Evaluation of an automated culture plate reading instrument for MRSA culture screening

Screening for MRSA colonisation is a routine task in the microbiology laboratory, with the standard method being culture on chromogenic media. The low prevalence (~1% in the UK) of colonisation means valuable time is spent reading and reporting negative culture plates. Use of an automated plate reader to screen and report negative culture plates could improve laboratory processes.

The APAS Independence infection control module for automated reading of MRSA culture plates was evaluated in a large diagnostic laboratory. MRSA culture plates from 3,719 routine diagnostic specimens (cultured on Brilliance II MRSA agar (BD) for 18 hours) were read by the APAS Independence and independently read by biomedical scientists (BMS) following laboratory protocol. The read designation (presumptive MRSA colony for review/No MRSA detected) of the APAS Independence was compared to the final culture result (manual read). Serial dilutions of stored clinical isolates were tested to assess performance with low bacterial colony counts.

43 of the 3,719 (1.2%) samples were positive for MRSA in the prospective comparison by the laboratory. The APAS Independence directed 220 plates for BMS review, and showed 100% sensitivity and 94.0 % specificity, with a negative predictive value of 100% and a positive predictive value of 16.5%. The results of this study indicate that 93% of plates could be reported as negative and directed to a waste stream without the need for a MBS to review. For the serial dilution samples on 12 occasions, the APAS Independence was able to call a plate with a single blue colony positive for presumptive MRSA.

Over 100 positive plates were correctly assigned as presumptive MRSA without missing a positive. The APAS Independence reliably detected MRSA colonies on chromogenic media and could streamline laboratory processing, removing vast majority of plate reading from the MRSA culture pathway.

Poster Presentation: by Health Services Laboratory

Conference: ECCMID 2021, Online

Date: July 2021

Authors: Williams A, Spratt

Citation: Williams A, et al. ECCMID 2021. Evaluation of an automated culture plate reading instrument for MRSA culture screening.