Amikacin treatment in patients with Enterobacterales bacteraemia: impact of MIC on mortality

BACKGROUND: Recently, breakpoints of Enterobacterales to amikacin were changed from MIC ≤ 16 mg/L to MIC ≤ 4 mg/L based mainly on laboratory data with little supporting clinical evidence. Our aim was to investigate the relation between MIC of Enterobacterales to amikacin and mortality among patients with Enterobacterales bacteraemia from a urinary tract source treated with amikacin. PATIENTS AND METHODS: This retrospective, single-centre study included patients with Enterobacterales urinary source bacteraemia treated with amikacin, with Low (MIC ≤ 4 mg/L) and High (MIC 8 or 16 mg/L) MICs. A cohort of patients treated with ertapenem was used to assess if amikacin MIC is a marker of severity independent of antimicrobial treatment. The primary outcome was 30-day mortality. Multivariate logistic regression analysis was done to assess risk factors for mortality. RESULTS: We included 85 patients, 46 (54.1%) were male, and mean age was 79.0 years (SD 11.7). Sixty-one patients (71.8%) had Low MIC and 24 (28.2%) had High MIC. Thirty-day mortality was 8.2% and 29.2% in the Low and High MIC groups, respectively (P = 0.031). Risk factors for 30-day mortality were age, infection by Enterobacterales other than Escherichia coli and high amikacin MIC. In a cohort of 88 patients treated with ertapenem, amikacin MIC was not associated with 30-day mortality. CONCLUSIONS: We demonstrated a relation between higher amikacin MIC levels (8 and 16 mg/L) and increased 30-day mortality in patients treated with amikacin for bacteraemia secondary to a urinary source. These findings support the new CLSI breakpoint change of Enterobacterales to amikacin.