COMPARISON OF ETEST(R) WITH AGAR DILUTION MINIMAL INHIBITORY CONCENTRATIONS FOR SUSCEPTIBILITY TESTING OF STREPTOCOCCUS-PNEUMONIAE TO PENICILLIN, AMOXICILLIN AND CEFOTAXIME - A MULTICENTER STUDY

In 1996-1997 a multicentre study was carried out on 450 Streptococcus pneumoniae strains to compare the MICs and susceptibility categories o btained with the Etest(R) (AB Biodisk) used under routine conditions i n 22 hospital laboratories in the Rhone-Alpes region, France, with tho se obtained by the reference technique of agar dilution performed in a single coordinating centre. Each laboratory detected penicillin resis tant pneumococci (PRP) by the oxacillin disk method (1 mu g and 5 mu g ) and determined the MICs of penicillin G (PG), amoxycillin (AMX) and cefotaxime (CTS) by the Etest(R). All the PRP strains were collected i n the coordinating centre where MICs were carried out. The strains wer e classified as susceptible (S), intermediate (I) and resistant (R) ac cording to the CASFM criteria (Comite de l'Antibiogramme de la Societe Francaise de Microbiologie). The concordance results based on suscept ibility categories are as follows: PC = 67,6 %, AMX = 63,6 %, CTX = 71 ,5 %. Minor errors are as follows: PG = 31,2 %, AMX = 36% CTX = 28,5 % . Major and very major errors are rare (0 % to 0,6 %). Agreement withi n 1 log2 dilution was obtained for about 80 % of the strains. The mino r errors result from strains clustering near the breakpoints 1 mg/l (P G) and 0,5 mg/l (AMX, CTX), and from practical difficulties in routine use of the Etest(R). These discrepancies may result in severe therape utic problems. This study confirms the limits of the Etest(R). The aut hors insist on standardization and rigorous use of the Etest(R) under routine conditions.