Ct）值以确定合适的截断值。对于差异分析，测定了常规mecA/mecC聚合酶链反应（PCR）和苯唑西林最低抑菌浓度（MIC）。
与Vitek 2相比，SR检测法正确鉴定了所有103株金黄色葡萄球菌分离株，但未能检测出3株MRSA分离株中的耐甲氧西林情况。所有55株MSSA分离株均被SR检测法正确鉴定。在一致性病例中，nuc、mecA和mec右端连接点（MREJ）的最高Ct值分别为25.6、22和22.2。因此，我们选择0至27的Ct值作为阳性范围，应用此截断值时，SR检测法检测金黄色葡萄球菌的灵敏度/特异性分别为为100%/100%，检测金黄色葡萄球菌和CNS中耐甲氧西林情况的灵敏度/特异性分别为97.9%/98.1%和99.0%/95.8%。
由于在阳性血培养物中MSSA和MRCNS的混合培养非常罕见（0.4%），我们提出了一个不考虑MREJ的nuc/mec检测法的Ct截断值。
