Some things that have helped me differentiate:
1. Considering the symptom of energy or "expansiveness" as the primary manic symptom versus irritability or just a high mood. Many of the patients which could carry a label of bipolar 2 have high levels of dissociation, which create these spells where they are super impulsive and have little self-awareness, usually in the context of interpersonal stress (argument) or crisis (breakup); as opposed to the BPD anger/aggression, they develop this sort of La Belle Indifférence and fly to Australia.
[4] The manic characteristics are not fulfilled in many cases clinically diagnosed with “mania.” The expansivity is missing in the next two examples of inventiveness only superficially resembling mania:
One patient reports good periods during which he is making inventions alternating with
gloomy periods with inactivity. A closer examination reveals periods dominated by negative
symptoms and better periods with fewer such symptoms. For several years he has been
working on a project to develop an improved laundry basket. In the good periods he spends much of his time on this task, taking out books from the library to solve technical problems and drawing sketches. In both types of period his affective mood is neutral.
Another patient in good mood brags a lot of creativity and inventiveness. As an example
of this he mentions that the moment his girlfriend told him she needed a coffee table at the sofa by the window he got a splendid idea of how to design it.
Laundry baskets and coffee tables are miserable expressions of expansive manic projects! We expect manic patients to have expansive plans agreeing with their manic mood. These are also missing in the following example:
A woman displaying pressure of speech and having certain grandiose ideas explains when
questioned about her plans for the future that she wants to apply for disablement pension so she can devote herself full-time to cooking for her friends.
2. Hypnotizability: The higher someone scores, the less confident I am in some of their subjective reports.
3. Conceptual development of diagnoses develops in parallel. More psychiatry's problem and less mine lol For instance, the borderline personality construct from Kernberg [1] s very different, theoretically, but overlapping symptomatically with the bipolar constructs from Akiskal [2]. I remember a study where they went and used something like the TEMPS scale or other bipolar measure on all the borderlines at Cornell Westchester and found that something like 40-60% met the criteria [too lazy to find reference]. So, is this comparing red skies or red apples [3]?
[1] Kernberg, O. F., & Yeomans, F. E. (2013). Borderline personality disorder, bipolar disorder, depression, attention deficit/hyperactivity disorder, and narcissistic personality disorder: practical differential diagnosis. Bull Menninger Clin, 77(1), 1-22.
[2] Akiskal, H. S., Hantouche, E. G., & Allilaire, J. F. (2003). Bipolar II with and without cyclothymic temperament:“dark” and “sunny” expressions of soft bipolarity. Journal of affective disorders, 73(1-2), 49-57.
[3] Ghaemi, S. N., & Barroilhet, S. (2015). Bipolar illness versus borderline personality: red skies versus red apples. Borderline Personality and Mood Disorders, 97-115.
[4] Jansson, L., & Nordgaard, J. (2016). The psychiatric interview for differential diagnosis (Vol. 270). Switzerland: Springer.