# Impact of Outdated Clinical and Laboratory Standards Institute (CLSI) Breakpoint Implementation on Antimicrobial Susceptibility Interpretation: A Retrospective Analytical Study

## TL;DR

Using outdated CLSI breakpoints for antimicrobial susceptibility testing leads to incorrect results, overestimating susceptibility and risking ineffective treatments.

## Contribution

This study quantifies the impact of outdated CLSI breakpoints on susceptibility misclassification using a large dataset of bacterial isolates.

## Key findings

- Outdated breakpoints significantly misclassified susceptibility for gentamicin and tobramycin in Enterobacterales and Pseudomonas aeruginosa.
- Revised CLSI breakpoints for linezolid reclassified many Staphylococcus aureus isolates from susceptible to intermediate.
- Older criteria overestimated susceptibility, masking emerging resistance in multiple bacterial species.

## Abstract

Introduction

Using outdated breakpoints can lead to a considerable overestimation of susceptibility, which can result in ineffective treatments; therefore, it's crucial to adopt current standards to improve patient outcomes.

Methods

This retrospective analytical study analyzed 9,279 bacterial isolates. Reporting errors in antimicrobial susceptibility testing (AST) by the Kirby-Bauer disk diffusion method due to the use of outdated breakpoints was assessed by comparing the revised cutoff breakpoints of specific antibiotics with previous Clinical and Laboratory Standards Institute (CLSI) guidelines. Chi-square testing and McNemar’s test were used to evaluate paired interpretive shifts between the older and updated CLSI breakpoints.

Results

Among Enterobacterales, analysis of 2,262 aminoglycoside zone diameters revealed significant misclassification when outdated CLSI breakpoints were applied. Gentamicin also showed a significant shift in susceptibility distribution (χ² = 95.27, p < 0.0001), with paired analysis confirming correction of susceptible isolates to the intermediate category using updated breakpoints (McNemar χ² = 36.0, p < 0.0001). Amikacin demonstrated a borderline overall shift (χ² = 6.00, p = 0.0499) but substantial paired misclassification (McNemar χ² = 36.0, p < 0.0001). In Pseudomonas aeruginosa, piperacillin-tazobactam (χ² = 6.62, p = 0.0366) and tobramycin (χ² = 77.94, p < 0.0001) demonstrated a significant redistribution of susceptibility categories, with older criteria overestimating susceptibility. Among Staphylococcus aureus, implementation of revised 34th-edition CLSI linezolid breakpoints introduced an intermediate category and resulted in significant reclassification from susceptible to intermediate (χ² = 17.45, p = 0.00016; McNemar χ² = 17.0, p < 0.0001).

Conclusion

Unregulated and delayed implementation of updated CLSI breakpoints results in significant misclassification of antimicrobial susceptibility, leading to overestimation of susceptibility and masking emerging resistance. Standardized and timely adoption of revised breakpoints, along with supportive laboratory systems, is essential to ensure accurate susceptibility interpretation, effective antimicrobial stewardship, and reliable antimicrobial resistance surveillance.