Abstract
Objective
We aimed to investigate patients discharged alive from hospital following an ICU admission for drug overdose and whether exposure of prior overdose-related ICU admission was associated with a future ICU readmission.
Design, setting, participants
This multicentre registry-based study included data from 167 ICUs across Australia that reported to the Australia New Zealand Intensive Care Society Adult Patient Database. We included all adult patients (≥16 years) discharged alive from the hospital after an ICU admission for an overdose between 1st July 2018 and 31st December 2023. The exposure of interest was an overdose-related ICU admission in the six months prior to the index ICU admission.
Main outcome measures
The primary outcome was all-cause ICU readmission within the following six months. The secondary outcome was ICU readmission due to an overdose within six months. We used mixed-effects multivariable logistic regression to adjust for confounders and identify risk factors for ICU readmission.
Results
Of 30,125 index episodes of ICU overdose admissions (25,604 individuals), 2555 (8.5%) had an overdose-related ICU admission in the prior six months. They had higher rates of all-cause ICU readmission compared to those without a prior overdose-related ICU admission (40.9% vs. 8.9%; p < 0.001). After accounting for other factors including younger age, female sex, Indigenous status, higher SOFA scores, chronic respiratory disease, cirrhosis, dialysis dependence and frailty, an overdose in the six months prior to the index ICU admission was associated with ICU readmission in the following six months (OR = 1.93; 95%CI: 1.59–2.34; p < 0.001).
Conclusions
This large Australian registry-based study found that in patients discharged alive from hospital following an ICU admission for overdose, patients with a history of prior ICU admission for overdose had almost double the risk for ICU readmission. These findings have implications for risk stratification, clinical decision-making and resource planning.
Keywords: Overdose, ANZICS, Readmissions related to overdose, Survival
1. Introduction
There is increased prevalence of patients hospitalised following a drug overdose requiring admission to the intensive care unit (ICU).[1], [2], [3] ICU-level care is typically required for complications including airway compromise, haemodynamic instability or need for extracorporeal therapies. Although morbidity and mortality vary depending on overdose type and intent,[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] these admissions place a significant burden on healthcare and institutional resources.3,12
Repeated episodes of drug overdose requiring ICU admission potentially highlight a complex interplay of psychiatric conditions, substance use disorders, and high-risk behaviours. A significant proportion of patients in ICU following a drug overdose have previous overdose-related hospitalisations, often linked to underlying psychiatric diagnoses.11 Each subsequent overdose can further exacerbate health complications, strain critical care resources, and increase the likelihood of long-term morbidity or mortality.
The repeated need for ICU-level care may reflect potential inequity in delivering treatment and intervention strategies for substance use disorder, emphasising the need for comprehensive, multidisciplinary approaches to prevent further overdoses and improve patient outcomes. In Australia, such inequities are particularly evident when observing the delivery of mental health services in regional and remote areas, when compared to metropolitan centres.13,14
Although outcomes after ICU admission for overdose have been described, patients with repeated ICU admissions remain poorly characterised. To address this gap, this registry-based study aimed to determine whether an overdose-related ICU admission within the prior six months was associated with ICU readmission for any cause in the six months following an index overdose admission, and to describe the characteristics of patients with recurrent admissions. We hypothesised that a recent prior overdose-related ICU admission would be associated with an increased risk of ICU readmission, with implications for risk stratification and healthcare resource allocation.
2. Methods
2.1. Ethics approval
The study was approved by the Alfred Hospital Research and Ethics Committee (Reference: 79/24) and the Australian and New Zealand Intensive Care Society (ANZICS) Centre for Outcome and Resource Evaluation Management Committee (CORE), and was conducted in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology guidelines.15
2.2. Data sources and definitions
We performed a retrospective cohort study using the ANZICS Adult Patient Database (APD). The ANZICS-APD is a bi-national clinical quality registry dataset collected by the ANZICS CORE on all admissions to 98% of adult ICUs in Australia and 67% of ICUs in New Zealand.16 Data collectors receive regular training and quality assurance reviews. Data is collected using a standardised data dictionary.17 Regular automated checks further ensure the validity of recorded data.18
The following information was extracted from the ANZICS-APD: age, sex, comorbidities, frailty, ICU and hospital admission source, diagnosis at the time of admission (categorized using the ANZICS modification of the APACHE IV diagnostic system), acute illness severity (APACHE-II,19 APACHE-III/IV, sequential organ failure assessment [SOFA] scores20 and Australian and New Zealand Risk of Death [ANZROD] mortality prediction model score21), treatment limitation at ICU admission, organ supports, ICU and hospital mortality, ICU and hospital length of stay (totalled across all hospitals if transferred) and discharge destination.17
2.3. Study population
All adult patients (≥16 years) discharged alive from hospital after an admission to an Australian ICU with a diagnosis of drug overdose between 1st July 2018 and 31st December 2023 were included. It was possible for patients to contribute multiple index episodes, but each index admission was unique, with separate assessments for the study outcome (Fig. 1). We excluded those aged <16 years, those who died in hospital and ICU readmissions within the same hospital stay. If a patient was transferred between ICUs, the second ICU admission episode was excluded from analysis, but the discharge destination at the end of the later hospitalisation was included as the patient's final outcome. We excluded patients with missing or inconsistent hospital and ICU admission and discharge times.
