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J Med Microbiol 55 (2006), 53-57; DOI: 10.1099/jmm.0.46196-0
© 2006 Society for General Microbiology
ISSN 0022-2615

PCR evaluation of false-positive signals from two automated blood-culture systems

Z. Ceren Karahan1, Ipek Mumcuoglu2, Haluk Guriz3, Deniz Tamer1, Neriman Balaban2, Derya Aysev3 and Nejat Akar1

1 ,3 Division of Paediatric Molecular Pathology and Genetics1 and Cebeci Hospital Central Microbiology Laboratory3 , Ankara University, School of Medicine, 06100-Cebeci, Ankara, Turkey

2 Microbiology and Clinical Microbiology Department, Ankara Numune Education and Investigation Hospital, 06100-Sihhiye, Ankara, Turkey

Correspondence
Z. Ceren Karahan
ckarahan{at}medicine.ankara.edu.tr

Received 9 June 2005
Accepted 21 September 2005

Rapid detection of micro-organisms from blood is one of the most critical functions of a diagnostic microbiology laboratory. Automated blood-culture systems reduce the time needed to detect positive cultures, and reduce specimen handling. The false-positive rate of such systems is 1–10 %. In this study, the presence of pathogens in ‘false-positive’ bottles obtained from BACTEC 9050 (Becton Dickinson) and BacT/Alert (Biomérieux) systems was investigated by eubacterial and fungal PCR. A total of 169 subculture-negative aerobic blood-culture bottles (104 BacT/Alert and 65 BACTEC) were evaluated. Both fungal and eubacterial PCRs were negative for all BACTEC bottles. Fungal PCR was also negative for the BacT/Alert system, but 10 bottles (9·6 %) gave positive results by eubacterial PCR. Sequence analysis of the positive PCR amplicons indicated the presence of the following bacteria (number of isolates in parentheses): Pasteurella multocida (1), Staphylococcus epidermidis (2), Staphylococcus hominis (1), Micrococcus sp. (1), Streptococcus pneumoniae (1), Corynebacterium spp. (2), Brachibacterium sp. (1) and Arthrobacter/Rothia sp. (1). Antibiotic usage by the patients may be responsible for the inability of the laboratory to grow these bacteria on subcultures. For patients with more than one false-positive bottle, molecular methods can be used to evaluate the microbial DNA in these bottles. False positives from the BACTEC system may be due to elevated patient leukocyte counts or the high sensitivity of the system to background increases in CO2 concentration.

Abbreviations: ANEIH, Ankara Numune Education and Investigation Hospital; AUSM-CH, Ankara University School of Medicine-Cebeci Hospital.






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