a ) , which provides that all psychiatric diagnoses must conform to the American Psychiatric Association ' s Diagnostic and Statistical Manual for Mental Disorders . 38 C . F . R . § 3 . 304 ( f ) . The evidence necessary to establish the occurrence of a stressor during service to support a diagnosis of PTSD will vary depending upon whether the Veteran engaged in " combat with the enemy , " as established by recognized military combat citations or other official records . See Cohen v . Brown , 10 Vet . App . 128 ( 1997 ) . If VA determines the Veteran engaged in combat with the enemy and the alleged stressor is combat - related , then the Veteran ' s