Fig. 1.
Inclusions and exclusions.
2.4. Outcomes
The primary outcome (‘All-cause readmission’) was defined as ICU readmission for any reason during a future hospitalisation episode at any hospital within six months of discharge (Fig. 1). The secondary outcome was ICU readmission due to an overdose within six months (‘Readmission with overdose’). Outcomes were identified up to 30th June 2024 (i.e. six months after the end of the study inclusion period). Additional outcomes reported included the cause of the readmission, time to readmission and the final discharge destination after readmission.
2.5. Exposure (‘prior overdose’)
The exposure of interest was an overdose-related admission to the ICU in any hospital in the six months prior to the index admission (Fig. 1, Appendix Fig. 1). Prior ICU admissions were identified from 1st January 2018 (i.e. six months before the beginning of the study inclusion period). An individual could be represented multiple times in the dataset, with each hospitalisation considered as an index patient-episode with an associated exposure window (prior overdose) and potential outcome (all-cause readmission).
2.6. Classification of drug overdose
Admission diagnoses listed as overdoses of sedatives (subcode: 703.08), analgesics (subcode: 703.05) or antidepressants (subcode: 703.06) were classified as ‘Overdose due to prescription drugs’. Admission diagnoses listed as alcohol (subcode: 702.04), street drugs (subcode: 703.09) or other toxins (subcode: 703.07) were classified as ‘Overdose due to non-prescription drugs’. Where no qualifying subcode information was available, the remaining patients were classified as ‘Overdose (drug unknown)’.
2.7. Remoteness
We used the Accessibility/Remoteness Index of Australia Plus (ARIA+),22 developed by The Australian Statistical Geography Standard Remoteness Structure, to categorise patients by their residential postcode into five classes of relative geographic remoteness: Major Cities, Inner Regional, Outer Regional, Remote, and Very Remote areas.
2.8. Socioeconomic status
Socioeconomic status was categorised by residential postcode linked to the Index of Relative Socio-economic Advantage and Disadvantage (IRSAD) score, obtained from the Australian Bureau of Statistics. Lower IRSAD scores indicated greater socioeconomic disadvantage, such as lower income, education, or employment levels and a lack of advantage in general.23 In comparison, higher scores suggested a relative lack of disadvantage and greater advantage in general. Patients were grouped into three categories representing the lowest three, middle four and upper three deciles of IRSAD score.
2.9. Statistical analysis
We performed data analysis using Stata 19.0 (StataCorp, College Station, Texas 77845 USA). We used a two-sided p-value of <0.05 to indicate statistical significance. Individual patients were encoded using a statistical linkage key (SLK-58124), a validated privacy-preserving identifier derived from components of a patient's name, date of birth, and sex, which has been widely used for linkage of Australian administrative health datasets.25,26 This allowed identification of the same individual at different hospitals. For descriptive reporting, each patient episode was considered as a distinct observation. We summarised categorical data using counts and percentages, and continuous data using mean (standard deviation) or median (interquartile range [IQR]) as appropriate, depending on the distribution. We compared between groups using the χ2 test, Student t-test, or log-rank tests as appropriate. We used mixed-effects multivariable logistic regression to adjust for confounders and identify associations of ICU readmission, including age, sex, ICU admission year, ICU and hospital admission source, comorbidities, frailty, hospital classification, socioeconomic status, categories of remoteness, ICU admission after a medical emergency team call, presence of treatment limitations at ICU admission and cardiac arrest in the 24 h before ICU admission. The patient identifier (SLK-581) was included as a random intercept to account for multiple admissions per individual. Covariates were selected a priori based on plausible relationships with both exposure and outcome; final model selection used Akaike and Bayesian Information Criteria. Results are reported using odds ratio (OR) with corresponding 95% confidence intervals (95%CI). A sensitivity analysis was undertaken examining time-to-readmission reported using Kaplan–Meier curves and using mixed effects multivariable Cox proportional hazards regression, adjusted for the same covariates, with results expressed as hazard ratios (HRs) with 95%CI.
