Digital imaging for reading of direct rapid antibiotic susceptibility tests from positive blood cultures

Delaying effective antibiotic therapy is a major cause of sepsis-associated mortality. The EUCAST rapid antibiotic susceptibility test (RAST) is performed from positive blood cultures to provide rapid results. Disc diffusion tests inoculated with positive blood culture broth are read at 4, 6, and 8 h and interpreted against species and time-specific criteria. Potential problems are the possibility of missing specific reading times for tests and slower growth in incubators that are frequently opened. The current study aimed to assess if digital visualization by the BD Kiestra™ total laboratory automation system is suitable for reading RASTs by capturing images at the correct times and retaining them for review. Utilizing the Kiestra™ InoqulA, 100 μl of positive blood culture broth was lawn-inoculated onto Mueller-Hinton agar and incubated at 35 °C for automated digital zone measurement at 4, 6, and 8 h. Aliquots from 135 positive blood cultures were tested against EUCAST-recommended and other drugs and assessed for readability of digital images. Microdilution MICs were determined in parallel to RASTs. All isolates except 7/10 enterococci yielded images of suitable quality for zone measurement. Of the 641 digitally read tests for other organisms, 207 (32.3%) were readable in 4 h, 555 (86.6%) in 6 h, and 641 (100%) in 8 h. For tests included in EUCAST criteria, 92.1% provided categorical agreement with microdilution MICs. Digital image reading of RASTs is a potentially viable, inexpensive tool for providing rapid susceptibility results which can help reduce sepsis-associated mortality.