Imipenem Susceptibility Breakpoint for E. coli in Male UTIs
The susceptibility breakpoint for imipenem against Escherichia coli in urinary tract infections—including in males—is not explicitly defined by CLSI with a urinary-specific threshold, but carbapenems including imipenem demonstrate near-universal susceptibility (≥99%) against E. coli from community-acquired UTIs, with no resistance reported in multiple surveillance studies. 1, 2, 3, 4
Standard Carbapenem Breakpoints
CLSI revised carbapenem interpretive breakpoints in 2010-2011 primarily to detect carbapenemase-producing Enterobacteriaceae (particularly KPC-producing Klebsiella pneumoniae), not to establish urinary-specific thresholds. 5
The new breakpoints created intermediate and resistant categories that better identify isolates with carbapenemases, obviating the need for routine Modified Hodge Testing except for epidemiological purposes. 5
These breakpoints apply uniformly across all infection sites and patient demographics—there is no sex-specific or urinary tract-specific modification for imipenem susceptibility testing. 6
Clinical Performance in UTIs
Imipenem demonstrates 99.9-100% susceptibility against E. coli isolated from community-acquired UTIs across multiple geographic regions and study populations. 1, 2, 3, 4
In a 2013 Italian study of 429 community-acquired UTI isolates, imipenem showed 100% sensitivity with favorable MIC distribution. 1
A 2018 Ethiopian study of uropathogenic E. coli reported 100% susceptibility to both imipenem and meropenem, even among multidrug-resistant (96.5%) isolates. 2
A 2016 Turkish seven-year surveillance (8,975 E. coli isolates) found zero resistance to imipenem or meropenem. 3
Even among ESBL-producing E. coli from community-acquired UTIs in China (38% ESBL prevalence), imipenem maintained >99% susceptibility. 4
Male-Specific Considerations
No breakpoint modification exists for male versus female patients—the microbiological susceptibility standard is identical regardless of sex. 6
While male UTIs differ clinically (often complicated, requiring longer treatment duration), these factors do not alter the laboratory interpretation of imipenem susceptibility. 6
The high ESBL prevalence in male patients (53.6% vs 35.2% in females in one Chinese cohort) reinforces the value of carbapenems when ESBL producers are suspected, but does not change the breakpoint itself. 4
Practical Application
For empirical therapy decisions in males with complicated UTI or suspected ESBL producers, imipenem remains highly reliable based on near-universal susceptibility patterns. 1, 2, 3, 4
In Taiwan, ertapenem (a related carbapenem) is widely used for ESBL-producing and multidrug-resistant Enterobacteriaceae, though newer CLSI breakpoints reclassified an additional 12% of ESBL-producing K. pneumoniae as non-susceptible. 5
The clinical efficacy of carbapenems against isolates with MICs in the new "intermediate" range remains uncertain due to lack of controlled outcome studies, though this rarely applies to E. coli from UTIs given the near-universal susceptibility. 5
Key Caveats
Carbapenem-resistant E. coli remains exceedingly rare in community-acquired UTIs (0.25% prevalence in one large Chinese study), but when present, represents a critical resistance mechanism requiring alternative therapy. 4
Routine susceptibility testing should still be performed rather than assuming susceptibility, particularly in healthcare-associated infections or patients with prior carbapenem exposure. 5
The absence of a urinary-specific breakpoint means laboratories apply the systemic infection standard, which is appropriate given imipenem's excellent urinary concentration and bactericidal activity. 7