3. Results
3.1. Patient demographics
During the study period, there were 987,768 ICU admissions reported to the ANZICS-APD. After exclusions, there were 25,604 individuals who had 30,125 ‘index’ ICU admissions due to an overdose and were discharged alive from the hospital. Of these, 22,901 (89.4%) had only one admission, with 1887 (7.4%), 462 (1.8%) and 171 (0.7%) having two, three and four overdose admissions respectively. Five patients (0.02%) had 20 or more overdose admissions, with one patient accounting for 62 admissions (Appendix Fig. 2).
Of these, 2555 admissions (8.5%) occurred in patients with the exposure of interest in the six months prior to the index admission (Fig. 1, Appendix Fig. 1), representing 1542 unique individuals. Among these 1542 patients with the exposure of interest (overdose-related ICU admissions in the six months prior to the index admission), 430 (27.9%) had at least one subsequent ICU readmission, 182 (11.2%) patients had two or more ICU readmissions, and 33 (2.1%) had more than five readmissions (Appendix Fig. 2). In comparison, the group without the exposure of interest comprised 25,547 individuals contributing 27,570 ICU admissions. Of these, 1683 (6.6%) had at least one ICU readmission, 273 (1.1%) had two or more readmissions, and no patient had more than five readmissions (Appendix Fig. 2).
Patients with the exposure of interest were younger and more commonly female, Indigenous, admitted from home or a mental health facility, and more commonly transferred to ICU from the emergency department rather than transferred from another hospital. They were more commonly from the highest socioeconomic area and residing in major cities. Compared with patients without exposure of interest, overdose admissions were more commonly attributed to prescription or unknown drugs as opposed to non-prescription drugs. These patients had lower frailty and acute illness severity scores, had less frequently developed delirium while in ICU, less commonly received invasive ventilation or renal replacement therapy. Despite this, they had more readmissions to ICU during the same hospitalisation. Hospital length of stay was shorter, and at discharge they were more commonly discharged to a mental health facility (Table 1).
Table 1.
Comparison of patient episodes with an ICU admission due to an overdose in the prior six months to those with no prior overdose-related ICU admission.
| Admission characteristics | ICU admission due to an overdose in the prior six months |
P Value | |
|---|---|---|---|
| No |
Yes |
||
| N = 27,570 | N = 2555 | ||
| Type of overdose in index admission | <0.001 | ||
| Overdose due to prescription drugs | 10,987 (39.9%) | 1101 (43.1%) | |
| Analgesic | 1396 (5.1%) | 172 (6.7%) | |
| Antidepressant | 2371 (8.6%) | 187 (7.3%) | |
| Sedative (incl antipsychotics, benzodiazepines) | 7220 (26.2%) | 742 (29.0%) | |
| Overdose due to non-prescription drugs | 12,576 (45.6%) | 1008 (39.5%) | |
| Alcohol | 1262 (4.6%) | 95 (3.7%) | |
| Street drugs (incl opiates, amphetamines) | 5428 (19.7%) | 296 (11.6%) | |
| Other (not analgesic, sedative or antidepressant) | 5886 (21.3%) | 617 (24.1%) | |
| Overdose (drug unknown) | 4007 (14.5%) | 446 (17.5%) | |
| Demographics | |||
| Age in years | 39.3 (28.1–52.3) | 35.0 (26.2–45.7) | <0.001 |
| Sex | <0.001 | ||
| Female | 13,820 (50.1%) | 1597 (62.5%) | |
| Male | 13,700 (49.7%) | 953 (37.3%) | |
| Other | 50 (0.2%) | 5 (0.2%) | |
| Pregnancy or post-partum | 98 (0.4%) | 8 (0.3%) | 0.73 |
| Indigenous status | 0.013 | ||
| Not indigenous | 24,761 (89.8%) | 2294 (89.8%) | |
| Aboriginal or Torres Straits Islander | 1800 (6.5%) | 191 (7.5%) | |
| Indigenous status unknown | 1009 (3.7%) | 70 (2.7%) | |
| Socio-economic and remoteness characteristics | |||
| Remoteness category by post code | <0.001 | ||
| Major city | 17,113 (62.1%) | 1664 (65.1%) | |
| Inner regional | 6476 (23.5%) | 537 (21.0%) | |
| Outer regional | 2846 (10.3%) | 296 (11.6%) | |
| Remote | 318 (1.2%) | 18 (0.7%) | |
