Objective:To investigate the role of blood ammonia in the evaluation of the prognosis of septic patients in the emergency department and to compare its value with mortality in emergency department sepsis (MEDS) score.Methods:A retrospective clinical study was conducted to septic patients who were diagnosed in the Emergency Department of West China Hospital of Sichuan University from June 2017 to May 2018, and met the diagnostic criteria established by the diagnostic criteria of the American College of Chest Physicians/Society of Critical Care Medicine in 2001. The subjects who had other diseases that affected blood ammonia level and were lost to follow-up were excluded. MEDS scores were collected, and the survival status of patients was followed up by telephone. The independent samples t test was used to compare the differences between the two groups, receiver operating characteristic (ROC) curve was used to assess the accuracy of the prediction of sepsis mortality, and the logistic regression model was used to explore the value of the combined use of blood ammonia and MEDS score.Results:Eighty subjects were finally included in the study and divided into the 1-week survival group ( n=52), 1-week death group ( n=28); 4-week survival group ( n=37), 4-week death group ( n=43); 12-week survival group ( n=33), 12-week death group ( n=47); 1-year survival group ( n=32), and 1-year death group ( n=48). There was no statistical difference in the demographic characteristics of subjects between the groups. The average blood ammonia level of all the subjects who died was higher than that of the patients who survived in the same period [(116.57 ± 85.33) μmol/L vs (77.63 ± 35.82) μmol/L, (108.53 ± 73.00) μmol/L vs (71.19 ± 32.53) μmol/L, (106.74 ± 71.59) μmol/L vs (69.21 ± 28.84) μmol/L, (105.77 ± 71.14) μmol/L vs (69.50 ± 29.25) μmol/L, P<0.05]. Based on death after one week, four weeks, twelve weeks and one year, the area under ROC curve (AUC) of blood ammonia was 0.668 (95% CI: 0.542-0.793, P=0.014), 0.706 (95% CI: 0.593-0.819, P=0.002), 0.705 (95% CI: 0.592-0.818, P=0.002), and 0.697 (95% CI: 0.582-0.811, P=0.003), respectively. Compared with the use of blood ammonia, lactic acid or MEDS score alone, the combined use of blood ammonia and MEDS score increased the accuracy of prognosis evaluation in sepstic patients ( P<0.05). Conclusions:Blood ammonia has a high value in predicting the short-term and 1-year prognosis of septic patients in the emergency department. The combined use of blood ammonia and MEDS score can further improve its predictive value.