Understanding how to differentiate borderline personality disorder, cyclothymic temperament, and bipolar disorder remains one of the most vital diagnostic challenges in psychiatry and psychotherapy. Accurate diagnosis often determines the success or failure of treatment plans.

While bipolar disorder typically responds well to medications, lithium, for example, can lead to symptom remission and long-term stabilization (Ghaemi, 2024) such pharmacological interventions do not address the core dysfunctions in borderline personality disorder. Mislabeling a borderline patient as bipolar may lead to a series of ineffective treatments and prolonged psychological distress.

The confusion largely stems from overlapping mood-related symptoms. However, two distinguishing features can clarify the diagnosis:

1. Borderline personality disorder is characterized by primitive defense mechanisms such as splitting, projection, and projective identification (Kernberg, 1975). For example, splitting leads individuals to perceive others as entirely good or entirely bad. This black-and-white thinking is reflected in the DSM-5’s criteria (“…alternating between extremes of idealization and devaluation”) (APA, 2013). These mechanisms are generally absent in bipolar disorders.
   
2. A key trait of borderline pathology is deep interpersonal hypersensitivity (Gunderson & Lyons-Ruth, 2008). Relationships, especially with a perceived “favorite person” drive most emotional turmoil in borderline patients. As Gunderson (1984) emphasized, relational instability is the defining feature of this condition when compared to other psychiatric disorders.
   

Although emotional dysregulation has been proposed as the core issue in borderline cases (Linehan, 1993), it is more accurate to view interpersonal sensitivity as the root cause, with mood instability being a secondary reaction.

A classic example: a borderline patient might spiral into abandonment depression, a profound state of despair and suicidal ideation, following a minor or perceived rejection by a close partner. Such episodes may resemble major depressive disorder but are reactive, not cyclical.

In contrast, those with cyclothymic temperament show long-standing, low-intensity mood fluctuations that are not triggered by interpersonal interactions and do not involve identity disturbance or primitive defenses.
Keeping these key elements in mind, defensive patterns and relationship-driven symptoms can significantly reduce diagnostic errors.