Failure of azithromycin therapy in gonorrhea and discorrelation with laboratory test parameters

Background: Azithromycin is efficacious in the treatment of chlamydial genital tract infection but less so in gonorrhea. However, MICs of azithromycin for gonococci from previously reported azithromycin treatment failures were consistently below the 'susceptible' MIC level of 2 mg/L. Goal of this Study: To examine gonococci not eliminated with 1 g azithromycin therapy to establish treatment outcome/MIC correlates in gonorrhea. Study Design: The MICs and phenotypes of gonococci isolated from five cases of treatment failure after 1 g azithromycin therapy were determined and compared with the MICs of a systematic sample of routine isolates. Results: Azithromycin MICs of gonococci from five cases of failed 1 g azithromycin treatment were 0.125 or 0.25 mg/L, well within the current 'susceptible' MIC range. None of the isolates were of the mtr phenotype. The MIC90 of a systematic sample of 219 gonococcal isolates was 0.25 mg/L. Conclusion: The antibiotic MIC/treatment outcome correlates that are usually found in gonorrhea do not apply for azithromycin. Current MIC criteria do not accurately define susceptibility or resistance of gonococci to azithromycin and by themselves do not predict the likely outcome of therapy. Pharmacokinetic factors may decrease the predictive value of MIC data.