*2.3. Independent Variables*

## 2.3.1. Demographics and Preinjury Comorbidity

Age (stratified into 18–29 years, 30–49 years, 50–64 years, 65–79 years, 80+ years), sex, living status at the time of injury (home independent, home with assistance, or institution (e.g., nursing home)), preinjury comorbidity (as classified by the American Society of Anesthesiologists Physical Status Classification System (ASA-PS)) [20], and any preinjury substance dependence (including alcohol and/or drugs).

#### 2.3.2. Injury Characteristics

Injury characteristics included the trauma mechanism (classified as falls, road traffic accidents (RTAs), other), whether high-energy trauma (defined as falls from a height ≥3 m, RTAs, or other high-energy accidents) was involved, the Glasgow Coma Scale score (GCS) (utilized lowest score documented prior to intubation or admission OUH), TBI severity according to the Head Injury Severity Score (HISS) (minimal: GCS 15 and no loss of consciousness or amnesia; mild: GCS 14 or 15 plus amnesia, or brief loss of consciousness (<5 min), or impaired alertness or memory; moderate: GCS 9–13 or loss of consciousness ≥5 min or focal neurological deficit; or severe: GCS ≤ 8) [21,22]. Computed tomography (CT) findings (primary CT head scan performed at OUH) and magnetic resonance imaging (MRI) (signs of traumatic axonal injury (TAI)) results were also collected. Minimal and mild TBI with traumatic findings on CT is referred to as complicated mild TBI [23,24].

## 2.3.3. Acute Treatment

Acute treatment involved the following: insertion of intracranial pressure (ICP) sensors and neurosurgical procedures including evacuation of the mass lesion (hematoma/hemorrhage), cerebrospinal fluid drainage, decompressive hemicraniectomy, repair of the dura or fractured skull (duraplasty/cranioplasty) and vascular surgery. Admission to the intensive care unit (ICU) included all patients admitted to the ICU, whereas uncomplicated short stays (<24 h) for TBI observation in the intermediate/step-down unit were registered as ward admissions. Calculation of length of stay (LOS) and days on ventilator were based on dates, with each date counted as a full day.
