How Reliable Is the Research into CPTSD and BPD? Understanding the Evidence

Research methods, measurement and why findings don’t always agree

When exploring the relationship between Complex Post-Traumatic Stress Disorder (CPTSD) and Borderline Personality Disorder (BPD), it is tempting to look for a definitive answer. Are these distinct conditions? How often do they occur together? Can research reliably distinguish between them?

Yet, as I discovered while undertaking my postgraduate Traumatology dissertation, answering these questions requires more than simply comparing the conclusions of published studies.

We also need to consider how the research was conducted, who took part, how symptoms were measured and whether different studies were investigating the same things.

In this sixth article in my research series, I explore some of the methodological challenges identified in my literature review and why they matter when interpreting evidence about CPTSD and BPD.

Why do research methods matter?

Research findings are shaped not only by the questions being asked but also by the methods used to answer them.

When studies reach different conclusions, this does not automatically mean that one is right and another is wrong. Differences may reflect the populations studied, the definitions used, the assessment tools selected or the way the findings were analysed.

My literature review identified several possible explanations for variation in research findings about CPTSD, PTSD and BPD.

These included differences in diagnostic criteria, assessment methods, sample characteristics and the ways researchers defined and measured trauma.

Understanding these differences is essential before drawing conclusions about the relationship between diagnoses.

Who took part in the research?

One issue I identified was that studies often involved quite different groups of people.

Some researchers studied people seeking psychotherapy, while others examined clients attending specialist trauma services. Other studies focused predominantly on women with histories of childhood sexual abuse.

These groups can provide valuable insights, but they do not necessarily represent everyone who experiences trauma-related difficulties.

For example, people who voluntarily seek therapy may differ from those who do not access treatment. People attending specialist services may have particular needs or presentations that are not representative of the wider population.

My review also raised concerns about the limited representation of some groups in certain studies, including people with more complex presentations and those from different educational backgrounds.

One example concerned a study that excluded participants with lower educational attainment to ensure they could understand self-report questionnaires. Although this may have been intended to support the quality of responses, it also raised the possibility that some relevant experiences were underrepresented.

This illustrates an important research question: who is included, who is excluded, and how might those decisions influence the conclusions?

How were symptoms and diagnoses measured?

Another important issue was the use of different assessment tools.

The literature I reviewed included studies using self-report questionnaires, structured clinical interviews and other methods of assessing PTSD, CPTSD and personality disorders.

For example, the International Trauma Questionnaire (ITQ) was used in research to assess PTSD and CPTSD symptoms as defined by ICD-11. The Personality Diagnostic Questionnaire–4+ (PDQ-4+) and the Structured Clinical Interview for DSM-5 Personality Disorders (SCID-5-PD) were among the measures discussed in relation to personality disorder assessment.

These tools serve different purposes. A self-report questionnaire can provide valuable information about a person’s experiences and symptoms, while a structured clinical interview offers a different way of examining diagnostic criteria.

However, a questionnaire indicating that someone meets a symptom threshold is not necessarily equivalent to a formal clinical diagnosis.

My review highlighted uncertainty about how often participants in some studies had received formal diagnoses, rather than being classified according to questionnaire scores.

This matters particularly when researchers are attempting to distinguish between conditions whose symptoms can overlap.

DSM and ICD: why diagnostic systems matter

The studies I reviewed did not all use the same diagnostic classification system.

Some used versions of the Diagnostic and Statistical Manual of Mental Disorders (DSM), while others used versions of the World Health Organisation’s International Classification of Diseases (ICD).

These systems have not always defined or organised trauma-related disorders and personality disorders in the same way.

Diagnostic criteria have also changed over time, including the development and recognition of CPTSD as a distinct diagnosis in ICD-11.

Consequently, findings from studies using different diagnostic frameworks may not be directly comparable.

This is not simply a technical detail. If researchers use different definitions to identify a condition, the groups they study may differ, making comparisons more difficult.

It also reinforces the importance of checking which diagnostic criteria a study used before interpreting its conclusions.

Why did findings about CPTSD and BPD differ?

The literature review identified variation in findings concerning the relationship between CPTSD, PTSD and BPD.

Some studies supported the distinction between CPTSD and BPD, while also recognising areas of symptom overlap. Other findings varied in the extent to which the diagnoses appeared to co-occur.

Several factors may help explain these differences:

• Different samples: studies involving specialist clinical populations may produce different findings from studies using community samples.
• Different assessment methods: self-report measures and structured clinical interviews do not operate in exactly the same way.
• Different diagnostic criteria: changes in the DSM and ICD can affect how participants are classified.
• Different definitions of trauma: studies may vary in the experiences they assess or the way trauma exposure is defined.
• Different sample sizes: the number of participants meeting particular diagnostic criteria can influence how confidently findings can be interpreted.

