Highlights
In more than 615,000 ICU admission episodes from Australia and New Zealand, frailty was common, affecting 19% of patients, while 4.1% required ICU readmission during the same hospitalization.
Frailty increased the likelihood of both ICU readmission and in-hospital death without readmission, underscoring its importance as a marker of vulnerability after critical illness.
ICU readmission was associated with a large increase in 60-day hospital mortality in both frail and nonfrail patients. The standardized absolute risk increase was nearly identical in the two groups: 14.6% in frail patients and 14.9% in nonfrail patients.
Although frailty did not appear to modify the incremental mortality effect of ICU readmission, frail patients who were readmitted had the worst observed outcomes overall, with a 22.7% 60-day hospital mortality.
Background
Frailty has become an increasingly important construct in critical care. It captures diminished physiologic reserve and increased vulnerability to stressors, and it often predicts poorer recovery after acute illness. Across ICU populations, frailty has been associated with higher mortality, greater functional decline, longer hospital stay, and greater likelihood of discharge to institutions rather than home. At the same time, ICU readmission remains a clinically significant event because it frequently signals unresolved organ dysfunction, premature transfer, new complications, or a mismatch between ward resources and patient needs.
Both frailty and ICU readmission are individually linked to adverse outcomes, but their joint relationship has been less well characterized. Clinicians commonly assume that readmission might be especially harmful in frail patients because frailty may amplify the consequences of recurrent physiologic deterioration. Yet this is not a trivial question methodologically. Readmission is a time-dependent event, and patients must survive long enough to experience it. Analyses that ignore timing can overestimate or misclassify risk. The present study is therefore notable not only because of its scale, but also because it uses time-dependent modeling and competing-risk methods to address a clinically relevant question more rigorously: does frailty modify the association between ICU readmission and 60-day hospital mortality?
Study Design
Design and data source
This was a retrospective registry-based cohort study using the Australian and New Zealand Intensive Care Society Adult Patient Database, one of the world’s largest high-quality critical care registries.
Setting and population
The investigators included all adult patients aged 18 years or older admitted to 203 ICUs in Australia and New Zealand between January 2017 and December 2022 who had a documented Clinical Frailty Scale score. Frailty was defined as a Clinical Frailty Scale score of 5 or greater, consistent with at least mild frailty.
The analysis comprised 615,719 ICU admission episodes. Of these, 115,453 patients, or 19%, were classified as frail.
Exposure and comparison
The principal exposure of interest was ICU readmission during the same hospitalization. Readmission occurred in 25,329 episodes, representing 4.1% of the cohort. The main comparison was between patients with and without frailty, while also evaluating whether frailty modified the mortality association of readmission.
Outcome measures
The primary outcome was 60-day hospital mortality. This outcome focuses on death in the hospital within 60 days of the index ICU admission. Secondary outcomes included hospital length of stay and discharge destination.
Statistical approach
The analytic strategy is one of the strengths of the study. The authors used a Cox proportional hazards model in which time to ICU readmission was treated as a time-dependent covariate. This matters because a patient is not “readmitted” at baseline; they become readmitted only if and when the event occurs. Modeling readmission as time-dependent reduces immortal time bias and aligns exposure status with clinical reality.
An interaction term between frailty state and readmission was included to test whether frailty altered the effect of readmission on mortality. To estimate absolute risk differences rather than only relative hazards, the investigators used regression standardization, with 95% confidence intervals derived by nonparametric bootstrap. They also performed a competing-risk analysis in which in-hospital death without ICU readmission was treated as a competing event.
Key Findings
Frailty was associated with greater vulnerability before considering readmission
Frailty was associated with an increased risk of ICU readmission, with a subdistribution hazard ratio of 1.10 and a 95% confidence interval of 1.07 to 1.14. Although the relative increase was modest, this finding supports the idea that frailty identifies patients at higher risk of post-ICU instability.
The larger signal was seen for death without ICU readmission. Frail patients had a subdistribution hazard ratio of 2.83 for in-hospital death without readmission, with a 95% confidence interval of 2.72 to 2.94. This suggests that among frail patients, poor outcomes after ICU discharge are not limited to those who re-enter intensive care; many may die before readmission occurs or may not be considered candidates for readmission, depending on goals of care and illness trajectory.
Observed mortality was highest in frail patients who were readmitted
By day 60, 16,353 patients, or 2.7% of the total cohort, had died in the hospital. The most clinically striking descriptive result was that frail patients who required ICU readmission had the highest observed 60-day mortality, reaching 22.7%.
This finding is important for bedside clinicians. It confirms that the combination of baseline vulnerability and recurrent critical illness identifies a particularly high-risk subgroup. In practical terms, a frail patient returning to the ICU should immediately trigger heightened prognostic awareness, multidisciplinary reassessment, and close attention to treatment goals, rehabilitation potential, and family communication.
