The primary reason for admission to the critical care unit will be recorded using the ICNARC Coding Method, a hierarchical coding method specifically designed for recording reasons for admission to critical care. For descriptive tables, primary reason for admission will be categorised according to the body system (respiratory, cardiovascular, gastrointestinal, neurological, genitourinary, endocrine or musculoskeletal/dermatological). For modelling, primary reason for admission will be categorised according to body system and pathological/physiological process (e.g. respiratory infection), with categories containing fewer than 100 admissions combined at the body system level.