[Clinical laboratory approach in estimating effective administrative dose of piperacillin. Reevaluation of MIC break points in 3 and 4 category systems of disc susceptibility test].
The reliability of the piperacillin (PIPC) disc susceptibility test in estimating approximate values of MICs was studied with various clinical isolates totaling 284 strains using Showa discs and the discs prepared in this laboratory (both 8 mm diameter containing 30 micrograms of PIPC). Clinical significance of a 4 category system for the interpretation of the PIPC 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. Break points in MIC values proposed for the classification of bacteria into 4 categories of susceptibility were: ( ) 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. A 3 category system for interpreting of disc test, which is generally used in the USA and Europe, was also evaluated. MIC break points in the 3 category system proposed for the classification of the PIPC test are sensitive (S) MIC less than or equal to 64 micrograms/ml and resistant (R) MIC greater than or equal to 256 micrograms/ml. In addition, British Society for Antimicrobial Chemotherapy recently proposed in vitro MIC break points for PIPC in therapeutic use, recommending MICs 16 and 64 micrograms/ml. MIC 16 micrograms/ml can be used for general infections treated with usual administrative doses for such infections and MIC 64 micrograms/ml may apply to increased dosage or to normal dosage when PIPC is locally concentrated (mainly urinary or biliary tract infections). 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 PIPC disc susceptibility test either with Showa discs or discs prepared in this laboratory were well correlated with MICs, showing the reliability of the disc method in estimating approximate values of MICs. In the 4 category classification system of the Showa disc test, 33 out of 284 strains (11.6%) tested showed false positive results and 5 strains (1.8%) false negative results. Similarly, in the test of 30 micrograms discs prepared in this laboratory, of 284 strains 26 (9.2%) showed false positive results and 8 (2.8%) false negative results. In the tests of Showa discs and discs prepared in this laboratory, both containing 30 micrograms of PIPC, no inhibitory zones were observed against the strains with MIC greater than 100 micrograms/ml.(ABSTRACT TRUNCATED AT 400 WORDS)
Matsuo
K
K
Department of Clinical Investigation, Kitano Hospital, Tazuke Kofukai Medical Research Institute.
Uete
T
T
D001419:Bacteria / Q000187:drug effects* / Q000254:growth & development; D005188:False Negative Reactions; D005189:False Positive Reactions; D008826:Microbial Sensitivity Tests / Q000379:methods; D010878:Piperacillin / Q000008:administration & dosage* / Q000494:pharmacology; D011237:Predictive Value of Tests
D010878:Piperacillin