Evaluation of the Etest for susceptibility testing of anaerobic bacteria

We compared the susceptibility test results of 220 anaerobes against 14 antimicrobials using the Etest (AB Biodisk, Solna, Sweden) with those using the National Committee for Clinical Laboratory Standards (NCCLS) standard agar dilution method (Wadsworth version). The Etest medium was brucella blood (whole) agar and the inoculum size was equivalent to a no. 1 McFarland standard. Thirty-six percent of Etest results were unreadable after 24 h of anaerobic incubation compared to only 5% after 48 h. Also, there were more results with categorical agreement with the NCCLS method after 48 h (97.9%) than at 24 h (89%) and more very major errors (VMEs) at 24 h (22% of resistant organisms) than at 48 h (3.2%). VMEs and major errors occurred most frequently with clindamycin, ceftriaxone, and trospectomycin (which should not be used with the Etest) and involved the Bacteroides fragilis group and/or Clostridium most commonly. The Etest is simple to perform and is a generally reliable method that is optimally read after 48 h of incubation. It should be an acceptable alternative to the agar dilution standard, although results with certain organism-antimicrobial combinations should be read very conservatively because of the frequency of VMEs.