more on diagnosis

http://www.springerlink.com/content/4af1prhmcw75n6df/

Apparently atypical depression can be an indicator of bipolar. Atypical depression involves a long-standing pattern of extreme sensitivity to rejection. This, I think, could be the equivalent of frantic attempts to avoid abandonment in borderline personality disorder.
Note: there is another way to interpret this. Atypical depression involves mood reactivity as well, and both mood reactivity and hypersensitivty to rejection could be equivalent to the mood reactivity in BPD.
So there is another way in which BP and BPD can overlap. That just leaves dissociation and splitting to differentiate BPD from BP. Although, a science direct search yields a lot of articles for "dissociation in bipolar disorder", and it makes sense that it could occur since bipolar disorder can involve psychotic symptoms, and dissociation is a sort of sub-threshhold psychotic symptom. As for splitting, it may be the one thing that really characterises BPD. I read some of a clinical guide on treating BPD and a lot of it seemed to be about dealing with splitting. Mostly it was about dealing with the difficulties that the patients bring, such as being insulted.

Plus there is the diagnostic category of bipolar not otherwise specified, which includes full hypomanic symptoms without depression, or a history of depression with hypomania that doesn't last for four days. Cycling between depression and hypomania throughout the day can be classified as BP-NOS. Wouldn't this look exactly like BPD, especially if the person had a long-standing pattern of hypersensitivity to rejection?

Of course there are still many other things that could differentiate BP: agitation, racing thoughts, increased activity in general and especially goal-directed activity, decreased need for sleep, flight of ideas, pressured speech, distractibility. None of these should really happen in BPD, at least not in a cluster. If they do happen in a cluster that also involves elated or irritable mood, even if the symptoms don't last that long, I guess that would indicate BP or BP-NOS. This is one way to differentiate BP from BPD, but it still leaves the problem of differentiating BPD from BP. BP can still "look like" BPD.

Apparently it's common for people with BPD to also have Axis I disorders like major depression. You can even have both BP and BPD. It's all very confusing. Obviously mental health diagnosis is not an exact science.