Rethinking the Epidemiology of Critical Illness: Beyond ICUs and Diagnoses

How many patients in your hospital are critically ill right now?
It’s a deceptively simple question — and until recently, there was no clear answer.

Traditionally, epidemiological data on critical illness has been derived from ICU admissions or diagnosis-specific registries (sepsis, trauma, pneumonia, etc.). But both of these approaches miss the bigger picture. Most critically ill patients never reach an ICU, and critical illness cuts across diagnostic categories. The result: our health systems lack a true understanding of the scale of critical illness, and policy-makers have little evidence on which to base priority setting.

Schell’s Hospital Burden of Critical Illness across Global Settings study sought to change this. His thesis developed and tested a new method for estimating the burden of critical illness — not through diagnoses or ICU beds, but through physiology.

A novel, physiology-based approach

Schell and colleagues defined critical illness as the presence of one or more danger vital signs — severely abnormal measures of respiratory rate, oxygen saturation, pulse, blood pressure, or consciousness level. These markers were chosen because they are:

• Universally measurable, even in low-resource settings

• Directly linked to vital organ dysfunction

• Strongly associated with mortality

• Immediately actionable in clinical care

This operational definition was applied in prospective point-prevalence and cohort studies across Malawi, Sri Lanka, and Sweden, capturing patients in general wards, high-dependency units, and ICUs alike.

What the data revealed

The findings are striking:

• 12% of all hospital in-patients were critically ill on any given day. That’s one in eight patients.

• In Sweden, the prevalence translated to 19 critically ill adults per 100,000 population.

• Mortality was high: 19% of critically ill patients died in hospital.

• Crucially, >90% of these patients were cared for in general wards, not ICUs.

These numbers challenge conventional assumptions. They suggest that critical illness is both more common and more widely distributed across hospitals than previously recognised.

Why this matters

From a clinical perspective, this physiology-based method gives frontline teams a feasible way to identify and track critical illness across specialties and wards. From a research perspective, it creates a foundation for comparable epidemiology — moving beyond diagnosis-bound studies that underestimate burden.

For policy-makers, the implications are even broader:

• Critical illness is common. It should be a recognised health priority.

• It is deadly. Nearly one in five patients with danger signs do not survive.

• It is system-wide. Interventions cannot be confined to ICUs — they must address care in general wards, where most critically ill patients are found.

Towards better data, better policy

Epidemiology drives priorities. The Global Burden of Disease project has shaped decades of investment, yet its diagnosis-based framework fails to capture severity of illness. By demonstrating a feasible method to measure critical illness directly, Schell’s work offers a pathway to reframe the need for critical care within global health metrics.

The message is clear: if we want to reduce preventable deaths in hospitals, we need to stop equating critical illness with intensive care, and start recognising it as a horizontal challenge across diagnoses and settings.

For clinicians, this means embedding vital signs monitoring and response into everyday practice. For researchers, it means refining and expanding epidemiological studies of critical illness. For health systems, it means adopting Essential Emergency and Critical Care (EECC) as a baseline, and ensuring it is provided across all wards and specialities — because without EECC, critically ill patients will keep dying unnecessarily and unseen.