[Clinical laboratory approach for estimating effective administrative dose of tobramycin. Reevaluation of in vitro MIC break points of disk susceptibility test].

The reliability of the tobramycin (TOB) disc susceptibility test in estimating approximate values of MICs was studied using various clinical isolates totaling 261 strains and using Showa discs (8 mm diameter containing 30 micrograms of TOB) and Difco discs (6 mm diameter containing 10 micrograms of TOB). Clinical significance of a 4 category system for the interpretation of the disc tests, which is widely used in Japan, and that of a 3 category system used in USA and Europe, were also evaluated to determine which system would be more suitable for the evaluation of proper dose levels of administration. Furthermore, the evaluation was made using these discs with respect to the in vitro MIC break points for therapeutic use of antibiotics proposed by the British Society for Antimicrobial Chemotherapy (J. Antimicr. Chemoth. 21:701-710, 1988). The results obtained with the disc method were compared with MICs determined using the agar dilution method at an inoculum level of 10(6) CFU/ml. The results of the TOB disc susceptibility test either with Showa or Difco discs were well correlated with MICs, showing the reliability of the disc method to estimate approximate values of MICs. Break points in MIC values proposed for the classification of bacteria into the 4 categories of susceptibility are () MIC less than or equal to 2 micrograms/ml, (++) MIC greater than 2-10 micrograms/ml, (+) MIC greater than 10-50 micrograms/ml, (-) MIC greater than 50 micrograms/ml. Those proposed in the 3 categories of susceptibility are Sensitive (S) MIC less than or equal to 4 micrograms/ml, Intermediate (I) MIC greater than 4-8 micrograms/ml, Resistance (R) MIC greater than 8 micrograms/ml. In the 4 category classification system of the Showa disc susceptibility test, 16 out of 261 strains (6.1%) tested showed false positive results and 7 (2.7%) did false negative results. If the classification was modified as follows: ( ) MIC less than or equal to 3 micrograms/ml, (++) MIC greater than 3-15 micrograms/ml, (+) MIC greater than 15-60 micrograms/ml, (-) MIC greater than 60 micrograms/ml, false positive results were markedly reduced. Only 6 out of 261 strains (2.3%) showed false positive results, and 7 (2.7%) did false negative results. With Difco disc, in the 4 category interpretation system, 8 out of 261 strains (3.1%) tested showed false positive and 35 (13.4%) did false negative results. No inhibitory zones were observed against a majority of strains with MIC greater than 25 micrograms/ml, thus unable to assess (+) susceptibility.(ABSTRACT TRUNCATED AT 400 WORDS)