Netilmicin disk susceptibility tests: Effect of cations on the MIC correlates

When testingPseudomonas aeruginosa against netilmicin, MICs were markedly affected by the concentration of cations added to the test medium. A susceptible disk test result (zone15 mm) corresponded to MIC4.0g/ml in unsupplemented broth, 12g/ml in broth with half the usual amount of cations and 32 g/ml in broth with the recommended concentration of cations. Tests with 30g netilmicin disks best predicted susceptibility as determined by MICs in broth without added cations. When the MICs were determined in cation supplemented broth, the number of interpretive discrepancies increased to an unacceptably high level.