Accuracy of methods used for susceptibility testing of Haemophilus influenzae in United Kingdom laboratories.

Antibiotic susceptibility test reports on 1841 strains of Haemophilus influenzae from 25 microbiology laboratories were compared with results obtained with the same strains at The London Hospital Medical College. Of strains found to be sensitive to the antibiotics tested, 0.5% were reported as tetracycline-resistant, 1.6% as ampicillin-resistant, and 6.2% as trimethoprim-resistant. Of strains found to be resistant to these antibiotics, 37% were reported as tetracycline-sensitive, 27% as ampicillin-sensitive, and 66.7% as trimethoprim-sensitive. Factors found to be of significance in improving accuracy of sensitivity reporting included use of chromogenic cephalosporin and low-content antibiotic discs for detection of ampicillin resistance, and use of lysed blood agar rather than chocolated blood agar to detect trimethoprim sensitivity.