BROTH DISK ELUTION METHOD FOR ANAEROBIC-BACTERIA - A COLLABORATIVE STUDY TO ASSESS ITS RELIABILITY FOR CLINICAL PURPOSES

A collaborative study involving seven laboratories was undertaken to e valuate the reproducibility and the reliability of the broth disk elut ion test against anaerobic bacteria by comparing with the reference ag ar dilution method. A two breakpoint broth test was also evaluated. As says were performed using the same testing conditions (i.e. medium, te mperature, atmosphere and incubation time). One hundred Gram-negative and Grampositive clinical isolates were initially studied. Overall agr eement of 98.5% and 97.5%, were found for disk elution and the two bre akpoint tests, respectively. Ln order to assess the reliability of the disk elution test, two different lots (LOT1 and LOT2) of disks of pip eracillin and clindamycin were selected, to obtain two final concentra tions after dilution (10 and 60 mg/mL; 1 and 4 mg/mL, respectively). T wo hundred and eighty assays were performed against one strain of both Bacteroides fragilis (piperacillin MIC, 8.0 mg/mL; clindamycin MIC, < 0.5 mg/mL) and Bacteroides thetaiotaomicron (piperacillin MIC, 16.0 m g/mL; clindamycin MIC, < 0.5 mg/mL). With LOT 1, considering both spec ies and both antibiotics, the agreement among six laboratories ranged from 85% to 100% (P > 0.05) with the higher concentration. Overall agr eement among all laboratories was 91%. No optimal agreement (>90%) for clindamycin-Bacteroides thetaiotaomicron using the LOT1 (77%) was fou nd. Since this finding was not observed with LOT2 (100% agreement), di screpancies were attributed to variation between lots. Overall agreeme nt with LOT2 was 100% for all centres. The present study indicates tha t the broth disk elution method proved to be a reliable and suitable a lternative for routine susceptibility testing for anaerobic bacteria, as a resistance screening method for clinical purposes. (C) 1997 Acade mic Press.