Rapid Determination of Antimicrobial Susceptibility of Gram-Negative Bacteria from Positive Blood Cultures Using an Innovative Method

A blood stream infection (BSI) presents a complex and serious health problem and is growing in light of the eminent antimicrobial resistance threat. Progress must be made towards rapid BSI diagnosis and antimicrobial susceptibility testing (AST) to reduce preventable death in BSI.

Positive blood cultures from Ninewells Hospital, Dundee were studied prospectively. Flagged positive blood cultures were processed by Gram-staining, Vitek identification and AST or disc diffusion for ceftriaxone susceptibility and by scattered light integrated collection device (SLIC). Susceptibility to a panel of five antibiotics as defined by EUCAST breakpointswere compared.The time to AST result and AST categorical agreement was compared for standard methods and SLIC, any discrepancies were resolved by the EUCAST broth micro-dilution reference method.

A total of 505 bacterial-antimicrobial combinations were analysed. A categorical agreement of 95.45% (482/505) was achieved between SLIC and Vitek/disk diffusion. The remaining 23 discrepancies were resolved by broth micro-dilution with 10 AST results agreeing with the SLIC result and 13 in agreement of Vitek/disk diffusion. Thus the overall AST agreement of the standard workflow was 98.01% (495/505) compared to 97.43% (492/505) using SLIC. The mean time for AST was 1.94 + 0.02 h and 10.53 + 0.46 h for SLIC and Vitek respectively. Overall SLIC was calculated to save 25hrs from positive blood culture to AST outcome.

SLIC has the capacity to provide AST at the same time as Gram stain or MALDI-TOF identification.This could improve the value of blood culture in clinical practice.