| Very remote | 166 (0.6%) | 3 (0.1%) | |
| Missing | 651 (2.4%) | 37 (1.4%) | |
| Acute illness severity | |||
| Glasgow coma score | 11.0 (5.0–14.0) | 12.0 (6.0–15.0) | <0.001 |
| Sequential organ failure assessment (SOFA) score | 3.9 (2.6) | 3.5 (2.5) | <0.001 |
| APACHE II score | 14.0 (8.0–19.0) | 12.0 (7.0–18.0) | <0.001 |
| APACHE III/IV score | 42.0 (28.0–65.0) | 37.0 (25.0–58.0) | <0.001 |
| ANZROD (percentage risk), mean (SD) | 1.1 (2.9) | 0.6 (2.0) | <0.001 |
| Therapies provided in the ICU | |||
| Invasive ventilation | 17,145 (62.2%) | 1498 (58.6%) | <0.001 |
| Duration of invasive ventilation (hours) | 15 (8–31) | 16 (9–31) | 0.11 |
| Renal replacement therapy | 719 (2.9%) | 50 (2.2%) | 0.050 |
| Discharge destination (incl. After transfer to another ICU) | <0.001 | ||
| Home | 22,409 (81.3%) | 2074 (81.2%) | |
| Another acute care hospital | 1487 (5.4%) | 111 (4.3%) | |
| Rehabilitation facility | 335 (1.2%) | 14 (0.5%) | |
| Chronic care facility or nursing home | 245 (0.9%) | 15 (0.6%) | |
| Other/unknown | 420 (1.5%) | 47 (1.8%) | |
| Mental health facility | 2674 (9.7%) | 294 (11.5%) | |
APACHE = Acute Physiology and Chronic Health Evaluation, ANZROD = Australian and New Zealand Risk of Death, CFS = Clinical Frailty Scale, ICU = Intensive Care Unit, IRSAD = Index of Relative Social Advantage and Disadvantage.
3.2. Primary outcome: ‘all-cause readmission’
‘All-cause readmission’ to ICU within six months was more common in patients with an overdose-related ICU admission in the six months prior to the index admission, compared to those without exposure (1045/2555 [40.9%] vs. 2460/27,570 [8.9%], p < 0.001) (Table 2, Fig. 1). Patients who were readmitted to the ICU were more commonly female, younger, from higher socioeconomic areas and major cities, admitted from home (as opposed to from another hospital) and transferred to the ICU from the emergency department. They were also more commonly to have been admitted initially following an overdose involving prescription or unknown drugs and had had lower acute illness severity scores at their index admission (Appendix Table 1).
Table 2.
Outcomes of all patient-episodes (Table 2a) and characteristics of the readmission episodes (Table 2b).
| Table 2a |
No prior overdose |
Prior overdose |
P Value |
|---|---|---|---|
| Outcomes in all patient episodes | N = 27,570 | N = 2555 | |
| Primary outcome (‘all-cause readmission’) | |||
| Readmission to ICU within the next 6 months (all causes) | 2460 (8.9%) | 1045 (40.9%) | <0.001 |
| Secondary outcome (‘readmission with overdose’) | |||
| Readmission to ICU within the next 6 months due to overdose | 1639 (5.9%) | 862 (33.7%) | <0.001 |
| Table 2b |
No prior overdose |
Prior overdose |
P Value |
| Outcome details for those readmitted | N = 2460 | N = 1045 | |
| Readmitted to the same ICU at the next hospitalisation | 1148 (46.7%) | 618 (59.1%) | <0.001 |
| Time to next ICU admission (months) | 2.0 (0.7–3.8) | 1.4 (0.3–3.2) | <0.001 |
| Time to next overdose admission (months) | 2.1 (0.7–4.1) | 1.5 (0.4–3.5) | <0.001 |
| Diagnosis category at next ICU admission | <0.001 | ||
| Overdose (total) | 1639 (66.6%) | 862 (82.5%) | |
| Overdose due to prescription drugs | 698 (28.4%) | 356 (34.1%) | |
| Overdose due to non-prescription drugs | 679 (27.6%) | 329 (31.5%) | |
| Overdose (drug unknown) | 262 (10.7%) | 177 (16.9%) | |
| Medical (not overdose) | 630 (25.6%) | 151 (14.4%) | |
| Emergency surgery | 100 (4.1%) | 28 (2.7%) | |
| Elective surgery | 91 (3.7%) | 4 (0.4%) | |
| Outcome of next ICU admission | <0.001 | ||
| Died in the hospital | 68 (2.8%) | 9 (0.9%) | |
| Home | 1882 (76.5%) | 865 (82.8%) | |
| Another acute care hospital (not ICU) | 138 (5.6%) | 39 (3.7%) | |
| Rehabilitation facility | 38 (1.5%) | 8 (0.8%) | |
| Chronic care facility or nursing home | 24 (1.0%) | 4 (0.4%) | |
| ICU In another hospital | 22 (0.9%) | 3 (0.3%) | |
| Other/unknown | 55 (2.2%) | 18 (1.7%) | |
| Mental health facility | 233 (9.5%) | 99 (9.5%) | |
ICU = intensive care unit.
