Often, people with borderline personality disorder (BPD) are misdiagnosed with bipolar disorder. In my experience, many people with BPD are first given a bipolar disorder diagnosis, only to later have the diagnosis revised to BPD.
A study published online last month in the Journal of Clinical Psychiatry by Mark Zimmerman MD and colleagues demonstrated that the most commonly used screening tool for bipolar disorder- the Mood Disorder Questionnaire (MDQ)- may lead some people be incorrectly diagnosed with bipolar disorder when the true diagnosis is BPD. In fact, in this study, a positive score on the MDQ (which is supposed to indicate a bipolar disorder) was as likely to occur for someone with BPD as for someone with true bipolar disorder.
Bipolar disorder and BPD do share some features, so it isn't surprising that a self-report measure like the MDQ does not do an adequate job of teasing apart the two conditions. This study provides further support for the idea that a thorough clinical interview is the best way to get an accurate BPD diagnosis.