Urine-to-Serum Osmolality Ratio as a Prognostic Marker in Traumatic Brain Injury

Background/Objectives: Prognostication in traumatic brain injury (TBI) remains challenging. The urine-to-serum osmolality (U/S) ratio may reflect hypothalamic–pituitary axis integrity, a critical but underexplored prognostic domain. We investigated whether the U/S ratio provides early prognostic value and enhances prediction when combined with conventional severity markers. Methods: This retrospective study included 128 adult TBI patients admitted to a neurosurgical intensive care unit (ICU) with simultaneous osmolality measurements within 6 h of admission. The primary outcome was ICU mortality; the secondary outcome was poor neurological outcomes (Glasgow Outcome Scale 1–3). Results: ICU mortality was 14.1% (18/128), and poor neurological outcome occurred in 41.8% (46/110). Non-survivors had significantly lower U/S ratios than survivors (1.09 ± 0.58 vs. 1.70 ± 0.68, p < 0.001). For ICU mortality, U/S ratios (AUC = 0.803) showed similar discriminative ability to GCS (AUC = 0.806). For poor neurological outcomes, the U/S ratio (AUC = 0.768) significantly outperformed both GCS (AUC = 0.641, p = 0.038) and the Acute Physiology and Chronic Health Evaluation (APACHE) II score (AUC = 0.553, p < 0.001). Combining the U/S ratio with GCS improved mortality prediction (AUC = 0.890), as did combinations with the APACHE II score (AUC = 0.847). The U/S ratio remained independently associated with ICU mortality and poor neurological outcomes after adjusting for GCS or APACHE II scores. Quartile analyses revealed a dose–response relationship, with ICU mortality of 34.4% in Q1 versus 3.1% in Q4 (p for trend < 0.001). Prognostic value was preserved in patients receiving osmotic therapy (n = 86). Conclusions: The U/S ratio is a simple, readily available biomarker that independently predicts mortality and poor neurological outcomes in TBI patients. Particularly for neurological outcome predictions, it outperforms GCS or the APACHE II score alone. Combined with established severity scores, it may serve as a practical bedside tool reflecting hypothalamic–pituitary function in neurocritical care.