Clarifying the Concept of "Unanticipated Transfer to the ICU": A Scoping Review

Unanticipated transfer to the intensive care unit (ICU), typically defined as an unscheduled escalation of care from inpatient general wards, is widely used in clinical research as a marker of adverse events, clinical deterioration, and system failure. However, its conceptual meaning, definition, and operationalization vary substantially across the literature, complicating interpretation and cross-study comparison. We conducted a scoping review following PRISMA-ScR guidelines to map how unanticipated ICU transfers are defined, framed, and used in empirical research. Searches of PubMed, CINAHL, and Web of Science identified 4,019 studies through August 7, 2025. After removing duplicates, 2,092 studies were screened for title and abstract, 565 underwent full-text review, and 219 studies met the inclusion criteria. Most studies were observational studies and terminology varied widely, with "unplanned admission to ICU" being the most common. Definitions were heterogeneous but generally focused on unscheduled ICU admission from inpatient wards, often excluding transfers from the emergency department or operating room and sometimes incorporating triggers such as rapid response activation or time-based thresholds. Unanticipated ICU transfer was predominantly used as a dependent variable and was overwhelmingly framed as a negative outcome or proxy for clinical deterioration. However, a smaller body of literature conceptualized these transfers as appropriate or beneficial escalations of care, particularly when occurring early in the course of deterioration. Our findings highlight definitional and conceptual inconsistency and suggest that unanticipated ICU transfer is a context-dependent process measure influenced by both patient condition and institutional factors. Greater standardization and a reframing toward appropriateness and timing of escalation may improve its utility in research and practice.