For patients who had had an overdose-related ICU admission in the prior six months, readmissions were more frequently to the same ICU and occurred sooner after discharge from hospital. These patients infrequently died in hospital during the readmission and were more commonly discharged home for the readmission episode (Table 2, Fig. 2).
Fig. 2.
Kaplan–Meier probabilities of all-cause readmission to ICU within six months of discharge.
After adjusting for confounders and accounting for multiple admission episodes in individual patients using a mixed-effects multivariable logistic regression model, a prior overdose-related ICU admission was associated with ‘all-cause readmission’ to ICU (OR = 1.93; 95%CI: 1.59–2.34; p < 0.001, Table 3). Patients who had been transferred from another hospital, received invasive ventilation during their initial ICU admission, or who had had an unknown destination at discharge from the hospital were less likely to be readmitted to the ICU (Table 3). A sensitivity analysis using mixed-effects Cox proportional hazards to model time-to-readmission showed similar findings (Appendix Table 2). In all models, there was no relationship between the likelihood of ‘all-cause readmission’ and the type of overdose (prescription vs. non-prescription), socioeconomic status, patient location (remoteness), source of admission to ICU, duration of mechanical ventilation, ICU length of stay, renal replacement therapy or the presence of discharge to a mental health facility.
Table 3.
Multivariable mixed-effects logistic regression model for associations with all-cause ICU readmission within six months of discharge from hospital.
| Variable | Adjusted odds ratio (95% confidence Interval) | P value |
|---|---|---|
| Prior overdose | ||
| (ICU admission due to overdose in the prior six months) | 1.93 (1.59–2.34) | <0.001 |
| Age in years | 0.992 (0.989–0.995) | <0.001 |
| Female | 1.22 (1.11–1.34) | <0.001 |
| Indigenous status | 0.001 | |
| Not indigenous | Reference category | |
| Aboriginal or Torres Straits Islander | 1.32 (1.11–1.57) | 0.002 |
| Indigenous status unknown | 0.74 (0.57–0.96) | 0.026 |
| Transferred from another hospital | 0.80 (0.70–0.92) | 0.001 |
| Invasive ventilation during index admission | 0.75 (0.68–0.84) | <0.001 |
| Acute illness severity and comorbidities | ||
| SOFA score | 1.03 (1.01–1.05) | <0.001 |
| Chronic respiratory disease | 1.68 (1.29–2.19) | <0.001 |
| Chronic renal disease (dialysis dependence) | 2.06 (1.24–3.44) | 0.005 |
| Chronic liver disease (cirrhosis) | 2.12 (1.38–3.24) | <0.001 |
| Frailty status | ||
| Not frail (CFS 1–4) | Reference category | |
| Frail (CFS 5–8) | 1.40 (1.14–1.72) | 0.001 |
| Frailty status unknown | 1.14 (1.02–1.27) | 0.020 |
| Discharge destination from the index ICU admission | ||
| Home | Reference category | |
| Another acute care hospital (not ICU) | 1.05 (0.86–1.29) | 0.61 |
| Rehabilitation facility | 0.82 (0.52–1.28) | 0.38 |
| Chronic care facility or nursing home | 0.63 (0.36–1.10) | 0.11 |
| Other/unknown | 0.58 (0.39–0.85) | 0.006 |
| Mental health facility | 0.95 (0.82–1.10) | 0.47 |
Area under the receiver operating characteristic = 0.96.
Brier score = 0.070.
SOFA = sequential organ failure assessment score, CFS = clinical frailty scale, ICU = intensive care unit.
3.3. Secondary outcome: ‘Readmission with overdose’
‘Readmission with overdose’ was more common in patients who had had an overdose-related ICU admission in the six months prior to the index admission (862/2555 [33.7%] vs. 1639/27,570 [5.9%]; p < 0.001, Table 2). Among patients who were readmitted with overdose, prescription or unknown drugs were more frequently implicated than non-prescription drugs. Although ‘readmission with an overdose’ was the most common reason for readmission in all patients, medical and surgical admissions were relatively more frequent among patients who had not had a prior overdose.
4. Discussion
4.1. Key findings
This registry-based study of 30,125 ICU overdose episodes over a five-and-a-half-year period found that a prior overdose-related ICU admission within six months was associated with all-cause ICU readmission within the next six months. Time between admissions was shorter in patients with prior overdose-related ICU admissions, and they more frequently had a repeat overdose-related ICU admission. However, there was no difference in the rates of discharge to a mental health facility during these admissions.
