SUSCEPTIBILITY TESTING OF HAEMOPHILUS-INFLUENZAE - AN INTERNATIONAL COLLABORATIVE STUDY IN QUALITY ASSESSMENT

In order to compare the prevalence of antibiotic resistance in differe nt geographical areas, it is necessary to ensure that agreement is ach ieved between laboratories on the assignment of strains to 'susceptibl e' and 'resistant' categories. An international quality assessment stu dy, involving 15 laboratories in eight countries, was performed to inv estigate the standard of performance of the susceptibility testing of Haemophilus influenzae. One hundred and fifty strains of H. influenzae were distributed from the London Hospital Medical College (LHMC) to a ll laboratories who were asked to test the susceptibility of the strai ns to ampicillin, chloramphenicol, tetracycline, trimethoprim, cephalo sporins and ciprofloxacin. Laboratories were also asked to provide the details of methodology to test the susceptibility. Significant discre pancy between the LHMC and the participating laboratories appeared in the detection of resistance to ampicillin (especially beta-lactamase-n egative strains resistant to ampicillin) as well as the assignment of susceptibility and resistance to chloramphenicol, tetracycline and tri methoprim. Often these reflected the use of inappropriate breakpoints which led to erroneous assignment of susceptibility. Other variations including disc content, medium and supplement, inoculum as well as fai lure to measure zone sizes properly also led to some repeating anomali es.