The Glasgow Coma Scale (GCS) is intended to be an objective, reliable measure of a patient's mental status. It is included as a metric for trauma registries, having implications for performance metrics and research. Our study compared the GCS recorded in the trauma registry (GCS-1) with that recorded in the neurosurgery consultation (GCS-2). This retrospective review compared GCS-1 with GCS-2. The Trauma Injury Severity Score (TRISS) method was used to calculate probability of survival (POS) for patients using both GCS-1 and GCS-2. GCS-1 score significantly differed from GCS-2 score (6.69 vs. 7.84, ± 2.553; P < 0.001). There were 172 patients (37.55%) with a GCS-1 score of 3 and 87 (19.00%) with a GCS-2 score of 3 (χ2P < 0.001). The POS calculated using TRISS methodology with GCS-1 (POS-1) was 74.7% ± 26.6% compared with GCS-2 (POS-2), which was 79.3% ± 24.4%. There was a statistically significant difference in the means of POS-2 and POS-2 (P < 0.001). The actual observed survival for the cohort was 71.0% (325/458). The immediate GCS score recorded on patient arrival after trauma differs significantly from the GCS score recorded at later times. This finding significantly altered the probability of survival as calculated by the TRISS methodology. This situation could have profound effects on risk-adjusted benchmarking, assessments of quality of care, and injury severity stratification for research. More studies into the optimal timing of GCS score recording or changes in GCS score and their impact on survival are warranted.