Broth microdilution testing of Pseudomonas aeruginosa and aminoglycosides: need for employing dilutions differing by small arithmetic increments.

The use of dilutions differing by small arithmetic increments was studied as a means for improving the definition and measurement of minimum inhibitory concentrations and precision parameters for testing Pseudomonas aeruginosa versus the aminoglycosides by the broth microdilution test. For five strains of P. aeruginosa versus gentamicin, tobramycin, and amikacin, comparisons were made of minimum inhibitory concentrations which were replicated in parallel by using three microdilution systems: small increment panels prepared by us, modified twofold dilution panels prepared by us, and similar modified twofold dilution panels obtained commercially. The small increment dilutions were prepared to differ by concentrations of 1.0 microgram/ml for gentamicin and tobramycin and by 2.0 micrograms/ml for amikacin. Use of the small increment dilutions resulted in the ability to measure minimum inhibitory concentrations at more closely spaced intervals than those dictated by modified twofold dilution schemes, and confidence limits were significantly improved. The average coefficient of variation for the small increment microdilution test results was 9.5%, with 99.5% of minimum inhibitory concentrations falling within +/- 2 small increment dilutions from their modal values.