It can be difficult to diagnose borderline personality disorders (BPD) because it generally has co-occurring disorders alongside it. These other disorders may then conceal the symptoms of BPD, causing those traits to go unnoticed for a period of time.

Borderline personality disorder is diagnosed due to continual emotion dysregulation that is reflected by unstable relationships, mood fluctuation and impulsivity of behavior. A co-occurring mood disorder may have the similar consequences, as well as the vacillation in mood appearing as depression during low points. A diagnosis of anxiety can display worry or avoidance that is also characterized by continual undermining of the self and/or panic in BPD.

The impulsivity of a BPD diagnosis can lead to excessive behaviors such as over use of alcohol or drugs or binge eating and purging, leading to an eating disorder or substance abuse. BPD is comprised of a combination of a genetic predisposition to a lower threshold of emotion regulation along with an invalidating environment.  In this invalidating environment, trauma may have occurred possibly leading to Post Traumatic Stress Disorder (PTSD).