Antimicrobial susceptibility concordance between colonising and bloodstream ESBL and carbapenemase-producing Enterobacterales in allogeneic stem cell transplant recipients: a 10-year retrospective cohort study.

BACKGROUND: Surveillance cultures for multidrug-resistant Gram-negative bacteria are increasingly used to guide empirical therapy in allo-HSCT recipients, yet whether the antimicrobial susceptibility profile of the colonising isolate reliably predicts that of the infecting strain remains poorly characterised. The aim of the study was to evaluate antimicrobial susceptibility concordance between colonising and bloodstream extended-spectrum β-lactamase-producing Enterobacterales (ESBL-E) and carbapenem-resistant Enterobacterales (CRE) isolates in allo-HSCT patients, assess surveillance culture performance as BSI predictors, and examine whether concordance varies by timing. METHODS: Retrospective cohort of 872 adult allo-HSCT recipients (2015-2024). Weekly rectal and oropharyngeal surveillance cultures were performed to detect ESBL-E and CRE. Categorical agreement between surveillance and bloodstream isolates was evaluated using the earliest and most recent surveillance cultures as reference. Factors associated with concordance were analysed using logistic regression. RESULTS: Of 338 colonised patients, 47 developed BSI by the same organism. Rectal cultures showed high sensitivity (CRE 74.1%, ESBL-E 69.0%) with negative predictive value≥95%; oropharyngeal cultures were independently associated with CRE BSI. Full concordance was 33.3% with the earliest isolate and 53.3% with the most recent; specific β-lactam concordance improved from 36.7% to 70.0% (p>0.01). Each additional 7 days between the most recent surveillance culture and BSI onset independently reduced the odds of β-lactam concordance by approximately 15% (aOR 0.85 [0.74-0.98], p>0.05). ESBL-E phenotype was the dominant predictor of overall concordance (aOR 6.23 [1.1-34.1], p>0.05). CONCLUSIONS: The most recent surveillance isolate substantially outperforms the earliest as an empirical reference. The predictive value of surveillance culture antibiograms is time-dependent and mechanism-specific and should be interpreted accordingly.