4.2. Comparison to previous studies
To our knowledge, the published literature is scarce on the patient population with repeat ICU admissions. A single-centre ICU study found that ≥50% of patients admitted to ICU for drug overdose had prior presentations to hospital for overdoses;11 another found that 17.7% of patients had more than one presentation to ED within the year studied and 7.3% had ≥3 episodes.27 Our study presents one of the largest analyses of overdose-related ICU readmissions, revealing that 8.5% of patients admitted to the ICU with drug overdose had a prior ICU admission for the same reason within six months. Furthermore, these patients were more likely to have an ICU readmission following a repeat overdose.
Though limited, prior research suggests that any overdose increases ICU admission risk, often reflecting the chronic nature of substance use disorders28,29 and underlying psychiatric comorbidities, including psychiatric conditions, limited treatment access, and lack of social support.1,11,22 Our study builds on this by identifying prior overdose as an association with repeat ICU admissions, highlighting the need for targeted strategies to address recurring presentations and identifying potential gaps in mental health care and social supports post-discharge. The absence of an association between socioeconomic status, geographic remoteness, and ICU readmission may reflect that overdose-related critical illness does not follow traditional socioeconomic gradients[30], [31], [32] and may be more prevalent in urban settings. Barriers to care may also relate more to stigma or limited engagement with treatment than to physical or financial access.
4.3. Study implications
Our study findings have important implications for healthcare resource planning and management. Identification of high-risk individuals, particularly younger women, Indigenous Australians, and those with prior overdose-related ICU admissions, can inform targeted risk prediction tools and interventions, including culturally tailored care and proactive follow-up for those with chronic disease or frailty. The high rates and costs of repeat ICU admissions3,12 among a small subset of patients suggest a need for proactive management of this cohort. Our study found that discharge to a mental health facility did not significantly reduce the risk of ICU readmission. Among readmitted patients, those with prior overdoses were not more likely to be discharged to a mental health facility, potentially highlighting a missed opportunity for harm reduction. Although the reasons for this are unclear, and the absence of data regarding overdose intent precludes conclusions about the appropriateness of psychiatric admission, these findings suggest opportunities to strengthen community-based services and better integrate ICU discharge planning with mental health and public health systems to reduce recurrent presentations and improve long-term outcomes. Previous studies have linked repeat overdoses with poor quality of life,33 unemployment, psychiatric history, and prior suicide attempts,27 reinforcing the importance of holistic care. Integrated community-focused interventions and early warning systems have shown promise,[34], [35], [36] and the high complexity of this patient group warrants a coordinated, multi-disciplinary response.
4.4. Strengths and limitations
This study had several key strengths. Firstly, to our knowledge, this study is the largest epidemiological study investigating associations of ICU readmission among patients discharged alive from an ICU due to drug overdose. It serves to characterise this group of patients with prior overdose-related ICU admissions in comparison to those without a recent history. Secondly, the ANZICS-APD is a high-quality database with near-universal coverage of all ICUs in Australia, allowing for an accurate representation of patients. Lastly, considerable effort was made in defining the cohort accurately; patient transfers were differentiated from re-admissions and multiple admissions of the same patient were managed effectively. All these strengthen the validity of the results and provide a high degree of confidence in the generalisability of the findings across the Australian ICU landscape.
However, we acknowledge there are several limitations. First, this is a retrospective study design using the ANZICS-APD registry data, limiting causal inferences. Some associations of readmission (e.g. younger age, absence of mechanical ventilation, lack of interhospital transfer) may appear counterintuitive from an acute severity perspective. However, these variables were included primarily for risk adjustment rather than causal interpretation, to account for baseline differences in readmission propensity when examining the main exposure. Such factors likely act as proxies for non-physiological domains (e.g. discharge thresholds, behavioural or social complexity, post-discharge support) that are not fully captured in administrative data. Second, although ANZICS-APD is known for its high-quality database, there is the possibility of data coding errors, and the effect of these errors cannot be assessed. Third, the ANZICS-APD did not collect data relating to baseline psychiatric diagnoses, classes of drugs, indication of ICU admission, inpatient psychiatry consultations or planned follow-up following discharge. This information can further delineate groups at high risk of ICU readmission. Moreover, the ANZICS-APD does not capture overdose intent, precluding distinction between deliberate self-harm and accidental or recreational overdoses. Consequently, differences in discharges to inpatient mental health facilities are included; however, absence of this should not be interpreted as absence of follow-up care in outpatient or community-based settings. Fourth, patient linkage was performed using SLK-581, a validated privacy-preserving linkage key commonly used in Australian administrative datasets. However, unlike linkage using unique patient identifiers, SLK-581 remains susceptible to linkage errors, which may have resulted in misclassification of some readmission events, particularly across healthcare networks and hospitals.25,26 Fifth, our study was limited to drug overdoses and excluded other forms of suicide attempts, potentially underestimating the broader burden of self-harm behaviours requiring ICU (re)admission. Sixth, although overdose-related ICU admissions have low in-hospital mortality, deaths occurring after discharge among patients not readmitted are not captured in this dataset. Finally, our data are context-specific, and the findings from this dataset cannot be generalised to non-critically ill populations. Similarly, the dataset requires replication in other national registries for validation.