These considerations make it difficult to reduce the evidence to a single, simple conclusion.

They also highlight why it is important to examine the methods and limitations of individual studies rather than relying solely on their headlines or final statements.

What about sample size and representation?

Sample size was another consideration in my review.

Some studies included relatively large numbers of participants, while the number meeting particular diagnostic criteria could be much smaller. This can be especially important when researchers are examining co-occurring diagnoses.

A study may include hundreds of participants overall but have only a small subgroup meeting criteria for both CPTSD and BPD. Conclusions about that subgroup may therefore require particular caution.

Representation matters too. Research based predominantly on one gender, a particular trauma history or people already accessing mental health services may not generalise to other populations.

My review suggested that further research would benefit from larger and more diverse samples, alongside more consistent assessment methods.

What did my review suggest was needed next?

Several priorities emerged from my examination of the literature.

First, researchers need clear and consistent ways of assessing trauma exposure and measuring symptoms. This would make it easier to compare findings across studies.

Second, research needs to include a broader range of participants and settings, so that conclusions are not based too heavily on a narrow group.

Third, greater clarity about whether participants have received formal diagnoses or have met questionnaire thresholds would help readers interpret results.

Finally, more research is needed into the relationship between CPTSD and the wider range of personality disorder presentations, rather than focusing exclusively on BPD.

These were priorities identified through the literature available for my dissertation. They should be understood in that historical context rather than as a complete account of research priorities today.

What does this mean for therapeutic practice?

For me, one of the most important lessons is that research evidence needs to be interpreted thoughtfully.

A published finding can inform our understanding, but its relevance depends on the question asked, the population studied, the methods used and the limitations acknowledged.

This is particularly important when considering diagnoses with overlapping features.

Clinical assessment should not rely on assumptions drawn from a single study, a questionnaire score or a person’s history alone. Instead, it should consider the individual’s presentation, experiences, circumstances and needs.

For therapists, this means maintaining curiosity, recognising uncertainty and avoiding the temptation to force complex human experiences into overly simple explanations.

It also means being willing to revise our understanding as research develops.

Reflections from my Traumatology dissertation

When I submitted my literature review in January 2022, I was interested not only in whether CPTSD and BPD could be differentiated, but also in how confidently the available research supported that distinction.

Examining the methods and limitations of the studies helped me appreciate why apparently conflicting findings need careful interpretation.

The evidence I reviewed supported further exploration of diagnostic differences, but it also highlighted limitations in measurement, sample representation and consistency between studies.

For me, this is an important part of evidence-informed practice: being interested in what research tells us, while remaining equally attentive to what it cannot yet tell us.

This article reflects the literature considered in my original dissertation rather than a comprehensive review of research published since 2022.

In the next article, I will turn to assessment and formulation, exploring how an understanding of the research might inform a more individualised approach to working with trauma-related difficulties.

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Lisa Harris
Psychotherapist | Clinical Supervisor | Consultant | Qualified Traumatologist
lisaharris.uk

References

Brewin, C. R., et al. (2017). Review of research concerning PTSD and CPTSD. Clinical Psychology Review. https://doi.org/10.1016/j.cpr.2017.09.001

Cloitre and colleagues (2014). Cloitre, M., Garvert, D., Weiss, B., Carlson, E., Bryant, R., 2014. Distinguishing PTSD, complex PTSD, and borderline personality disorder: A latent class analysis. Eur. J. Psychotraumatol. 5, 25097. https://doi.org/10.3402/ejpt.v5.25097.  

Berroni (2020). Research examining the relationship between PTSD, CPTSD and BPD, including assessment methods and diagnostic differentiation.

Knefel and colleagues (2016). Knefel, M., Tran, U.S., Lueger-Schuster, B., 2016. The association of posttraumatic stress disorder, complex posttraumatic stress disorder, and borderline personality disorder from a network analytical perspective. J. Anxiety Disord. 43, 70–78. https://doi.org/ 10.1016/j.janxdis.2016.09.002.  

Jowett and colleagues (2019/2020). Jowett, S., Karatzias, T., Shevlin, M., Albert, I., 2020. Differentiating symptom profiles of ICD-11 PTSD, complex PTSD, and borderline personality disorder: A latent class analysis in a multiply traumatized sample. Personal. Disord. Theory Res. Treat. 11, 36–45. https://doi.org/10.1037/per0000346.