Bipolar disorder, previously known as manic depression, involves dramatic swings in mood - ranging from debilitating depression to euphoric recklessness.
In the original 2008 study, researchers at Brown University School of Medicine found that of 145 adults who said they had been diagnosed with bipolar disorder, 82 (57%) turned out not to have the condition when given a comprehensive diagnostic interview.
In this latest study, published in the Journal of Clinical Psychiatry, the researchers used similar standardised interviews to find out which disorders those 82 patients might have.
Overall, they found, nearly half had major depression, while borderline personality disorder, post-traumatic stress disorder (PTSD), generalised anxiety and social phobia were each diagnosed in roughly one-quarter to one-third.
When the researchers then compared the patients with 528 other psychiatric patients who had never been diagnosed with bipolar disorder, they found that those in the former group were nearly four times more likely to have borderline personality disorder.
They were also 70% more likely to have major depression and twice as likely to have PTSD.
Other less common diagnoses
Some of other diagnoses were less common but still seen at elevated rates among the patients previously diagnosed with bipolar disorder. These included antisocial personality disorder and impulse-control disorder.
Over diagnosis of bipolar disorder is concerning, in part, because it is typically treated with mood-stabilising drugs that can have side effects - including effects on the kidneys, liver, and metabolic and immune systems, explained lead researcher Dr Mark Zimmerman, an associate professor at Brown and director of outpatient psychiatry at Rhode Island Hospital.
In addition, he told Reuters Health in an email, over diagnosis means some patients are likely not getting the appropriate care for the problems they do have.
Bipolar disorder shares certain characteristics with some other psychiatric conditions. Borderline personality disorder, for instance, is marked by unstable mood, impulsive behaviour and problems maintaining relationships with other people.
But Zimmerman and his colleagues suspect that some doctors are over diagnosing bipolar disorder because - unlike certain other causes of mood disturbance - it does have effective drug therapies.
No drug treatment
There are no medications approved specifically for treating borderline personality disorder, for instance, but research suggests that some forms of "talk therapy" are effective.
"We believe that clinicians are inclined to diagnose disorders that they feel more comfortable treating," Zimmerman explained.
"The increased availability of medications that have been approved for the treatment of bipolar disorder might be influencing clinicians who are unsure whether or not a patient has bipolar disorder or borderline personality disorder to err on the side of diagnosing the disorder that is medication-responsive," he added.
This "bias", Zimmerman said, is reinforced by drug company marketing, which highlights certain studies that have suggested that bipolar disorder goes unrecognized in many people.
Source: Journal of Clinical Psychiatry, online July 28, 2009.