This paper examines the prevalence and comorbidity of complex post-traumatic stress disorder (CPTSD) and borderline personality disorder (BPD) among individuals with probable personality disorder, using baseline data from the Structured Psychological Support clinical trial. The clinical characteristics and personality functioning of participants are summarised and compared between those meeting criteria for BPD, CPTSD, both or neither condition. Among 292 participants, 97% reported significant trauma exposure, and over half met the criteria for CPTSD. Those with CPTSD exhibited higher levels of social dysfunction and depression compared with those with BPD, despite both groups showing elevated emotion dysregulation and anxiety. Comorbidity of CPTSD and BPD was high, with 50% of the sample meeting criteria for both conditions. Participants with comorbid CPTSD and BPD displayed poorer baseline scores across all measures of mental health and functioning than those who met criteria for BPD alone. No statistically significant differences were found in suicidal behaviour or treatment-seeking between groups. There were no significant differences in International Classification of Diseases-11 personality trait domains between participants with CPTSD and BPD, but people with comorbid CPTSD and BPD displayed higher levels of trait negative affectivity than those with BPD alone. The findings highlight the need for trauma-informed assessments in clinical settings and a better understanding of the impact of CPTSD on treatment outcomes for people with personality disorder, including how existing treatments may need to be modified to better meet the needs of people with these highly comorbid conditions.

Trial Registration
Current controlled trials ISRCTN13918289 (registered 11/11/2022)