Evaluation of an automated microarray-based nucleic acid test for multiplex identification of gram-positive bacteria from positive blood culture broths

Abstract

Bacteria can cause serious illness to patients once entry into the bloodstream. Early detection of bloodstream pathogens enables early treatment and better survival. This study evaluated the performance of the Verigene BC-GP assay, which is an automated microarray based assay to specifically detect nucleic acid of common Gram positive bacteria present in the blood culture in hours.

A total of 64 blood culture samples showing Gram positive cocci and bacilli in Gram smear were collected from three local hospitals during January 2014 to February 2015. The three local hospitals were Princess Margaret Hospital (PMH), Pamela Youde Nethersole Eastern Hospital (PYNEH) and United Christian Hospital (UCH). Concordance study was performed to compare the results of BC-GP assay with routine identification methods in the microbiology laboratories of the three hospitals. Any disconcordant result was resolved by 16S rDNA PCR sequencing, which served as a gold standard method in this study. Results showed that the BC-GP assay demonstrated an overall concordance of 90.77% and 92.19% concordance for in panel targets of the assay. The assay was able to detect majority of Gram positive pathogens present in the blood culture, including (30/30) Staphylococcus aureus, (11/12) coagulase negative Staphylococci, (5/5) Streptococcus pneumoniae, (1/1) Streptococcus pyogenes, (1/1) Streptococcus agalactiae, (4/4) Streptococcus dysgalactiae, (1/2) Streptococcus bovis, (1/1) Streptococcus mitis, (3/5) Enterococcus faecalis, (1/1) Enterococcus faecium and (1/1) Listeria monocytogenes The assay was not able to detect one Kocuria species present in the study, which is out of the detection panel of the assay.

Among 64 positive blood cultures, 62 (97%) were pure culture and 2 (3%) were mixed culture. The performance of Verigene BC-GP assay was indeed better in pure culture as compared to mixed culture, with 93.55% concordance in pure culture compared to 33.33% in mixed culture. The assay was able to detect the predominant coagulase negative Staphylococcus and E.coli in the two identified mixed cultures, while missing the scanty Enterococcus faecalis. This result imposed extra attention to laboratory technicians when using the assay under mixed conditions. Nevertheless, the Verigene BC-GP assay was able to provide rapid detection, with 42.35 hours reduction of time to results when comparing with routine identification methods. This enabled the assay to provide rapid detection and possible early treatment of common Gram positive bloodstream pathogens.