Background: Previous research exploring the psychometric properties of the scores ofmeasures of posttraumatic stress disorder (PTSD) suggests there is variation in their functioningdepending on the target population. To date, there has been little study of theseproperties within UK veteran populations.Objective: This study aimed to determine optimally efficient cut-off values for the Impact ofEvent Scale-Revised (IES-R) and the PTSD Checklist for DSM-5 (PCL-5) that can be used toassess for differential diagnosis of presumptive PTSD.Methods: Data from a sample of 242 UK veterans assessed for mental health difficultieswere analysed. The criterion-related validity of the PCL-5 and IES-R were evaluated againstthe Clinician-Administered PTSD Scale for DSM-5 (CAPS-5). Kappa statistics were used toassess the level of agreement between the DSM-IV and DSM-5 classification systems.Results: The optimal cut-off scores observed within this sample were 34 or above on thePCL-5 and 46 or above on the IES-R. The PCL-5 cut-off is similar to the previously reportedvalues, but the IES-R cut-off identified in this study is higher than has previously beenrecommended. Overall, a moderate level of agreement was found between participantsscreened positive using the DSM-IV and DSM-5 classification systems of PTSD.Conclusions: Our findings suggest that the PCL-5 and IES-R can be used as brief measureswithin veteran populations presenting at secondary care to assess for PTSD. The use of ahigher cut-off for the IES-R may be helpful for differentiating between veterans who presentwith PTSD and those who may have some sy`mptoms of PTSD but are sub-threshold formeeting a diagnosis. Further, the use of more accurate optimal cut-offs may aid clinicians tobetter monitor changes in PTSD symptoms during and after treatment.