The incremental mortality impact of ICU readmission was similar regardless of frailty
The central analytic result of the study is that frailty did not meaningfully modify the absolute mortality effect associated with ICU readmission. After standardization, the increase in 60-day mortality associated with readmission was 14.6% in frail patients, with a 95% confidence interval of 13.7% to 15.6%, compared with 14.9% in nonfrail patients, with a 95% confidence interval of 13.4% to 16.6%.
Clinically, this means two things can be true at once. First, frail readmitted patients have the worst overall prognosis because their baseline risk is already high. Second, the additional absolute mortality burden associated with readmission itself appears to be of similar magnitude in frail and nonfrail patients. In other words, readmission is a bad prognostic event for everyone, not uniquely or disproportionately for the frail.
This is a nuanced but useful distinction. If one focuses only on crude mortality among readmitted patients, it is easy to conclude that frailty magnifies the harm of readmission. However, when outcomes are interpreted relative to comparable nonreadmitted patients within each frailty stratum, the excess mortality associated with readmission is remarkably similar.
Implications of the competing-risk findings
The competing-risk analysis adds depth to interpretation. Frail patients were more likely both to be readmitted and to die without readmission. This pattern highlights an underappreciated issue in post-ICU outcome studies: not all deterioration culminates in ICU readmission. Some patients may deteriorate too rapidly, may have treatment limitations, or may be managed outside the ICU because readmission is judged nonbeneficial. Thus, readmission rates alone can be an incomplete quality signal, especially in older or frail populations.
For health systems, this means that using ICU readmission as a standalone performance metric may be misleading unless paired with mortality, goals-of-care context, and ward-based deterioration data.
Clinical Interpretation
What should intensivists and hospital teams take from this study?
First, frailty should remain a routine part of ICU risk assessment. The Clinical Frailty Scale is simple, widely used, and clinically informative. In this cohort, frailty identified patients with greater risk of readmission and much greater risk of in-hospital death without readmission.
Second, an ICU readmission should be regarded as a major adverse prognostic event irrespective of frailty status. The absolute increase in 60-day mortality associated with readmission was approximately 15% in both groups. This is a substantial signal, one that likely reflects residual illness severity, post-ICU complications, discharge timing, and transitions-of-care failures.
Third, the findings support a broad rather than narrow prevention strategy. If readmission carries similar incremental risk in frail and nonfrail patients, interventions designed to reduce readmission should not target only frail individuals. Instead, hospitals may need layered approaches: universal discharge readiness practices for all ICU survivors, plus enhanced support for those with frailty because their baseline event rates are higher.
Potential practice applications
These results can inform several areas of practice. ICU discharge decisions may benefit from more structured assessment of physiologic stability, delirium, mobility, nutrition, and nursing requirements. Patients with frailty may warrant additional ward monitoring, early geriatric or rehabilitation input, and clearer advance-care planning discussions. For all patients, rapid response system vigilance after ICU discharge remains essential.
The study also has implications for prognostic conversations. Families of readmitted patients, especially those with frailty, should be informed that readmission signals a markedly increased short-term risk of death. At the same time, because the incremental effect of readmission was similar regardless of frailty, clinicians should avoid framing readmission in frail patients as uniquely futile solely on the basis of frailty status.
Strengths and Limitations
Major strengths
The most obvious strength is scale. More than 615,000 ICU admission episodes across 203 ICUs provide exceptional statistical power and broad representation of real-world practice in two healthcare systems.
The second major strength is methodological. Treating readmission as a time-dependent covariate is the correct analytic choice for this type of question and improves credibility compared with simpler fixed-exposure models. The use of regression standardization and bootstrap confidence intervals also makes the results easier to interpret clinically because absolute risk differences are directly relevant to decision-making.
The competing-risk analysis is another important asset. It acknowledges that death without readmission can preclude the occurrence of readmission and is particularly relevant in frail populations.
Important limitations
As with all retrospective registry studies, residual confounding remains possible. The study can demonstrate association, not causation. Readmission is likely a marker of complex underlying illness severity and discharge context, and not necessarily a direct cause of mortality.
The analysis was limited to patients with a documented Clinical Frailty Scale, which raises the possibility of selection effects if frailty scoring was not uniform across sites or patient groups. In addition, frailty was dichotomized using a threshold of 5 or greater, which is clinically reasonable but may obscure gradients of risk across the frailty spectrum.
