# Evaluation of the BL-RED test and comparison with the β-LACTA test for rapid detection of resistance to third-generation cephalosporins in Enterobacterales on early culture of positive blood cultures

## TL;DR

This study compares two rapid tests for detecting antibiotic resistance in bacteria from blood cultures, aiming to improve early treatment decisions.

## Contribution

The study evaluates and compares the performance of BL-RED and β-LACTA tests for rapid detection of 3GC-resistant Enterobacterales.

## Key findings

- Both BL-RED and β-LACTA tests showed high specificity and positive predictive value for detecting 3GC resistance.
- The tests were less effective for resistance caused by Ambler class C beta-lactamases.
- A diagnostic algorithm combining these tests with MALDI-TOF MS is proposed for early targeted therapy.

## Abstract

The dissemination of multidrug-resistant Enterobacterales, particularly those resistant to third-generation cephalosporins (3GC), poses a significant public health challenge. This study evaluated the performance of the Beta-Lactamase Rapid Electrochemical Detection (BL-RED) test (CORIS BioConcept, Belgium) alongside the β-LACTA test (Bio-Rad, France), another routinely used technique in our laboratory, for the rapid detection of 3GC-resistant Enterobacterales from 4 h subcultures of positive blood cultures. A total of 178 isolates were included: 55 clinical isolates with characterized resistance mechanism and 123 isolates from positive blood cultures prospectively collected between April and July 2023. Parallel testing with the BL-RED and β-LACTA tests was performed, and the results were compared to those of conventional antibiotic susceptibility testing by disk diffusion, following the 2024 EUCAST recommendations. The sensitivity, the specificity, the observed positive predictive value, and the observed negative predictive value for 3GC resistance detection in the prospective cohort were 74%, 100%, 100%, and 95%, respectively, for both the BL-RED and β-LACTA tests. Both tests demonstrated good performance in detecting 3GC-resistant Enterobacterales, particularly those expressing common Ambler class A beta-lactamases. However, they were less effective in detecting 3GC resistance due to the presence of other Ambler class families, notably overproduced chromosomal and plasmid-mediated Ambler class C beta-lactamases. The ease of use of these tests makes them suitable for routine microbiology laboratory testing. We propose a diagnostic algorithm combining the use of either the BL-RED and β-LACTA tests with that of MALDI-TOF MS in order to initiate targeted antibiotic therapy early.

Antimicrobial resistance, especially among Enterobacterales resistant to third-generation cephalosporins (3GC), represents a major public health threat. Delayed detection of these resistant pathogens can lead to inappropriate empirical antibiotic therapy, with risk of treatment failures and further spread of resistance. This study underscores the value of rapid diagnostic tools, specifically the BL-RED and β-LACTA tests, for the rapid identification of 3GC-resistant Enterobacterales directly from positive blood cultures. By enabling quicker therapeutic decision-making, these assays provide a practical solution for microbiology laboratories to enhance patient management and contribute to global efforts in mitigating the impact of antibiotic-resistant infections.