E-test method of antimicrobial susceptibility testing of Neisseria gonorrhoeae for routine diagnostic service

The minimising inhibitory concentrations of four antimicrobial agents for 64 clinical isolates of Neisseria gonorrhoeae including 26 penicillinase producing strains (PPNG) as determined by E test, a recently developed method for sensitivity testing, were compared with those of agar dilution method using isosensitest (IST) agar. The medium was supplemented with either 5% lysed horse blood alone or with both lysed horse blood and 1% vitox defined supplement. The E test MICs compared closely with those obtained by agar dilution with essential agreements within +/- 1 log2 dilution being over 90% with all test antibiotics on medium that did not contain vitox, and between 71 and 93% on medium containing vitox. The Pearson's correlation coefficients ranged from 0.84 to 0.96 on either medium formulation. Excellent categorical agreement was obtained for all isolates with ceftriaxone, ciprofloxacin and tetracycline, while the E test gave a minor categorical discrepancy for two isolates with penicillin. We conclude that the E test is as reliable as conventional agar dilution method for MIC testing of N. gonorrhoeae in a routine laboratory.