Background and Aim American College Of Emergency Medicine (ACEM) guideline has a recommendation for early diagnosis of head injuries following mild trauma. In this study we examined the prediction power, sensitivity, and specificity of this clinical guideline in the need for computed tomography (CT) scan 
Methods & Materials This cross-sectional study was performed for 6 months on patients over 18 years old referred to the emergency department of Vali-e-Asr Hospital in Arak who met ACEM criteria for head CT scan for suspected mild trauma. Demographic characteristics, clinical symptoms, trauma mechanism, physical injuries caused by head trauma, and history of drug abuse were recorded. The consciousness level (Glasgow Coma Scale) was checked every two hours. Patients underwent treatment if there was a pathology in CT images, and those with no clear pathology were discharged after 6 hours and, followed up by phone for two weeks, and in case of any abnormality in the level of consciousness, they were re-examined by CT scanning.
Ethical Considerations This study has an ethical approval obtained from Arak University of Medical sciences (code: IR.ARAKMU.REC.1396.227).
Results 500 patients, 335 male (67%) and 165 females (33%) with the mean age of 46.39± 2.01 years were studied; the sensitivity the ACEM guideline for predicting the need for CT scan in patients with mild head trauma were 100% with a specificity of 3.46% (for the second recommendation, the sensitivity was 100% with a specificity of 6.7%) which indicated that the test was highly sensitive to diagnosing the patients, but its specificity was low.
Conclusion The ACEM guideline had high sensitivity to predicting the need for CT in patients with mild head trauma, but had very low specificity which makes it an unacceptable criterion for rejecting or performing CT scan in these patients.