title: Influence of disk separation distance on accuracy of the disk approximation test for detection of inducible clindamycin resistance in Staphylococcus spp.
abstract: We undertook this study to assess the accuracy of the clindamycin-erythromycin disk approximation test (D-test) for detection of inducible clindamycin resistance in Staphylococcus spp. One hundred sixty-three Staphylococcus aureus and 68 coagulase-negative Staphylococcus (CoNS) spp. which were erythromycin nonsusceptible but clindamycin susceptible were tested using the D-test performed at both 15-mm and 22-mm disk separations and compared with genotyping as the "gold standard." The rate of inducible clindamycin resistance was 96.3% for S. aureus and 33.8% for CoNS spp. The sensitivities of the D-tests performed at 15 mm and 22 mm were 100% and 87.7%, respectively, and specificities were 100% for both. The use of 22-mm disk separation for the D-test to detect inducible clindamycin resistance results in an unacceptably high very major error rate (12.3%). All isolates with false-negative results harbored the ermA gene, and the majority were methicillin-resistant Staphylococcus aureus. False-negative results were associated with smaller clindamycin zone sizes and double-edged zones. We recommend using a disk separation distance of </=15 mm. There is wide geographic variation in the rates of inducible clindamycin resistance, and each laboratory should determine the local rate before deciding whether to either perform the D-test routinely or else report that all erythromycin-resistant S. aureus isolates are also clindamycin resistant.
authors.affiliation: Centre for Infectious Diseases and Microbiology, Institute of Clinical Pathology and Medical Research, Westmead, New South Wales, Australia. maosulli@nsccahs.health.nsw.gov.au
authors.forename: Matthew V N
authors.initials: MV
authors.lastname: O'Sullivan
authors.forename: Yongwei
authors.initials: Y
authors.lastname: Cai
authors.forename: Fanrong
authors.initials: F
authors.lastname: Kong
authors.forename: Xianyu
authors.initials: X
authors.lastname: Zeng
authors.forename: Gwendolyn L
authors.initials: GL
authors.lastname: Gilbert
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002981:Clindamycin / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D005838:Genotype; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods*; D013210:Staphylococcus / Q000145:classification / Q000187:drug effects* / Q000235:genetics; D013211:Staphylococcus aureus / Q000187:drug effects
chemical_list: D000900:Anti-Bacterial Agents; D002981:Clindamycin