# Performance evaluation of early growth isolates for automated and manual broth microdilution antimicrobial-susceptibility testing

## TL;DR

This study shows that using early growth isolates for antimicrobial testing can significantly reduce waiting times without compromising accuracy.

## Contribution

The study introduces a cost-effective method to reduce AST turnaround times by using early growth isolates with existing commercial systems.

## Key findings

- Early growth AST showed high concordance with standard methods for both gram-positive and gram-negative bacteria.
- Using early growth isolates reduced incubation time by up to 18 hours without additional costs.
- The method is practical for clinical labs and maintains high accuracy with minimal errors.

## Abstract

Prolonged turnaround times (TAT) represent a major limitation to current automated susceptibility testing systems and manual susceptibility-testing methods such as broth microdilution. As a result, targeted therapy for patients may be delayed, portending suboptimal clinical outcomes. One contributing factor is the 18–24 h of incubation prior to antimicrobial-susceptibility testing (AST) recommended by the Clinical Laboratory Standards Institute (CLSI) and some automated AST manufacturers. This study evaluates the performance of AST by manual broth microdilution (Thermo Fisher Sensititre) and an automated AST system (BD Phoenix) on isolates incubated for 6 h (early growth AST, egAST) compared with 18–24 h (standard growth AST, sgAST). An initial proof-of-concept study conducted on gram-negative (n = 5) and gram-positive (n = 2) quality control strains incubated for 6 or 24 h prior to Sensititre and Phoenix demonstrated 100% essential agreement. Subsequently, we evaluated the performance of egAST on gram-positive (n = 49, Phoenix; n = 46 Sensititre) and gram-negative (n = 81 Phoenix; n = 61 Sensititre) patient-derived isolates with diverse resistance profiles compared with sgAST. In total, 1,666 organism–drug combinations were tested by Sensititre (560 gram-positive and 1,106 gram-negative) and 1,927 by Phoenix (409 gram-positive and 1,518 gram-negative). For Enterobacterales, egAST using Phoenix revealed 1.3% minor errors (MiE), 0.17% major errors (ME), and 1.1% very major errors (VME) compared with sgAST. Phoenix egAST performance for Pseudomonas aeruginosa revealed 0.18% MiE and no ME or VME. Similarly, egAST of Enterobacterales by Sensititre revealed 1.5% MiE and no ME or VME, whereas 2.8% MiE, 6.6% ME, and no VME were observed for P. aeruginosa. For Staphylococcus spp. and Enterococcus spp., there were no MiE, ME, or VME on the Phoenix system, whereas early growth Sensititre showed 3.14% MiE, 0.3% ME, and 2.99% VME for Staphylococcus spp. and 6.8% MiE, 0.09% ME, and no VME for Enterococcus spp. Taken together, these data suggest that egAST represents a viable strategy to reduce the prolonged incubation period currently recommended by CLSI and select automated AST manufacturer guidelines without incurring any additional costs while simultaneously maintaining high concordance with reference standard methods.

Traditional antimicrobial-susceptibility testing (AST) methods typically span several days from the time of organism isolation. The majority of this time is spent waiting for a cultured isolate to incubate up to 1 day prior to AST. There exists an unmet need to provide more rapid AST as various rapid methods have been shown to reduce exposure to broad-spectrum antibiotics that select for antimicrobial resistance, shorten hospital stays, and improve clinical outcomes. Simultaneously, there is a need to ensure that rapid AST approaches are readily implemented in the clinical microbiology laboratory, with little to no added financial burden. This study demonstrates a cost-considerate and practical approach to reduce AST turnaround times by up to 18 h through the use of early growth isolates in combination with two commercial AST systems. The findings from this study complement those of previous reports describing the largely acceptable performance of disk diffusion using early growth isolates.