Reconsidering azithromycin disc diffusion interpretive criteria for in view of azithromycin MIC creep among typhoidal and nontyphoidal salmonella.

Purpose: Enteric fever continues to be an important public health challenge for the developing world. With the emergence of fluoroquinolone resistance in spp. azithromycin is increasingly being used for oral treatment of enteric fever. We investigated the antibiotic susceptibility pattern of azithromycin in spp. isolates from a tertiary care hospital to detect emerging resistance.

Methods: The study assessed the reliability of disc diffusion as a screening test to detect azithromycin resistance by comparing it with the minimum inhibitory concentrations (MICs) of the drug in 100 spp. strains. The strains of spp. showing resistance to azithromycin were further investigated for resistance markers - , , and genes.

Results: This study was conducted on 100 strains recovered from blood culture samples between 2013 and 2017. Among these isolates, 18 showed resistance to azithromycin by disc diffusion methodology with zones of inhibition <13 mm. MIC of 6 of these isolates were ≥32 mg/L. The mean MIC of azithromycin increased from 5 mg/L in 2013 to 24 mg/L in 2017. Azithromycin consumption as defined daily doses per 1000 patient days also showed an increase over the past 4 years.

Conclusion: Azithromycin disc diffusion diameter interpretations as recommended by Clinical and Laboratory Standards Institute can mislabel a few sensitive strains as resistant. Azithromycin resistance is emerging in typhoidal and nontyphoidal . gene is associated with high MICs in nontyphoidal spp.