External quality assessment for clinical microbiological laboratories in Norway 1982. 2. Evaluation of the antibiotic...

An external quality assessment for microbiology included antibiotic sensitivity testing of the 24 bacterial strains submitted. The number of antibiotics selected when sensitivity testing bacteria from different sources varied considerably between the laboratories. The discrepancy between actually reported sensitivity groups and the sensitivity groups calculated on the basis of the reported inhibition zones using the recommended regression lines was as great as 7% of a total of 1922 tests. This was mainly due to the use of outdated zone size break-points, indicating that information concerning changing recommendations may be inadequate. Discrepancies between the results reported by the laboratories and the results of a microdilution method giving the minimal inhibitory concentration for six antibiotics were recorded in 18.6% of the tests. Approximately one third of these discrepancies were characterized as "major" or "very major".