The abstract does not detail the covariates included in the final models, nor does it provide subgroup analyses by diagnostic category, elective versus emergency admission, or treatment limitation status. These factors could influence both readmission and mortality. Similarly, “60-day hospital mortality” does not capture deaths occurring after discharge, functional status, cognitive outcomes, or health-related quality of life, all of which are highly relevant in frail ICU survivors.
Finally, the meaning of ICU readmission is context dependent. Readmission rates are shaped by ICU bed availability, ward monitoring capability, local discharge practices, and institutional thresholds for readmission. Therefore, generalizability outside Australia and New Zealand should be thoughtful rather than automatic.
Relation to Existing Literature
The study aligns with prior critical care literature showing that frailty is associated with worse short- and long-term outcomes. International guideline groups and major observational studies have increasingly supported routine frailty assessment in ICU populations. The Clinical Frailty Scale, developed by Rockwood and colleagues, has become one of the most practical bedside tools for this purpose. Earlier multicenter work has also shown that frailty predicts mortality and dependence after critical illness.
ICU readmission has long been recognized as an adverse marker, but prior studies have often been smaller, single-center, or methodologically limited by inadequate handling of exposure timing. The present analysis advances the field by integrating frailty and readmission within a contemporary, large-scale registry framework and by emphasizing absolute risk differences, which are often more clinically actionable than relative measures alone.
One broader lesson emerging from this and related work is that post-ICU outcomes should be viewed through two lenses: baseline vulnerability and trajectory after discharge. Frailty captures the first. Readmission captures the second. Patients do poorly when either is present, and worst when both are present, but the additional risk imposed by trajectory failure appears substantial across the board.
Research and Policy Implications
Several next steps follow logically from these data. Future studies should determine which components of ICU discharge readiness and ward follow-up most strongly mediate readmission risk. It would be particularly useful to separate potentially preventable readmissions from those that reflect unavoidable disease progression.
Research should also move beyond mortality alone. In frail survivors of critical illness, outcomes such as functional decline, new institutionalization, symptom burden, and caregiver impact may be equally or more important. Prospective studies incorporating treatment goals, frailty trajectories, and patient-reported outcomes would deepen the clinical meaning of these observations.
At a policy level, the findings caution against simplistic benchmarking based only on ICU readmission rates. Because frail patients are also at increased risk of dying without readmission, institutions with lower readmission rates are not necessarily delivering better care. Composite metrics that incorporate mortality, case mix, treatment limitations, and post-ICU support may offer a fairer assessment of quality.
Conclusion
This large binational registry study provides a clinically important clarification. Frailty identifies ICU patients at higher baseline risk for deterioration and death, and frail patients who are readmitted have the highest observed 60-day hospital mortality. However, the additional absolute mortality burden associated with ICU readmission is essentially the same in frail and nonfrail patients, at about 15%.
For clinicians, the message is straightforward: ICU readmission is a serious warning sign for all patients, while frailty remains a crucial determinant of overall prognosis. The practical response should combine universal efforts to improve ICU discharge safety with targeted post-ICU support for frail patients, whose underlying vulnerability leaves them with the least margin for error.
Funding and ClinicalTrials.gov
The abstract does not report a specific funding source. No ClinicalTrials.gov registration number is listed, which is expected for a retrospective registry-based observational study.
Citation
Walker HGM, Vo TK, Santamaria J, Serpa Neto A, Subramaniam A, Brown AJ, Australian and New Zealand Intensive Care Society Center for Outcome and Resource Evaluation (ANZICS CORE) Committee. The Effect of Readmission to the ICU on 60-Day Hospital Mortality in Patients With and Without Frailty: A Binational Registry-Based Study. Critical Care Medicine. 2026 May 19. PMID: 42153779. Available at: https://pubmed.ncbi.nlm.nih.gov/42153779/
Selected References
Rockwood K, Song X, MacKnight C, et al. A global clinical measure of fitness and frailty in elderly people. CMAJ. 2005;173(5):489-495.
Bagshaw SM, Stelfox HT, McDermid RC, et al. Association between frailty and short- and long-term outcomes among critically ill patients: a multicentre prospective cohort study. CMAJ. 2014;186(2):E95-E102.
Muscedere J, Waters B, Varambally A, et al. The impact of frailty on intensive care unit outcomes: a systematic review and meta-analysis. Intensive Care Medicine. 2017;43(8):1105-1122.
Pilcher DV, Duke GJ, George C, et al. Assessment of a novel marker of ICU discharge, the readmission rate. Critical Care and Resuscitation. 2013;15(1):13-19.
Guidet B, de Lange DW, Boumendil A, et al. The contribution of frailty, cognition, activity of daily life and comorbidities on outcome in acutely admitted patients over 80 years in European ICUs: the VIP2 study. Intensive Care Medicine. 2020;46(1):57-69.
