Discussions

We found that almost three-quarters of all patients with NI acquired resistant bacterial infections over the study period. The number of cases with AMR NI in our study was consistent with findings in a recent study in Thailand [Reference Lim16]. The distribution of the type of bacterial infections was also in accordance with the national antibiogram data provided by the National Antimicrobial Resistance Surveillance Center Thailand (NARST) [17]. The relative increased in inpatient mortality from AMR NI was estimated using a survival analysis controlling for confounding factors [Reference Shurland18–Reference Huang21], including CCI score in order to adjust patients’ clinical baseline as other recent studies had done [Reference Pena22–Reference Nguyen24]. Compared with patients in the non-AMR group, the mortality chances for patients in the AMR group with A. baumannii, E. coli, P. aeruginosa and S. aureus were 92%, 25%, 60% and 84% higher, respectively. Because resistant E. coli in our study was ESBL-producing organisms which can be treated by widely available carbapenem antimicrobials, mortality of infection caused by this bacteria remained low. Another explanation is that the most common site of E. coli infection was UTI, which generally is not significantly associated with fatal outcome. On the other hand, A. baumannii mostly causes infection of the respiratory system which tends to carry higher mortality background. Only a few antimicrobial agents such as colistin, a nephrotoxic agent and tigecycline, a bacteriostatic agent with unpromising efficacy in the literature, are available for treatment. As a result, mortality associated with A. baumannii infection in our study was high. There were several studies that reported the mortality chances of these bacterial infections in Thailand and worldwide. Lim et al. published in 2016 [Reference Lim16] estimated the mortality attributable to multidrug resistance in hospital-acquired infection in Thailand for A. baumannii, E. coli, P. aeruginosa and S. aureus as 70%, 19%, 65% and 44%, respectively. Yang et al. published in 2013 [Reference Yang23], a retrospective 10-year study was conducted at a 2900-bed teaching hospital located in Northern Taiwan, reported crude 14-day mortality rates of A. baumannii in hospital-acquired PNI and ventilator-associated PNI were 43.8% and 31.3%, respectively. Huang et al. published in 2012 [Reference Huang21], a retrospective cohort study was conducted in Taipei Veterans General Hospital, Taiwan, estimated odd ratio of mortality caused by carbapenem-resistant A. baumannii bacteraemia was 1.03 (95% CI 0.48–2.20) relative to carbapenem-susceptible A. baumannii bacteraemia. Abernethy et al. published in 2015 [Reference Abernethy25], a large national study in England, reported a 30-day all-cause mortality rate of E. coli was 18.2% (95% CI 17.8–18.7%). Schreiber et al. published in 2011 [Reference Schreiber, Chan and Shorr19] reported the mortality rate in patients with bacteraemia complicated S. aureus PNI was 39% compared with non-bacteria. The meta-analysis of Cosgrove et al. published in 2003 [Reference Cosgrove26], enrolled studies between January 1980 and December 2000 reported a significant increase in odd ratio (1.93; 95% CI 1.54–2.42; P < 0.001) of mortality associated with methicillin-resistant S. aureus bacteraemia compared with methicillin-susceptible S. aureus bacteraemia. Our study estimated higher expected hazard ratios of mortality for AMR NI relative to non-AMR NI than other studies. Overall, mortality of AMR NI in our study was not statistically significant different from those of non-AMR. It might be due to the fact that these patients were all critically ill which posed them at risk of dying. Furthermore, with relatively more available resources for treatment as compared with other non-teaching hospitals, mortality of AMR NI might be lowered as shown by some studies that effective antibiotics played an important role in improving outcomes of these infections [Reference Wang27, Reference Gutiérrez-Gutiérrez28].

This study estimated that the annual relative increased in inpatient mortality in Thailand would be 48 258 cases out of 111 295 (over 50%) cases with AMR NI. The study of Lim et al. published in 2016 [Reference Lim16] estimated the burden of MDR bacteria in Thailand to be 19 122 of 45 209 (43%) deaths in 2010. The preliminary study of health and economic impact of AMR in Thailand published in 2012 [Reference Phumart14] found 38 481 additional deaths of 87 481 (43%) AMR cases in 2010. However, our annual number of deaths is higher than those reported by others. We identified the annual number of deaths as the relative difference hazard ratio of mortality for the AMR and non-AMR groups based on the Cox model fitted with individual-level data of our study samples. To the best of our knowledge, this is the most updated data collected from patients in Thailand. The conditional mortality on having AMR was derived using the hazard ratio of mortality provided by the Cox model. Kritsotakis et al. in 2017 [Reference Kritsotakis11] also used the multivariate Cox proportional hazard model to estimate hazard of death between patients with hospital-acquired infections and uninfected patients in terms of adjusted hazard ratios and associated 95% confidence bounds. Even though, Lim et al. in 2016 [Reference Lim16] used a 30-day all-cause mortality rate while the preliminary study in 2012 [Reference Phumart14] used a decision tree model. Moreover, our study took place in the critical care units of a large tertiary hospital. Most of the patients in our study had more severe illnesses than other hospital types. Consequently, the samples used may not necessarily be representative of the whole NI population in Thailand. This may result in an overestimate of AMR burden estimation for all hospitalised patients in the country. However, to the best of our knowledge, hospital-acquired infection surveillance in most hospitals in Thailand now focus on the ICU population according to targeted surveillance strategies.