Qian Q, Tang Y W, Kolbert C P, Torgerson C A, Hughes J G, Vetter E A, Harmsen W S, Montgomery S O, Cockerill F R, Persing D H
Division of Clinical Microbiology, Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota 55905, USA.
J Clin Microbiol. 2001 Oct;39(10):3578-82. doi: 10.1128/JCM.39.10.3578-3582.2001.
In a previous study which evaluated the BACTEC 9240 automated blood culture system (Becton Dickinson Diagnostic Instrument Systems, Sparks, Md.), we noted a 1.3% "instrument false-positive" rate. That is, the BACTEC system signaled that a bottle (BACTEC Plus Aerobic/F bottle or BACTEC Anaerobic Lytic/10 bottle) culture was positive but a Gram stain was negative and there was no growth of bacteria or yeasts on subculture to chocolate agar. Furthermore, from the same sample of blood, cultures for fungi using the Isolator blood culture system (Wampole Laboratories, Cranbury, N.J.) were negative for growth. For the present study, we evaluated 76 instrument false-positive samples for the presence of 16S ribosomal DNA using the MicroSeq 500 kit (PE Biosystems, Foster City, Calif.). These samples also were negative for fungi by the Isolator method. This kit has a PCR module and sequencing module for the amplification and sequencing of the 16S RNA gene and provides a database for sequence alignment and identification of bacteria. To optimize the assay, we evaluated the effect of adding 0.5% bovine serum albumin to the sample from blood culture bottles and found that it decreased the effects of inhibitors on the PCR. Two control groups of blood culture specimens were also evaluated. One group (n = 45) were "instrument true positives"; the instrument signaled positive, and subsequent Gram stains were positive and subcultures on chocolate agar grew bacteria. The other group (n = 20) were "instrument true negatives"; the instrument signaled negative, the Gram stain was negative, and subcultures on chocolate agar and from the Isolator tube on fungal media showed no growth. None of the 76 instrument false-positive samples had evidence for 16S rRNA gene sequences. All of the instrument true-positive samples and all of the instrument true-negative specimens were positive and negative, respectively, using the MicroSeq 500 kit. Total peripheral white blood cell counts were statistically significantly higher for patients who had instrument false-positive results than for patients who had instrument true-positive or true-negative results (P = 0.001). We conclude that instrument false positives signaled by the BACTEC 9240 system are not due to bacteria in the blood culture samples.
在之前一项评估BACTEC 9240自动血培养系统（Becton Dickinson诊断仪器系统公司，马里兰州斯帕克斯）的研究中，我们注意到有1.3%的“仪器假阳性”率。也就是说，BACTEC系统提示某一瓶（BACTEC Plus需氧/F瓶或BACTEC厌氧溶解/10瓶）培养物呈阳性，但革兰氏染色为阴性，且转种至巧克力琼脂上无细菌或酵母菌生长。此外，从同一血液样本中，使用Isolator血培养系统（Wampole实验室，新泽西州克兰伯里）进行的真菌培养未生长出真菌。在本研究中，我们使用MicroSeq 500试剂盒（PE生物系统公司，加利福尼亚州福斯特城）评估了76份仪器假阳性样本中16S核糖体DNA的存在情况。通过Isolator方法检测，这些样本的真菌结果也均为阴性。该试剂盒有一个用于16S RNA基因扩增和测序的PCR模块和测序模块，并提供一个用于序列比对和细菌鉴定的数据库。为优化检测方法，我们评估了向血培养瓶样本中添加0.5%牛血清白蛋白的效果，发现其可降低抑制剂对PCR的影响。还评估了两组血培养标本对照。一组（n = 45）为“仪器真阳性”；仪器提示阳性，随后革兰氏染色为阳性，转种至巧克力琼脂上生长出细菌。另一组（n = 20）为“仪器真阴性”；仪器提示阴性，革兰氏染色为阴性，转种至巧克力琼脂上以及从Isolator管接种至真菌培养基上均未生长。76份仪器假阳性样本均未发现16S rRNA基因序列的证据。使用MicroSeq 500试剂盒时所有仪器真阳性样本和所有仪器真阴性标本分别呈阳性和阴性。仪器假阳性结果患者的外周血白细胞总数在统计学上显著高于仪器真阳性或真阴性结果患者（P = 0.001）。我们得出结论，BACTEC 9240系统提示的仪器假阳性并非由于血培养样本中存在细菌。
