Antimicrobial susceptibility of Neisseria gonorrhoeae isolates determined by the agar dilution, disk diffusion and Etest methods: comparison of results using GC agar and chocolate agar

Although the use of GC agar for determining Neisseria gonorrhoeae antimicrobial susceptibilities is suggested by Clinical and Laboratory Standard Institute (CLSI) guidelines, chocolate agar is still used in some regions owing to its low cost and availability. To determine the differences in susceptibilities determined using GC and chocolate agars, 163 non-duplicate N. gonorrhoeae isolates were tested. Minimum inhibitory concentrations (MICs) and percent susceptibilities determined using the GC agar dilution method, respectively, were as follows: ceftriaxone, 0.004–0.125 mg/L, 100%; cefixime, 0.002 mg/L to >32 mg/L, 98.2%; and ciprofloxacin, 0.002 mg/L to >32 mg/L, 3.1%. Comparison of ceftriaxone MICs determined by the Etest using GC agar and chocolate agar showed that use of GC agar tended to result in lower MICs than GC agar dilution, whilst use of chocolate agar tended to result in higher MICs (concordance, 55.8% and 82.8%, respectively). Disk inhibition zones obtained using GC agar and chocolate agar (and their correlation coefficients) were, respectively: ceftriaxone, 35–55 mm and 25–50 mm (0.46); ciprofloxacin, 6–55 mm and 6–43 mm (0.84); and penicillin, 6–47 mm and 6–50 mm (0.93). Use of chocolate agar with the disk diffusion method for ceftriaxone was associated with a 5.5% false resistance rate. In summary, compared with GC agar, susceptibility testing using chocolate agar tends to yield higher MICs with the Etest and smaller disk inhibition zones with disk diffusion methods. Clinical microbiology laboratories should strictly adhere to CLSI recommendations by using GC agar instead of chocolate agar when performing susceptibility testing for N. gonorrhoeae.