Antimicrobial susceptibility testing of enterococci: results of a survey conducted by the United Kingdom National External Quality Assessment Scheme for Microbiology

Six strains of enterococci were distributed to participants in the United Kingdom National External Quality Assessment Scheme for Microbiology with a request that they be tested for susceptibility to ampicillin/penicillin and vancomycin and for high-level resistance to gentamicin. More than 98% of laboratories correctly reported the resistant strains as resistant to penicillin and ampicillin. However, there was a poorer correlation for the susceptible strains, with only 1-6% and 69-83% of laboratories reporting the strains as susceptible to penicillin and ampicillin respectively. Those laboratories which used 5-25 μg ampicillin discs or breakpoint methods reported proportionally more results correctly for the susceptible strains than those using 2 μg discs