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Mortality differences between ICUs that are regarded as 'in control': a longitudinal register-based study in the
Fabian Termorshuizen1,2,3, Sylvia Brinkman4,2,3, Sesmu M Arbous4,5,6
1National Intensive Care Evaluation (NICE) Foundation, Amsterdam, The Netherlands F.Termorshuizen@amsterdamumc.nl.
BMJ Open
|April 20, 2026
Summary
Standardized mortality ratios (SMRs) within control limits may mask significant intensive care unit (ICU) mortality differences. A longitudinal analysis revealed persistent excess mortality risks in ICUs not flagged as outliers, emphasizing the need for comprehensive performance assessment.
Area of Science:
- Critical care medicine
- Health services research
- Biostatistics
Background:
- Funnel plots are standard for identifying intensive care units (ICUs) with high risk-adjusted mortality.
- ICUs within control limits (e.g., 99.8% confidence limits) are typically considered 'in control', potentially overlooking persistent mortality issues.
- Random fluctuations may obscure true differences in mortality rates over time between ICUs.
Purpose of the Study:
- To investigate whether statistically significant and clinically relevant differences in mortality exist over time among ICUs considered 'in control' using funnel plots.
- To assess the limitations of traditional funnel plot analysis in detecting persistent excess mortality.
- To propose improved methods for evaluating ICU mortality performance.
Main Methods:
- A longitudinal, register-based study design.
- Inclusion of 88 ICUs in the Netherlands from 2013-2023, analyzing 759,498 admissions.
- Hospital death was analyzed using multivariable logistic regression with random intercepts, translating variance to the median odds ratio (MOR).
Main Results:
- The median odds ratio (MOR) increased from 1.12 for ICUs within the 95% confidence limits to 1.20 for those within the 99.8% confidence limits.
- Analysis of 877 ICU-calendar year combinations indicated persistent excess mortality risks not identified by standard outlier detection.
- Similar findings were observed across separate calendar years and diagnostic groups, confirming the robustness of the results.
Conclusions:
- Mortality differences exist between ICUs even when they are not flagged as outliers by standard funnel plots.
- Current assessment methods may fail to capture persistent patterns of excess risk.
- Integrating cross-sectional funnel plots, MOR, and longitudinal SMR trends is recommended for a more thorough evaluation of ICU mortality performance.
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