Evaluation of interpretive criteria of agar dilution and disk diffusion susceptibility tests for Neisseria...

Using strains of Neisseria gonorrhoeae from Western New York, the National Committee for Clinical Laboratory Standards' (NCCLS) interpretive criteria for disk diffusion susceptibility testing was evaluated on chocolate-Mueller-Hinton agar (CMH) and GC agar. The reference method for comparison was the NCCLS agar dilution minimum inhibitory concentration (MIC) method. Even though the zone sizes were significantly smaller on the CMH agar, the interpretations were not significantly different on either GC or CMH agars except for tetracycline. On the CMH agar, the number of tetracycline-resistant strains was greater than on GC agar: 3% of the strains failed to grow on CMH agar and 4% did not produce interpretable zone sizes for ceftriaxone. Therefore, the use of CMH is not recommended. There was a significant difference between the interpretative criteria of the MIC and the disk diffusion method only for tetracycline. Therefore, the NCCLS zone-diameter interpretation criteria for tetracycline does not seem applicable for N. gonorrhoeae isolates in the Buffalo, New York, area.