Development of revised quality control limits for disk diffusion susceptibility tests of selected cephem antibiotics with Haemophilus influenzae and description of a new control strain.

Inconsistent quality control results in disk diffusion testing of cefaclor, cefamandole, cefonicid, and cefuroxime with Haemophilus influenzae ATCC 49247 and Haemophilus test medium (HTM) prompted a search for an alternative control strain that would provide more reliable results. A five-laboratory study was conducted to evaluate two candidate H. influenzae strains as possible alternatives to the aforementioned strain. Repetitive testing of the candidate strains and H. influenzae ATCC 49247 over several days with a total of six different lots of HTM documented consistent performance of the two candidate strains and confirmed inconsistent results for some of the antibiotics with H. influenzae ATCC 49247. In particular, certain lots of HTM failed to yield cefaclor and cefamandole zone sizes within the quality control range advocated by the National Committee for Clinical Laboratory Standards. Because of the greater consistency offered by the new strains, one was selected (now designated H. influenzae ATCC 49766) to be recommended for routine quality control testing of cefaclor, cefamandole, cefonicid, cefuroxime, and the related carbacephem loracarbef. The new control strain and zone size ranges proposed here have been approved by the National Committee for Clinical Laboratory Standards in place of the previously recommended strain and zone size limits for testing of these five cephem antibiotics.