667 - Multicenter Assessment of Enterobacterales, Salmonella spp. and Pseudomonas aeruginosa Using Updated CLSI Levofloxacin Breakpoints on MicroScan Dried Gram Negative MIC Panels

Background: Data from a multicenter clinical study with Enterobacterales, Salmonella spp. and P. aeruginosa on a MicroScan Dried Gram-negative MIC (MSDGN) Panel was evaluated with updated US FDA/CLSI levofloxacin breakpoints. MIC results were compared to results obtained with frozen broth microdilution panels prepared according to CLSI methodology.

Methods: A total of 839 Enterobacterales, Salmonella spp. and P. aeruginosa clinical isolates were tested at three clinical sites in efficacy and challenge combined. MSDGN panels were evaluated with turbidity and Prompt^® methods of inoculation. MIC values obtained from the MSDGN panels were compared to MICs utilizing a CLSI broth microdilution reference panel. To assess reproducibility, a subset of 15 organisms were tested on MSDGN panels at each site. MSDGN panels were incubated at 35 ± 1^ºC and read on the WalkAway System, the autoSCAN-4 instrument, and visually. Read times for the MSDGN panels were at 16-20 hours. Frozen reference panels were prepared and read according to CLSI methodology. FDA and CLSI breakpoints (µg/mL) used for interpretation of MIC results were: Enterobacterales ≤ 0. 5 S, 1 I, ≥ 2 R; Salmonella spp. ≤ 0.12 S, 0.25-1 I, ≥ 2 R; P. aeruginosa ≤ 1 S, 2 I, ≥ 4 R.

Results: Essential and categorical agreement were calculated compared to frozen reference panel results. Results for isolates tested during efficacy and challenge with Prompt inoculation and manual read are as follows:

Read Method	Essential Agreement (EA) %	Categorical Agreement (CA) %	Very Major Error (VMJ) %	Major Error (MAJ) %
Enterobacterales	96.2 (638/663)	96.7 (641/663)	1.5 (2/133)	0.0 (0/515)
Salmonella spp.	100 (83/83)	98.8 (82/83)	0.0 (0/19)	0.0 (0/38)
P. aeruginosa	94.6 (88/93)	93.6 (87/93)	0.0 (0/34)	1.9 (1/54)

Conclusion: Levofloxacin MIC results for Enterobacterales, Salmonella spp., and P. aeruginosa obtained with the MSDGN panel correlate well with MICs obtained using frozen reference panels using updated FDA/CLSI interpretive criteria in this multicenter study.
* PROMPT^® is a registered trademark of 3M Company, St. Paul, MN USA.
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