. 303 . See Arzio v . Shinseki , 602 F . 3 d 1343 , 1347 ( Fed . Cir . 2010 ) . Establishing service connection for PTSD requires ( 1 ) a diagnosis of PTSD ; ( 2 ) credible supporting evidence that the claimed in - service stressor actually occurred ; and ( 3 ) medical evidence of a causal nexus between current symptomatology and the specific claimed in - service stressor . 38 C . F . R . § 3 . 304 ( f ) . A mental health diagnosis should conform to the Diagnostic and Statistical Manual of Mental Disorders , Fifth Edition ( DSM - V ) ( or , prior to 2014 , the Fourth Edition [ DSM -