Assessment of lefamulin 20 µg disk versus broth microdilution when tested against common respiratory pathogens

Objective

To evaluate the performance of the disk diffusion test with lefamulin 20 µg compared with the Clinical and Laboratory Standards Institute (CLSI) reference broth microdilution (BMD) method.

Methods

A total of 572 clinical stains, including 240 Staphylococcus aureus, 211 Streptococcus pneumoniae, and 121 Haemophilus influenzae, isolated from 71 medical centres from the China Antimicrobial Surveillance Network in 2020. BMD method and disk diffusion methods were performed according to CLSI. Categorical agreement (CA), major error (ME), and very ME (VME) were calculated.

Results

Lefamulin showed potent activity against S. aureus, S. pneumoniae, and H. influenzae. Using the BMD method, lefamulin inhibited 97.1% of S. aureus isolates at 0.25 mg/L; seven isolates were not susceptible. For S. pneumoniae and H. influenzae, the percentage of susceptibility to lefamulin was 100% and no non-susceptible strains were found in this study. Compared with the reference BMD method, the CA of the lefamulin 20 µg disk testing was 99.8% (571/572), with 14.3% (1/7) VME and no ME. In our study, VME was determined in S. aureus. For S. pneumoniae and H. influenzae, the VME was not determined due to the lack of lefamulin non-susceptible strains.

Conclusions

The lefamulin 20 µg disk diffusion testing showed excellent CA and ME with the reference BMD method for S. aureus, S. pneumoniae, and H. influenzae. The VME exceeding CLSI recommendations may be a bias due to fewer lefamulin non-susceptible isolates. Our results suggest that lefamulin non-susceptible isolates detected by disk diffusion should be confirmed by the reference BMD.