[Clinical laboratory approach for estimating effective administrative dose of cephalothin. Reevaluation of MIC break points in 4 category system of disc susceptibility test].

The reliability of the cephalothin (CET) disc susceptibility test in estimating approximate values of MICs was studied using various clinical isolates totaling 248 strains and using Showa discs (8 mm diameter containing 30 micrograms of CET) and Difco discs (6 mm diameter containing 30 micrograms of CET). Clinical significance of a 4 category system for the interpretation of the CET disc tests, which is widely used in Japan, was reevaluated to determine whether this system would be suitable or not for the evaluation of proper dose levels of administration. The results obtained with the disc methods were compared with MICs determined using the agar dilution method at an inoculum level of 10(6) CFU/ml. The results of the CET disc susceptibility test 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 4 categories of susceptibility are ( ) 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. With the Showa disc susceptibility test, 15 out of the 248 strains (6.0%) tested showed false positive results and 6 strains (2.4%) showed false negative results. With the Difco disc test, 18 out of the 248 strains (7.3%) tested showed false positive results and 6 (2.4%) showed false negative results. Excluding Enterococcus faecalis from the test, results because better in the quantitative estimation of MICs, resulting false positive rates of 3.2% (Showa), and 4.4% (Difco). A 3 category system of the interpretation of disc test is generally used in the USA and Europe. MIC break points proposed for the classification of the CET test are sensitive, MIC less than or equal to 8 micrograms/ml, and resistance, MIC greater than or equal to 32 micrograms/ml. With the Showa disc susceptibility test, 14 out of the 248 strains (5.6%) tested showed false positive results and 6 strains (2.4%) showed false negative results. With the Difco disc test 7 out of the 248 strains (2.8%) showed false positive results and 21 strains (8.5%) showed false negative results. In this study, MIC70S of CET against Staphylococcus aureus and Staphylococcus epidermidis were 1.56 and 0.78 micrograms/ml, respectively. CET was not so effective against Gram-negative rods except Klebsiella pneumoniae, Proteus mirabilis and Escherichia coli. MIC70S against K. pneumoniae, P. mirabilis, and E. coli were 6.25, 3.13, and 3.13 micrograms/ml, respectively.