5. Conclusion
This large Australian registry-based study found that patients discharged alive from hospital following an overdose-related ICU admission who have had an overdose-related ICU admission within the prior six months represent a particularly high-risk group for recurrent ICU admission. These findings identify an important opportunity for targeted interventions, improved discharge planning, and integration with mental health, addiction, and community support services to reduce recurrent overdose-related critical illness and subsequent ICU utilisation.
Ethics approval and consent to participate
All experimental protocols were approved by The Alfred Hospital Ethics Committee (Reference: 79/24, Project Title Outcomes of patients with drug overdose requiring intensive care admission in Australia: A retrospective registry-based study, approved January 2024), which approved this study with a waiver of informed consent. The ANZICS Centre for Outcome and Resource Evaluation Management Committee granted access to the ANZICS-APD following standing protocols in March 2024. All methods were carried out following the relevant guidelines and regulations of the Declaration of Helsinki.
Consent for publication
Not applicable.
Data availability statement
The datasets generated and/or analysed during the current study are available on request by a member centre to the Australia New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Committee (anzics@anzics.org), but it is not publicly available.
Author contributions
MW, DP and AS conceived the presented idea. MW researched supporting literature and wrote the article under the supervision of AS. RRL and DP performed the data analyses. PS, MB, DP and AS developed the methodology of the research. All authors discussed the results and contributed to the final manuscript.
Funding
No financial support, including any institutional departmental funds, was used for the study.
Conflicts of interest
RRL acknowledges research support outside of this work from the Clinician Scientist Development Unit, Yong Loo Lin School of Medicine, National University of Singapore.
DP and AS are Associate Editors of Critical Care and Resuscitation.
All other authors declare no competing interests.
Acknowledgements
The authors and the ANZICS CORE management committee would like to thank clinicians, data collectors and researchers at the following centers for their contribution: Albury Wodonga Health ICU, Alfred Hospital ICU, Alice Springs Hospital ICU, Angliss Hospital ICU, Armadale Health Service ICU, Ashford Community Hospital ICU, Austin Hospital ICU, Ballarat Health Services ICU, Bankstown-Lidcombe Hospital ICU, Bathurst Base Hospital ICU, Bendigo Health Care Group ICU, Blacktown Hospital ICU, Bowral Hospital HDU, Box Hill Hospital ICU, Brisbane Private Hospital ICU, Broken Hill Base Hospital & Health Services ICU, Buderim Private Hospital ICU, Bunbury Regional Hospital ICU, Bundaberg Base Hospital ICU, Caboolture Hospital ICU, Cabrini Hospital ICU, Cairns Hospital ICU, Calvary Adelaide Hospital ICU, Calvary Hospital (Lenah Valley) ICU, Calvary Mater Newcastle ICU, Campbelltown Hospital ICU, Canberra Hospital ICU, Casey Hospital ICU, Central Gippsland Health Service (Sale) ICU, Coffs Harbour Health Campus ICU, Concord Hospital (Sydney) ICU, Dandenong Hospital ICU, Dubbo Base Hospital ICU, Echuca Regional Hospital HDU, Epworth Eastern Private Hospital ICU, Epworth Freemasons Hospital ICU, Epworth Geelong ICU, Epworth Hospital (Richmond) ICU, Fairfield Hospital ICU, Fiona Stanley Hospital ICU, Flinders Medical Centre ICU, Flinders Private Hospital ICU, Footscray Hospital ICU, Frankston Hospital ICU, Gold Coast Private Hospital ICU, Gold Coast University Hospital ICU, Gosford Hospital ICU, Goulburn Base Hospital ICU, Goulburn Valley Health ICU, Grafton Base Hospital ICU, Grampians Health Horsham ICU, Greenslopes Private Hospital ICU, Griffith Base Hospital ICU, Hervey Bay Hospital ICU, Hollywood Private Hospital ICU, Holmesglen Private Hospital ICU, Hornsby Ku-ring-gai Hospital ICU, Ipswich Hospital ICU, John Fawkner Hospital ICU, John Flynn Private Hospital