Comparison of the Etest and agar dilution methods for susceptibility testing of resistant clinical isolates of Haemophilus influenzae from Saudi Arabia

Sir, We read with interest the article by Rowe et al.1 concerning the use of the Etest to determine the susceptibility of respiratory isolates from the Central African Republic. The availability of the Etest has increased the clinical laboratory's ability to determine quantitative antibiotic susceptibilities for many fastidious pathogens, including Haemophilus influenzae. Although the Etest is less tedious and time-consuming than the standard agar dilution method, several investigators have recently observed that MICs of certain antibiotics measured by the Etest were different from those obtained by the broth or agar dilution methods.2,3 In this study the Etest was compared with the agar dilution method for testing the susceptibility of 26 antibiotic-resistant strains of H. influenzae to antimicrobial agents. These strains were resistant to ampicillin, chloramphenicol, tetracycline or roxithromycin according to the current National Committee for Clinical Laboratory Standards (NCCLS) criteria4 when tested by the agar dilution method. The Etest was performed in accordance with the manufacturer's instructions. The antibiotic concentration gradient for each strip was from 0.016 to 256 mg/L. All plates were incubated for 18–24 h at 37°C in an atmosphere containing 5% CO2. H. influenzae reference strain ATCC 49247 was included as a control. The MIC was defined as the intercept of the zone of inhibition with the graded Etest strip. Acceptable Etest accuracy for an antimicrobial agent should provide >90% agreement with the MICs determined by the agar dilution method.5 The Etest results showed excellent correlation with the agar dilution results (Table). Agreement between the MICs (within one log2 dilution) obtained by the two separate methods was 92.3% (for ampicillin and tetracycline) and 96.2% (for chloramphenicol and roxithromycin). The overall agreement was 94.3%. These results confirm that the Etest is comparable to the agar dilution method for susceptibility testing of antibiotic-resistant isolates of H. influenzae. Comparison of Etest and agar dilution test results for four antimicrobial agents tested against 26 H. influenzae isolates Comparison of Etest and agar dilution test results for four antimicrobial agents tested against 26 H. influenzae isolates