REVISED DISK DIFFUSION INTERPRETIVE CRITERIA FOR CEFACLOR, LORACARBEF, CEFPROZIL AND CEFIXIME WHEN TESTING HAEMOPHILUS-INFLUENZAE ON HAEMOPHILUS TEST MEDIUM

The aim of the current five-center collaborative study was to reassess the interpretive criteria for cefaclor, loracarbef, cefprozil and cef ixime previously adopted or proposed by the National Committee for Cli nical Laboratory Standards (NCCLS) for disk diffusion susceptibility t ests with Haemophilus influenzae on Haemophilus Test Medium (HTR I) ag ar. MICs and zones of inhibition were determined using NCCLS methods, HTM and two collections of strains of Haemophilus influenzae. One grou p of strains consisted of 118 stock organisms taken largely from vario us recent U.S. antibiotic resistance surveillance studies. The emphasi s in this selected group of organisms was on strains that were beta-la ctamase negative but ampicillin resistant (BLNAR) by some other mechan ism. The second collection of test organisms consisted of 50 recent cl inical isolates of Haemophilus influenzae obtained from each of the fi ve participating study centers. This group was considered representati ve of the type of Haemophilus influenzae currently recovered from clin ical sources in the USA. Frequency distribution assessment and error-r ate bounded analysis of scattergram comparisons of MICs and zone sizes were used to develop the following zone diameters interpretive for di sk diffusion susceptibility tests with Haemophilus influenzae on HTM a gar: cefaclor, greater than or equal to 20 mm (susceptible, S) and les s than or equal to 16 mm (resistant, R); loracarbef, greater than or e qual to 19 mm (S) and less than or equal to 15 mm (R); and cefprozil, > 18 mm (S) and < 14 mm (R). The respective MIC correlates for all thr ee antimicrobial agents were less than or equal to 8 mu g/ml (S) and 3 2 mu g/ml (R). For cefixime, a zone diameter size of greater than or e qual to 21 mm was recommended to define the susceptible category, with an MIC correlate of less than or equal to 1 mu g/ml. Only a susceptib le category was defined for cefixime testing. These new interpretive c riteria ha, e recently been adopted by the NCCLS.