Aggregate MDRO proportion and species-specific resistance trends in a tertiary general hospital: a five-year laboratory-based surveillance study of priority resistance phenotypes

Aggregate multidrug-resistant organism (MDRO) proportions and species-specific resistance proportions capture different aspects of antimicrobial resistance. We evaluated these measures and temporal resistance patterns in a tertiary general hospital during 2021–2025. This single-center, retrospective, laboratory-based surveillance study included inpatient bacterial isolates deduplicated using a 30-day first-isolate rule. The aggregate MDRO category included isolates meeting general multidrug-resistance criteria or nationally prioritized resistance phenotype definitions. Aggregate MDRO proportions used all included isolates as the denominator. Resistance proportions for methicillin-resistant Staphylococcus aureus (MRSA), carbapenem-resistant Klebsiella pneumoniae (CRKP), carbapenem-resistant Pseudomonas aeruginosa (CRPA), carbapenem-resistant Acinetobacter baumannii (CRAB), and carbapenem-resistant Escherichia coli (CREC) used species-specific denominators. Annual and quarterly patterns and ward and specimen distributions were described. Quarterly trends were evaluated using data-driven single-breakpoint segmented Poisson regression; monthly trends and calendar-month variation were assessed using Poisson generalized linear models. Among 10,708 non-duplicate clinical isolates, 4,850 (45.29%) met the aggregate MDRO definition, including 1,265 isolates belonging to the five priority resistance phenotypes. Overall species-specific resistance proportions were 53.67% for CRAB, 38.35% for MRSA, 21.03% for CRPA, 13.70% for CRKP, and 0.75% for CREC. Aggregate MDRO proportions remained approximately 48%–50% during 2021–2023 and were lower in 2024–2025. Clear support for an internal breakpoint was identified only for CRKP, after 2023Q1. CRKP resistance increased by 21.21% per quarter before the breakpoint (95% CI, 8.22% to 35.76%; P < 0.001); the subsequent downward trend was not statistically significant (− 4.48% per quarter; 95% CI, − 8.85% to 0.10%; P = 0.055). CRPA resistance increased by 2.04% per month (95% CI, 1.05% to 3.04%; P < 0.001), with significant calendar-month variation ( P = 0.003). MRSA, CRKP, and CRAB showed no significant monthly trends. Aggregate MDRO and species-specific resistance proportions provide complementary surveillance information. CRKP showed a nonlinear temporal pattern, whereas CRPA showed an overall upward trend with calendar-month variation. Integrating species-specific denominators and analyses at multiple temporal scales may improve surveillance interpretation and inform infection prevention and antimicrobial stewardship.