ICU, John Hunter Hospital ICU, Joondalup Health Campus ICU, Kareena Private Hospital ICU, Knox Private Hospital ICU, Latrobe Regional Hospital ICU, Launceston General Hospital ICU, Lingard Private Hospital ICU, Lismore Base Hospital ICU, Liverpool Hospital ICU, Logan Hospital ICU, Lyell McEwin Hospital ICU, Mackay Base Hospital ICU, Macquarie University Private Hospital ICU, Maitland Hospital ICU, Maitland Private Hospital ICU, Manly Hospital & Community Health ICU, Manning Rural Referral Hospital ICU, Maroondah Hospital ICU, Mater Adults Hospital (Brisbane) ICU, Mater Private Hospital (Brisbane) ICU, Mater Private Hospital (Sydney) ICU, Mater Private Hospital (Townsville) ICU, Mildura Base Public Hospital ICU, Monash Medical Centre–Clayton Campus ICU, Mount Isa Hospital ICU, Mulgrave Private Hospital ICU, National Capital Private Hospital ICU, Nepean Hospital ICU, Newcastle Private Hospital ICU, Noosa Hospital ICU, North Canberra Hospital ICU, North Shore Private Hospital ICU, North West Regional Hospital (Burnie) ICU, Northeast Health Wangaratta ICU, Northern Beaches Hospital ICU, Norwest Private Hospital ICU, Orange Base Hospital ICU, Peninsula Private Hospital ICU, Pindara Private Hospital ICU, Port Macquarie Base Hospital ICU, Prince of Wales Hospital (Sydney) ICU, Prince of Wales Private Hospital (Sydney) ICU, Princess Alexandra Hospital ICU, Queen Elizabeth II Jubilee Hospital ICU, Redcliffe Hospital ICU, Robina Hospital ICU, Rockhampton Hospital ICU, Rockingham General Hospital ICU, Royal Adelaide Hospital ICU, Royal Brisbane and Women's Hospital ICU, Royal Darwin Hospital ICU, Royal Hobart Hospital ICU, Royal Melbourne Hospital ICU, Royal North Shore Hospital ICU, Royal Perth Hospital ICU, Royal Prince Alfred Hospital ICU, Ryde Hospital and Community Health Services ICU, Shoalhaven Hospital ICU, Sir Charles Gairdner Hospital ICU, South East Regional Hospital ICU, South West Healthcare (Warrnambool) ICU, St Andrew's Hospital (Adelaide) ICU, St Andrew's Hospital Toowoomba ICU, St Andrew's Private Hospital (Ipswich) ICU, St Andrew's War Memorial Hospital ICU, St George Hospital (Sydney) ICU, St George Private Hospital (Sydney) ICU, St John Of God Health Care (Subiaco) ICU, St John Of God Hospital (Ballarat) ICU, St John of God Hospital (Bendigo) ICU, St John of God Hospital (Berwick) ICU, St John Of God Hospital (Geelong) ICU, St John Of God Hospital (Murdoch) ICU, St John of God Midland Public & Private ICU, St Vincent's Private Hospital Northside ICU, St Vincent's Hospital (Melbourne) ICU, St Vincent's Hospital (Sydney) ICU, St Vincent's Hospital (Toowoomba) ICU, St Vincent's Private Hospital (Sydney) ICU, Sunnybank Hospital ICU, Sunshine Coast University Hospital ICU, Sunshine Coast University Private Hospital ICU, Sunshine Hospital ICU, Sutherland Hospital & Community Health Services ICU, Sydney Adventist Hospital ICU, Sydney Southwest Private Hospital ICU, Tamworth Base Hospital ICU, The Bays Hospital ICU, The Chris O'Brien Lifehouse ICU, The Memorial Hospital (Adelaide) ICU, The Northern Hospital ICU, The Prince Charles Hospital ICU, The Queen Elizabeth (Adelaide) ICU, The Wesley Hospital ICU, Toowoomba Hospital ICU, Townsville University Hospital ICU, Tweed Valley Hospital ICU, University Hospital Geelong ICU, Wagga Wagga Base Hospital & District Health ICU, Warringal Private Hospital ICU, Werribee Mercy Hospital ICU, Western District Health Service (Hamilton) ICU, Westmead Hospital ICU, Westmead Private Hospital ICU, Wollongong Hospital ICU, Wollongong Private Hospital ICU, and Wyong Hospital ICU.
Footnotes
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccrj.2026.100208.
Contributor Information
Michael Wu, Email: michaelywu92@gmail.com.
Ashwin Subramaniam, Email: ashwin.subramaniam@monash.edu.
Supplementary data
The following is the Supplementary data to this article:
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The datasets generated and/or analysed during the current study are available on request by a member centre to the Australia New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Committee (anzics@anzics.org), but it is not publicly available.