Validation and reproducibility assessment of tigecycline MIC determinations by Etest

Abstract

A multicenter study was conducted to validate Etest tigecycline compared to the Clinical Laboratory Standards Institute reference broth microdilution and agar dilution methodologies. A large collection of gram-negative (n = 266) and gram-positive (n = 162) aerobic bacteria, a collection of anaerobes (n = 385), and selected collections of nonpneumococcal streptococci (n = 369), Streptococcus pneumoniae (n = 372), and Haemophilus influenzae (n = 372) were tested. Strains with reduced susceptibility to tigecycline were used with all test methods. The Etest showed excellent inter- and intralaboratory reproducibility for all organism groups tested regardless of the test methodology. The essential agreement values with the reference method (+/-1 dilution) were >99% for the collection of gram-negative and gram-positive aerobes; >98% for the S. pneumoniae, H. influenzae, and anaerobe collections; and 100% for the group of nonpneumococcal streptococci. These results validate the performance accuracy and utility of Etest tigecycline and verify the reproducibility of this convenient predefined gradient methodology for tigecycline susceptibility determination.

FIG. 1.

Error rate-bounded analysis comparing tigecycline reference broth dilution with tigecycline Etest strips against 221 gram-negative isolates. The datum points that fall along the diagonal line of the scattergram represent EA between the Etest result (x axis) and the broth microdilution result (y axis). Dotted horizontal and vertical lines demarcate the susceptible (≤2 μg/ml), intermediate (4 μg/ml), and resistant (≥8 μg/ml) categories along both axes. The datum points falling within the quadrant lines represent minor errors. Note that an Etest result that falls between twofold dilutions must be rounded up to the next upper twofold value before categorization. The gram-negative collection includes: Citrobacter spp., Enterobacter cloacae, E. coli, Klebsiella oxytoca, K. pneumoniae, Morganella morganii, Providencia stuartii, and Serratia spp.

FIG. 2.

Error rate-bounded analysis comparing tigecycline reference broth dilution with tigecycline Etest strips against 386 anaerobes. The datum points that fall along the diagonal line of the scattergram represent EA between the Etest result (x axis) and the AD result (y axis). Dotted horizontal and vertical lines demarcate the susceptible (≤4 μg/ml), intermediate (8 μg/ml), and resistant (≥16 μg/ml) categories. The datum points falling within the quadrant lines represent minor errors. Note that an Etest result that falls between twofold dilutions must be rounded up to the next upper twofold value before categorization. The anaerobe collection includes: Bacteroides caccae, B. distasonis, B. fragilis, B. ovatus, B. thetaiotaomicron, B. uniformis, B. ureolyticus, B. vulgatus, and other Bacteroides spp.

FIG. 3.

Error rate-bounded analysis comparing tigecycline reference broth dilution with tigecycline Etest strips against 374 H. influenzae isolates. The datum points that fall along the diagonal line of the scattergram represent the EA between the Etest result (x axis) and the broth microdilution result (y axis). Horizontal and vertical lines demarcate the hypothetical nonsusceptible MIC (MIC ≥ 0.5 μg/ml) along both axes. Note that an Etest result that falls between twofold dilutions must be rounded up to the next upper twofold value before categorization.