Criteria of time and antibiotic susceptibility in the elimination of duplicates when calculating resistance frequencies.

We studied the effects of various systems of eliminating repeat isolates on the absolute number and susceptibility of Pseudomonas aeruginosa, Acinetobacter baumannii and Staphylococcus aureus isolates over a 2 year period. The criterion of time is objective and reproducible, whereas that of variation in antibiotic susceptibility detects variations in the susceptibility of microorganisms that acquire resistance during treatment, but may be affected by methodological errors in determining the antibiotic susceptibility. These tools are useful in the control of multi-resistant bacteria and enable the true situation regarding antibiotic resistance in each geographical area to be determined.