**2. Materials and Methods**

#### *2.1. Study Setting and Participants*

Oslo University Hospital (OUH) is the only Level 1 trauma center with neurosurgical services in the southeastern region of Norway, serving 3.0 million inhabitants. OUH also serves as the primary trauma referral hospital for Oslo residents (population ≈ 700,000). Trauma care in Norway is organized through public hospitals with an equal access policy and is free of charge. In 2007, OUH established early specialized rehabilitation and a continuous chain of treatment for severe TBI.

Data were retrieved from the Oslo TBI Registry—Neurosurgery, a quality control database maintained by the neurosurgical department at OUH since 2015. The registration is prospective; data are derived manually from electronic medical records and stored in a Medinsight database [18]. The inclusion criteria for the Oslo TBI Registry—Neurosurgery were (i) traumatic brain injury; (ii) cerebral CT/CTA or cerebral MRI/MRA with findings of acute trauma (hemorrhage, fracture, traumatic axonal injury, vascular injury); (iii) admission to OUH within seven days postinjury; and (iv) Norwegian social security number. A more thorough description of patients in this registry has been published previously [19]. For this study, we included adult patients (age ≥18 years) who were residents of the southeastern region, admitted to OUH between 1 January 2015 and 31 December 2019, and discharged alive from the acute care units at OUH.

#### *2.2. Endpoint*

The endpoint in this study is discharge destination from the acute care units at OUH. Acute care units are defined as ICUs and surgical wards. Discharge destinations are categorized as home, specialized rehabilitation, local hospital, general rehabilitation, nursing home and other. The endpoint variable was binary: direct transfer to rehabilitation or not. Only patients discharged to specialized inpatient rehabilitation were categorized as "yes". All other discharge places are considered as "no". Information regarding inpatient rehabilitation at later stages of TBI was not available, thus not addressed in this study.
