Assessment of laboratory performance with Streptococcus pneumoniae antimicrobial susceptibility testing in the United States: a report from the College of American Pathologists Microbiology Proficiency Survey Program.

OBJECTIVE: To assess the performance of clinical microbiology laboratories in the United States when conducting in vitro susceptibility tests with Streptococcus pneumoniae.
METHODS: The results of a nationwide College of American Pathologists Proficiency Survey test sample, in which susceptibility testing of an isolate of S. pneumoniae was performed, were assessed with respect to precision and accuracy.
RESULTS: Wide variability was noted among participating laboratories with both minimum inhibitory concentration procedures and disk diffusion susceptibility tests when both methods were applied to S. pneumoniae. Despite this high degree of variation, categorical interpretive errors were uncommon. Numerous laboratories reported results for antimicrobial agents that are not recommended by the National Committee for Clinical Laboratory Standards for tests with S. pneumoniae.
CONCLUSIONS: Current susceptibility testing practices with S. pneumoniae in the United States indicate limited precision and a tendency for laboratories to test and report results obtained with antimicrobial agents of questionable therapeutic value against this organism. Continued efforts to standardize susceptibility testing of S. pneumoniae in the United States are warranted. In addition, modifications of existing interpretive criteria may be necessary.