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

Background: Azithromycin is efficacious in the treatment of chlamydial geni tal tract infection but less so in gonorrhea. However, MICs of azithromycin for gonococci from previously reported azithromycin treatment failures wer e consistently below the 'susceptible' MIC level of 2 mg/L. Goal of this Study: To examine gonococci not eliminated with 1 g azithromyc in 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 co mpared with the MICs of a systematic sample of routine isolates. Results: Azithromycin MICs of gonococci from five cases of failed 1 g azith romycin treatment were 0.125 or 0.25 mg/L, well within the current 'suscept ible' 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 usuall y found in gonorrhea do not apply for azithromycin. Current MIC criteria do not accurately define susceptibility or resistance of gonococci to azithro mycin and by themselves do not predict the likely outcome of therapy. Pharm acokinetic factors may decrease the predictive value of MIC data.