When there's a fine line between BPD and Bipolar?

Just curious as to how you tell the difference if someone is displaying possible signs of both? I recently caught up on an old talk show episode (not going to say which one, because I don't want anybody commenting on a specific case) and one of the guests they had on I found really interesting to watch, because on the way hand I was getting major BPD vibes from her, but then on the other hand there was also a lot of stuff that might have been observed as being part of a Bipolar disorder... When there's a fine line between BPD and Bipolar?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums