Background <p>Overcrowding in emergency departments (EDs) due to injured patients who do not need urgent treatment can lead to less efficacious healthcare outcomes for those that do. The trauma referral audit (TRA) was developed to support medical staff and ensure that the injured receive treatment timely and efficiently.</p> Methods <p>Data on 14,399 injured patients referred to Maharaj Nakorn Chiang Mai Hospital between 2007 and 2017 were analyzed to examine the impact of the full TRA program implemented at the end of 2010. Injury severity was measured by using the injury severity score (ISS) whereby a score &gt; 9 points infers severe injury. The impact of the establishment of TRA was examined using the trend of referrals and an interrupted time-series analysis of monthly mortality among patients with severe injury.</p> Results <p>The median ISS of the patients in 2010 was 9 [4–18] and slightly increased to the highest score of 16 [8–25] in 2017. The proportion of patients with less severe injuries (ISS ≤ 9) decreased after the full implementation of TRA (55.4% in 2010 compared to 42.3% in 2017). Overall mortality was 6% (5.5% vs. 6.2% for the pre- and post-full TRA periods, respectively), and the mortality rate tended to increase from 4.77% in 2011 to 7.59 in 2017. The monthly mortality rate was estimated at 7.22% [95% confidence interval (CI) = 5.89–8.56%] with a significant increase of mortality in the post-full TRA period by 1.57% [0.16-2.98%] and in the annual trend of 0.11% [0.05-0.16%]. However, when considering severely injured patients only, the level and trend of the mortality rate were no different.</p> Conclusions <p>Although the TRA program could help reduce patient overcrowding in EDs, it does not reduce the risk of mortality. Revision of the referral and in-hospital care guidelines accounting for these relevant factors might lead to a decrease in mortality.</p> Trial registration <p>Clinical trial number: Not applicable.</p>